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ELT MASS/DIMM instrument for atmospheric turbulence measurements. Optical and mechanical design. Alignment.
Kornilov V., Potanin S., Shatsky N., Safonov B., Voziakova O. August 23, 2006


Contents
1 Optics design 1.1 Basic principles . . . . . . . . . . . . . . . . . . . . . . 1.2 Principal geometry of MASS­DIMM device . . . . . . 1.2.1 Entrance and exit pupils. System magnification 1.2.2 Fabry lens . . . . . . . . . . . . . . . . . . . . . 1.2.3 Geometry of exit pupil . . . . . . . . . . . . . . 1.3 MASS optics . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Pupil segmentation unit . . . . . . . . . . . . . 1.3.2 MASS channels A, B, C, and D . . . . . . . . 1.3.3 MASS sp ectral resp onse . . . . . . . . . . . . . 1.4 DIMM sub-device . . . . . . . . . . . . . . . . . . . . . 1.5 Field ap erture and viewer . . . . . . . . . . . . . . . . 7 7 8 8 10 10 11 11 14 14 15 17 18 18 20 21 22 23 23 24 24 25 25 26 26 26 27 28 29 29 29 30 31

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2 Mechanical design 2.1 General description . . . . . . . . . . . . . . . . . . . . . 2.1.1 Device skeleton . . . . . . . . . . . . . . . . . . . 2.1.2 Optical b ench . . . . . . . . . . . . . . . . . . . . 2.1.3 Fabry lens unit . . . . . . . . . . . . . . . . . . . 2.1.4 Viewer . . . . . . . . . . . . . . . . . . . . . . . . 2.1.5 Optical plate . . . . . . . . . . . . . . . . . . . . 2.1.6 Electronics mo dule design . . . . . . . . . . . . . 2.2 Alignment p ossibilities . . . . . . . . . . . . . . . . . . . 2.2.1 Common optics . . . . . . . . . . . . . . . . . . . 2.2.2 MASS sub-device optics . . . . . . . . . . . . . . 2.2.3 DIMM sub-device optics . . . . . . . . . . . . . . 2.3 Disassembling and assembling . . . . . . . . . . . . . . . 2.3.1 Disassembly sequence for alignment, maintenance 2.3.2 Disassembly of the electronics mo dule . . . . . . 2.3.3 Assembly . . . . . . . . . . . . . . . . . . . . . . 3 Alignments 3.1 Preliminary alignments . . . . . . . . 3.1.1 MASS PSU alignment . . . . 3.1.2 DIMM sub-device preliminary 3.2 Device alignments at the telescop e . ...... ...... alignment ...... . . . . . . . . . . . . . . . . . . . .

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3.2.1 3.2.2

Fabry lens p osition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Viewer alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31 33 34 34 34 35 35 37 38 38 39 40

4 Critical parameters determination 4.1 System magnification . . . . . . . . . . . . . . . . . . . . . . . . 4.2 DIMM scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 MASS detectors parameters . . . . . . . . . . . . . . . . . . . . 4.3.1 PMT optimal voltage and discrimination determination 4.3.2 Non-linearity and Non-p oissonity determination . . . . . A Optical A.1 The A.2 The the

parts sp ecifications sp ecifications for MASS/DIMM purchased optical elements . . . . . . . . . . sp ecifications for MASS/DIMM sp ecial optical elements manufactured by contractor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B List of mechanical parts

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List of Figures
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 2.1 2.2 2.3 2.4 2.5 2.6 2.7 3.1 3.2 3.3 4.1 4.2 4.3 View of the ELT MASS/DIMM device . . . . . . . Fo cal length of a complex Fabry lens . . . . . . . . Geometry of exit pupil . . . . . . . . . . . . . . . . Optical layout of MASS/DIMM device in ZY plane Optical layout of the MASS sub-device in ZX plane MASS replica segmentator . . . . . . . . . . . . . . Sp ectral resp onse of the MASS device . . . . . . . Optical layout of DIMM sub-device in ZX plane . . Side view of the MASS/DIMM device . Cross-section view of the MASS/DIMM Optical b ench unit . . . . . . . . . . . . Fabry lens unit . . . . . . . . . . . . . . Optical plate views . . . . . . . . . . . . View of the electronic mo dule . . . . . . Alignment of the optical plate . . . . . . .... device. .... .... .... .... .... .. . .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 11 12 12 13 14 16 16 19 20 21 22 23 24 26 30 31 32 35 36 37

Photo catho des p osition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DIMM channel alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Exit pupil to ol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CCD image of a binary star . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Counting functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dep endence of non-Poisson parameter p on flux F . . . . . . . . . . . . . . . . .

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List of Tables
1.1 1.2 1.3 Measured PSU segment dimensions and entrance segments . . . . . . . . . . . . . MASS sp ectral resp onse in relative photon units . . . . . . . . . . . . . . . . . . DIMM channel basic characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 13 15 17

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Intro duction
This do cument describ es the optical and mechanical design of a low-resolution turbulence profiler (MASS) combined with the DIMM device in a single instrument, mo dified for ELT site testing program, according to the Prop osal to Europ ean Southern Observatory (ESO) [1]. The pro ject is implemented in frame of the ESO contract No. PO007467/GWIE. The MASS/DIMM optical scheme was sp ecially calculated for the use with Celestron 11 feeding telescop e. Nevertheless, the two comp onents Fabry lens unit p ermits to use the instrument with other similar telescop es. The principles of the work of MASS and DIMM comp onents of the combined instrument are describ ed in [6], [4], and [11]. The combined MASS/DIMM instrument for CTIO and TMT site testing op erations was designed three years ago [9]. Meanwhile, according to the exp erience obtained in a year-long exploitation of these devices, some changes have b een intro duced in the geometry of the main optical comp onent of MASS ­ the pupil segmentation unit. The Chapter 1 of the do cument presents the final optical parameters of elements together with the tolerances for the critical measures. In addition, the tables give the full sp ecifications for the optical elements, b oth for the standard ones for purchasing in commercial companies and the sp ecial elements manufactured by the contractor. The Chapter 2 describ es the general mechanical design of the instrument. The dimensions are given for CCD camera ST2000MX which was planned for use with MASS/DIMM instrument. The next chapter is a guide for alignment of the optical scheme elements ­ the op eration which is mandatory after the device assembly or while installing the device on the telescop e. Exit pupil optics tuning (MASS segmentator and DIMM mirrors), fo cusing and lateral p ositioning of the Fabry lens, checking the entrance pupil p osition are the sub jects of particular attention. Lastly, the Chapter 4 helps to compute the principal parameters of the device resulted from the finished alignment pro cedure. This is critical for correct interpretation of the scintillation data which is p erformed by the MASS and DIMM software. App endices which follow give technical parameters of the optical and mechanical device comp onents. The electronics of the device and details related to it are presented in a separate do cument [8]. Also, separate do cuments contain Turbina Software reference guide, Turbina user guide [13], and Sup ervisor user guide [10], which complete the full description of the MASS/DIMM instrument and its control software.

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Bibliography
[1] Kornilov V., Combined MASS/DIMM instrument for measurements of the atmospheric optical turbulence. A Proposal to European Southern Observatory (ESO). Septemb er 21, 2005 [2] Kornilov V., Potanin S., Shatsky N., Shugarov A., Voziakova O., ELT MASS/DIMM instrument for atmospheric turbulence measurements. Optical parameters and general design. Octob er 16, 2005 [3] Kornilov V., Combined MASS/DIMM instrument for atmospheric turbulence measurements. A Proposal to Cerro Tololo Inter-American Observatory. Septemb er 27, 2002 [4] Kornilov V., Potanin S., Shatsky N., Voziakova O., Zaitsev A. Multi-Aperture Scintil lation Sensor (MASS). Final design report. February 2002. [5] Kornilov V., Potanin S., Shatsky N., Voziakova O., Shugarov A. Multi-Aperture Scintil lation Sensor (MASS) Upgrade. Final report. January 2003. [6] Kornilov V., Tokovinin A., Voziakova O., Zaitsev A., Shatsky N., Potanin S., Sarazin M. MASS: a monitor of the vertical turbulence distribution. Pro c. SPIE, V. 4839, p. 837-845, 2003 [7] A.Tokovinin, V.Kornilov, N.Shatsky, O.Voziakova, Restoration of turbulence profile from scintil lation indices, MNRAS 2003, V. 343, P. 891 [8] Kornilov V., Shatsky N., Shugarov A., Voziakova O. Combined MASS/DIMM instrument for atmospheric turbulence measurements. Electronics and Device control. Novemb er 2003. [9] Kornilov V., Potanin S., Shatsky N., Shugarov A., Voziakova O. Combined MASS/DIMM instrument for atmospheric turbulence measurements.Optical and mechanical design. Alignment. September 2003 [10] Kornilov V., Shatsky N., Voziakova O. Supervisor program User Guide. SV version 0.22, January 2004. [11] Sarazin M., Ro ddier F., The E.S.O Differential Image Motion Monitor Astron. Astrophys. 227, 294-300 (1990). [12] Tokovinin A. Polychromatic scintil lation. JOSA(A), 2003, V. 20 P. 686-689 [13] Kornilov V., Potanin S., Shatsky N., Voziakova O. MASS Software User Guide Version 2.04. Decemb er, 2003.

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Chapter 1

Optics design
1.1 Basic principles

Here we briefly remind the principles of a Multiap erture scintillation sensor (MASS) instrument. MASS measures four scintillation indices in small central circular ap erture and 3 concentric annular ap erture as well as 6 differential scintillation indices for all p ossible pairwise ap erture combinations. Scintillation indices pro duced by a turbulent layer at some altitude h dep end on the turbulence intensity, on the ap erture geometry and on the sp ectral range (this dep endence is reflected by the so called weighting function W (h) [6, 7, 12]). Using these 10 measured index values, calculation of some integral characteristics of the atmospheric turbulence and restoration of the vertical turbulence profile with low-resolution (5 ­ 6 fixed layers) are p ossible. All the weighting functions drop to zero at zero altitude, so the ground layer is not sensed. DIMM (Differential image motion monitor) measures fluctuations of the angular distance b etween two images pro duced by two circular ap ertures ab out 8 ­ 10 cm diameter separated by ab out 20 cm. Theory states that the rms image motion is prop ortional to the turbulence integral along the full light path in the atmosphere. So, the weighting function for DIMM do es not dep end on altitude. The idea to combine two different turbulence measuring devices in one was brought from the following facts: ­ Multiap erture scintillation sensor (MASS) do es not sense a turbulence lo cated b elow 1 km ab ove ground (b oundary, ground layers). Possible solution -- a generalized mo de of the MASS measurement, was tested with original MASS device. It was found that this metho d is capable of giving reasonable results with large ap erture feeding telescop e only. On the other hand, DIMM equally sensitive to b oth low and high turbulence. ­ When a Cassegrain-typ e small telescop e is used to feed a DIMM device, only two circular parts of the entrance pupil (ab out 15% of area) are used. MASS device uses even less part of the entrance pupil. Note that the original MASS with a set of entrance ap ertures having 13 cm largest ap erture requires 40 cm or larger Cassegrain-typ e telescop e. ­ Numerical simulations show [7] that reducing the largest MASS annular entrance ap erture (the segment D) down to 8.5cm improves the metho d sensitivity for middle altitudes. This

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means that a non-exp ensive (amateur class) telescop e with a diameter 25 ­ 30 cm can b e used to feed the MASS device. The following general solution was chosen and implemented in the combined MASS/DIMM instrument for turbulence measurements: ­ to re-image the plane of entrance pupil of feeding telescop e to the exit pupil plane; ­ to separate sub-ap ertures in that plane, one for the MASS channel and two for the DIMM; ­ for the MASS sub-device, to split the light with help of a segmentator unit (see [4]) onto four MASS channels, and to re-image the exit pupil at photo catho des of MASS detectors; ­ for the DIMM sub-device, to re-image the star in the plane of the CCD detector, while simultaneously moving apart the images pro duced by each of two DIMM sub-ap ertures to obtain the needed separation. The following Sections describ e this pro cess in detail.

1.2

Principal geometry of MASS­DIMM device

Principal geometry of ELT MASS/DIMM device do es not differ from optical scheme used in CTIO MASS/DIMM device. As in CTIO device, the Fabry lens (LF) is placed b efore the fo cal plane F of the telescop e. In this case the Fabry lens shifts the fo cal plane to the new p osition F' (fo cal plane of the instrument) where the field diaphragm is placed. This plane is fixed relative to the feeding telescop e at distance c from the p ole (nominal lo cation) of the primary mirror M1. Also, the plane F' is fixed with resp ect to the segmentator S at distance a. MASS/DIMM device contains complex pupil segmentator unit (PSU) including b oth the MASS channel segmentator and DIMM channel mirrors. These segmentators direct light towards the MASS channel and DIMM channel, resp ectively. The p osition and fo cal length of LF must satisfy two conditions simultaneously: · Precise fo cusing of the image of a target star in the field ap erture critically needed for DIMM channel. · Coincidence of the exit pupil plane with plane of the segmentators. Our calculations (see [2]) show that in order to meet these two demands for the C11 telescop e the two comp onents Fabry lens must b e used.

1.2.1

Entrance and exit pupils. System magnification

To make an image of the entrance pupil, a Fabry lens with some fo cal length F abry must b e placed on the optical axis of the instrument. In fact, the Fabry lens re-builds not the original entrance pupil but its image pro duced sequentially by the primary and secondary mirrors of a telescop e. The dimension and lo cation of this image dep end on the exact geometry of a telescop e, and slightly change when a telescop e is refo cused. Note, that in case of a simple refractor with one ob jective lens, entrance and exit pupil practically coincide, so the magnification of the telescop e itself is 1.

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Figure 1.1: ELT MASS/DIMM device at the telescop e C11. View from the side of the viewer and electronics b ox.

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In case of two mirrors optical system, the telescop e itself pro duces some magnification, b ecause its entrance pupil is larger than exit one. With enough accuracy, the telescop e magnification KT = F1 /F2 . For the Celestron 11, this value lies b etween -3.668 and -3.214 due to uncertainty of the assumed telescop e parameters. The ratio of the diameter of an ap erture in the entrance pupil plane to the diameter of the resp ective physical element placed in the exit plane, is the magnification K of the instrument. It dep ends on the telescop e and device geometry as well. For the design, the lo cation of the entrance pupil plane was chosen to b e 100 mm in front of the secondary mirror (at top edge of the telescop e tub e). Such a p osition is very convenient for instrument check and alignment. The nominal value of K = 15.75 was chosen to fill the whole p ossible entrance ap erture of the C11 telescop e (K = 15.00 for MASS/DIMM for TMT program). To provide b oth the adopted entrance plane lo cation and the needed system magnification, the Fabry lens fo cal length and its p osition with resp ect to the exit plane can b e adjusted moving the comp onents of the LF unit. The exact value of the system magnification influences the final results of b oth MASS and DIMM b ecause the geometry of entrance pupil is used in computation of the turbulence intensity from directly measured values.

1.2.2

Fabry lens

The optical layout of the MASS/DIMM instrument (without feeding telescop e) is presented in Fig. 1.4. The co ordinate system which is used here and further, is defined as follows: ­ Z-axis go es along optical axis of the instrument ­ Y-axis lies in plane of symmetry of the instrument towards viewer. ­ X-axis is p erp endicular to b oth Z- and Y-axes. In order to diminish overall dimensions of the device, the Fabry lens LF is placed b efore the fo cal plane of the telescop e. In this case the Fabry lens shifts the fo cal plane to a new p osition FP where we have a real image of a target star. In this plane the field ap erture FA is placed. Additionally, such LF placement protects the interior optics of the device from dust. Such design requires re-fo cusing the telescop e each time the Fabry lens is shifted along the axis. The p ossibility to move Fabry lens along optical axis is necessary in order to adjust the system magnification, which can differ from its nominal value due to lens fo cal length tolerance and uncertainties in the telescop e geometry. As we show in Optical design rep ort [2], to avoid risk of to o small or to o large ap ertures, the two-elements Fabry lens is necessary, including negative and p ositive achromatic lens. Exact equivalent fo cal length of the system will b e adjusted by alignment of inter-lens distance d. The dep endence of equivalent fo cal length on d is shown on Fig. 1.2 for the chosen lens pair. Also, the separation b etween principal p oints is increased.

1.2.3

Geometry of exit pupil

The size of full exit pupil imaged by Fabry lens dep ends on magnification as well as on the entrance ap erture of telescop e. With adopted values (K = 15.75 and the telescop e C11) outer diameter of the exit pupil is equal to 17.7±0.5 mm, inner diameter is equal to 6.4±0.2 mm, so the clear segment is 5.65±0.1 mm. The drawing of the exit pupil is shown in Fig. 1.3. The further 10


f

eqv

h, h', mm
30 25

140 130 120 110 100 10 20 30 40

20 15 10 5 0

d, mm

10

20

d, mm

30

40

Figure 1.2: Dep endence of the equivalent fo cal length of a complex Fabry lens (left) and p osition of principal p oints (solid -- direct path, dashed -- reverse) on the lens separation d light separation (segmentation) b etween four channels of MASS sub-device and two channels of DIMM sub-device is pro duced with help of MASS pupil segmentation unit (PSU) and two DIMM mirrors DM1 and DM2. Evidently, these elements must b e within the exit pupil. In order to provide this, the lateral shifts of the exit pupil are foreseen in the design (see Sect. 2.2). The presented numb ers force to adopt the maximal outer diameter of the MASS pupil segmentation unit and maximal diameter of DIMM ap erture as large as 5.50±0.05, the same as in MASS-LITE and CTIO MASS/DIMM. For the case of a telescop e with larger diameter, there is more freedom to select size and p osition of the DIMM ap ertures inside exit pupil, but the telescop e secondary mirror spider must not cause any vignetting of either PSU or DIMM ap ertures.

1.3

MASS optics

The optical scheme of the MASS sub-device is shown in Fig. 1.4 as ZY plane view, and in Fig. 1.5 as ZX plane view (from viewer side). The main optical element of the MASS sub-device is the pupil segmentation unit (PSU). PSU forms four reflected b eams and reflects them in different directions.

1.3.1

Pupil segmentation unit

The PSU is lo cated off the instrument optical axis (see Fig. 1.3 and Fig. 1.5) at distance of 6.5 mm, to avoid the central obscuration in the exit pupil. This requires to align the exit pupil by shifting the Fabry lens in Y direction by ±0.5 mm. This is done during Fabry lens alignment. To provide the light reflection from the PSU segments in needed directions to the re-imaging mirrors RA, RB, RC, and RD, the segment mirrors are pro duced tilted by 8.0 to the PSU rotation axis. Then, the segments of the PSU are rotated around PSU axis so that the rotation angle b etween the adjacent segments equals 30 . In order to comp ensate partially for the large 11


Figure 1.3: Geometry of exit pupil for telescop e C11. MASS PSU is shown by yellow. DIMM masks are shown by cyan. Black -- the placement of DIMM re-imaging mirrors.

Figure 1.4: Optical layout of MASS/DIMM device in ZY plane. Common parts: LF -- Fabry lens, FP -- instrument fo cal plane, FA -- field ap erture, ExP -- plane of exit pupil. MASS sub-device: PSU -- pupil segmentation unit, RA, RB, RC, RD -- re-imaging mirrors, PMTs -- MASS detectors. DIMM sub-device: DMs -- two DIMM re-imaging mirrors, MR -- folding mirror, CCD -- plane of CCD detector. Viewer is not shown.

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Figure 1.5: Optical layout of the MASS sub-device in ZX plane (corresp onds to the b ottom view in Fig 1.4). DIMM sub-device and viewer are not shown. Common part: LF -- Fabry lens, FP -- instrument fo cal plane, FA -- field ap erture, ExP -- plane of exit pupil. MASS: PSU -- pupil segmentation unit, RA, RB, RC, RD -- re-imaging mirrors of the A-, B-, C-, and D-channels, PMT -- four detectors. segments tilt and to place the re-imaging mirrors closer to the instrument optical axis, the PSU as a whole is inclined by 4.75 around X axis. Note that the incident b eam is inclined by -2.8 with resp ect to the instrument axis, to o. Angles b etween the segment normals and incident/reflected b eam are 6.9 for outer channels A,D and 6.4 for inner channels B,C. Segments have concave surface with a curvature radius of 250 mm that ensures non-divergent b eams after the reflection from segmentator. This p ermits to use small re-imaging mirrors. Despite the segments tilt, their pro jections are circular with high accuracy. Using the optimal set of MASS ap ertures [7] and geometry of exit pupil (see Sect. 1.2.3 ) the dimensions of the PSU segments were chosen as listed in [2]. Table 1.1: Measured PSU segment dimensions and entrance segments. All values are in millimeters Segment/Channel Segment Segment Segment Segment D C B A Physical diameter inner outer 3.89 2.19 1.30 5.41 3.86 2.16 1.27 Entrance diameter inner outer 62.2 35.0 20.8 86.6 61.8 34.6 20.3

Contrary to previous designs, the PSU is not fabricated from hard bronze but is replicated by A. Tokovinin, having applied right oriented bronze PSU as master and p olystyrene for PSU replica. The replica is covered by Aluminum layer, and a protective S iO overcoating. The microphotograph of the PSU is shown in Fig. 1.6. Final diameters of PSU segments, measured 13


Figure 1.6: On the left: Side view of the one of the MASS segmentator mounted on its holder. On the right: Top view of the finished segmentator. The PSU is illuminated by scattered light. with the help of such microphotographies for all pro duced segmentator, agree well (±0.03 mm) with the nominal diameters, except outer diameter of segment D. Measurement shows that this real diameter is less by 0.1 mm than the nominal one. In the Table 1.1 the measured values are listed. Entrance segment diameters were calculated with K = 16.0 which is typical (and greater than nominal) for measurements during MASS/DIMM instruments commissioning in March 2006.

1.3.2

MASS channels A, B, C, and D

MASS Pupil segmentation unit pro duces four reflected b eams. Each b eam falls on the corresp onding re-imaging spherical mirror RA, RB, RC, and RD. MASS re-imaging mirrors are chosen to b e the same as in original MASS, i.e. 12.5 mm diameter and 51 mm fo cal length. The distances from the PSU to mirrors are equal to 120 mm for all mirrors, the distances from mirrors to corresp onding PMTs are 88.7 mm. The angles b etween the mirror normals and incident/reflected b eams are 11.1 for outer channels A,D and 8.3 for inner channels B,C. Re-imaging mirrors are coated by protected Aluminum layer, reflecting up to 80%. Re-imaging mirrors are tilted slightly to direct light to PMT photo catho des. Light falls onto photo catho des under relatively large angles (13 to 15 ), contributing to the distortion of PSU segment images. The effect of star motion into field ap erture was estimated, and no significant energy re-distribution at photo-catho des was found. The size of the segment images on the photo catho des is reduced by 0.73, so the largest image (D segment) has the diameter ab out 4.0 mm.

1.3.3

MASS sp ectral resp onse

Sp ectral resp onse of the MASS detectors affects the restoration of the vertical turbulence profile through weighting functions (see [12]), therefore the sp ectral resp onse must b e known well. Compact photomultipliers R7400P from Hamamatsu are used as light detectors. These PMTs have bi-alkali photo catho de of 7 mm diameter. The sp ectral sensitivity is typical of 14


bi-alkali photo catho des. We did not foresee the glass sp ectral filters to sp ecify the short-wave cutoff of the MASS sp ectral resp onse. The reason is to improve statistical accuracy in A and B channels where star flux is small. The sp ectral resp onse of MASS is shap ed by the PMT sp ectral sensitivity at its red side, the transmittance of the optic parts at blue side (flint glass SF5 and lens and telescop e optics visual anti-reflection coating, mainly). The final sp ectral resp onse is shown in Fig. 1.7 (right) and numerical data are presented in the Table 1.2. Such sp ectral resp onse pro duces a dep endence of MASS magnitude on star color. In Fig. 1.7 (left) the dep endence is plotted. Transformation from standard V magnitude is describ ed as follows: M AS S = V + 0.71(B - V ) - 0.091(B - V )
2

This equation may b e used for control of the MASS sp ectral resp onse. Such control is needed b ecause the transmittance of the telescop e entrance correction plate is not well known. Table 1.2: MASS sp ectral resp onse in relative photon units. Wavelengths in nanometers 330 340 350 360 370 380 390 400 S () 0.010 0.020 0.050 0.100 0.170 0.280 0.430 0.580 410 420 430 440 450 460 470 480 S () 0.720 0.830 0.890 0.950 0.990 1.000 1.000 0.970 490 500 510 520 530 540 550 560 S () 0.920 0.830 0.740 0.640 0.550 0.472 0.397 0.320 570 580 590 600 610 620 630 640 S () 0.255 0.203 0.160 0.125 0.090 0.070 0.050 0.040

The integral parameters of the MASS sp ectral resp onse are: effective wavelength ef f for A0 star 467 nm (496 nm for TMT instruments with cutoff filter), sp ectral bandwidth 1/2 ab out 100 nm (85 nm). Effective wavelength for other star can b e approximated by dep endence ef f = 467 + 29 · (B - V ). So, the ELT MASS sp ectral resp onse mimics rather the B photometric band than V. More precise control of the MASS sp ectral sensitivity requires the carefully prepared star list, since: · some program stars have variability 0 m .05 Â 0m .1. For example µ1 Sco have amplitude 0m .3. · a slop e of (VM AS S - V ) versus (B - V ) color dep ends on effective wavelength mainly, a curvature of this one dep ends on width of the sp ectral band, so not only blue and red stars must b e measured, but uniformly distributed over color range.

1.4

DIMM sub-device

Two spherical mirrors DM1 and DM2 covered by two-ap erture mask are placed in the exit pupil. These mirrors transfer the stellar image from the instrument fo cal plane to two images on the 15


MASS-V
0,8 0,6 0,4

S()

1,0 0,8 0,6

0,2 0,4 0,0 0,2 -0,2 -0,5 0,0 0,5 0,0 300

B-V

1,0

1,5

400

, nm

500

600

700

Figure 1.7: On left: Color equation b etween MASS magnitude and star color index B-V. On right: Sp ectral resp onse of the ELT MASS device.

Figure 1.8: Optical layout of DIMM sub-device in ZX plane (corresp onds to the top view in Fig 1.4). The MASS sub-device and viewer are not shown. Common part: LF -- Fabry lens, FP -- instrument fo cal plane, FA -- field ap erture, ExP -- plane of exit pupil and mask. DIMM: DM1 and DM2 -- DIMM re-imaging mirrors, MR -- folding mirror. CCD detector surface. The distance b etween the mask holes defines the DIMM base (see Fig. 1.4 and Fig. 1.8). The diameter of the re-imaging mirror is equal to 10.8 mm with clear diameter 9.8 mm (see Fig. 1.3). Such a diameter p ermits to fix the DIMM base with help of the DIMM mask only. The distance b etween mirror centers is 15.0 mm. The fo cal length of such a mirror must b e 67.5 ± 1 mm to provide minimal ab errations and the needed distance to CCD. The mirrors are made very thick for their size -- 5 mm, in order to provide a stability of the image. The mirrors are coated by Aluminum with protective S iO film. The main characteristics of the DIMM sub-device are presented in the Table 1.3. The physical dimensions are determined by fabricated mask, entrance dimension are calculated for 16


the case of K = 16.0. Table 1.3: DIMM channel basic characteristics. All values are presented in millimeters Segment/Channel Diameter of ap erture Base bD Scale on CCD Physical dimension 5.5 12.1 Entrance dimension 88 194 88 ± 5 /mm

The distance b etween the DIMM mirrors and the folding mirror MR (see Fig. 1.8) is 100 mm. In this case the reflected b eam cross-sections at the MR mirror are equal to 2.5 mm and the distance b etween them is ab out 5 mm. The minimal tilt of the DIMM mirrors is 1.25 in YZ plane, which pro duces a clearance of ab out 0.5 mm b etween the incident b eam and the edge of MR for the worst case of star p osition in the field ap erture (shift 0.7 mm to the CCD edge). At the mirror surface, the clear space from the edge is ab out 0.8 mm in this case. The incident angle varies from 0.95 to 1.55 dep ending on the star p osition in the ap erture, and this can change slowly due to abb erations as well. In the plane XZ, the DIMM mirrors are tilted by ±3.19 . Note, that these angles must b e adjustable very finely to provide the needed distance b etween two star images. Re-imaging pro duces an additional scale change with magnification ab out 1.2.

1.5

Field ap erture and viewer

A field ap erture is lo cated in the fo cal plane of the instrument. It serves to limit the contribution of sky background to the light measured by MASS detectors. On the other hand, the ap erture limits the field of view for DIMM sub-device. As a compromise, the size of the field ap erture as large as 2.2 mm (4 ) was chosen. The ap erture is made as a hole in a flat thin steel plate. The size of a wide field of view for star finding is ab out 9 mm or 16 . To view this field, a moving mirror MV is shifted onto the optical axis of the device. The selected star must b e placed into the central hole (it is seen as red circle when FOV illumination is on) of a glass plate CC, which is co-aligned with the field ap erture to b etter than 0.2 mm (ab out 20 ). In this case, after removing of the mirror, light passes to DIMM and MASS detectors. Further star centering must b e done by the DIMM software. The viewer is not used in a normal work since the DIMM sub-device provides the star detection after telescop e was p ointed at the star and guiding during measurements. In practice, the viewer serves as an auxiliary to ol in extraordinary cases. Removable mirror MV has dimensions 12 â 18 mm. When inserted in the b eam, it is placed at 40 angle with resp ect to the optical axis of the device. This provides the viewer axis tilt equal to 80 with resp ect to the instrument optical axis. The re-imaging system of the viewer consists of two achromatic lenses with fo cal length 50 mm and repro duces the instrument fo cal plane with magnification -1. The lenses are separated by 75 mm distance. Standard 1 1 inches eye-piece with fo cal length 12 ­ 15 mm is used with the viewer. The 4 eye-piece is lo cated at 220 mm from the axis of the device and telescop e and ab out 40 mm from the back plane of telescop e to provide easy access for the observer. 17


Chapter 2

Mechanical design
2.1 General description

Mechanical design of the combined MASS/DIMM device is done on the base of CTIO and TMT MASS/DIMM development. We keep the main dimensions and general structure of the device. But a numb er of units were redesigned to provide matching with Celestron 11 telescop e. Also, some mo difications were implemented if previous design was found not optimal. External view of the ELT MASS/DIMM instrument is shown in Fig. 1.1 attached to Celestron 11 telescop e. Practically all parts of the device are fabricated from hard aluminum alloy, black-ano dized. Only few critical parts are made from stainless steel. The dimensions of the instrument and its weight are minimized. The full length of the device is 175 mm. The width and depth of the device without a viewer tub e and CCD camera is ab out 80 mm â 90 mm. The eyepiece of the viewer is 165 mm apart from the device side. Total weight of the MASS/DIMM instrument is ab out 1.5 Kg. The instrument is fixed on the feeding reflector instead of a 2 inch Back fo cus barrel with help of a thread on the sp ecial mount ring 02A. Main b ox can b e aligned with resp ect to this ring with four pairs of p op-pushing screws PP2, to provide the needed p osition of the telescop e exit pupil on segmentator unit. The needed p ositional angle of the device is fixed by the counter-blo cking screw. At the device front side 01A, the Fabry lens unit 02 is attached. The side cover 01B of the MASS device b ox is detachable for access to the optics of the instrument fixed on the transversal b eam 01E which is called hereafter "optical b ench". This b ench b ears the field diaphragm FA, the removable mirror MV and the glass CC with the centering hole of the viewer, the main blind 06C and four re-imaging mirrors RA - RD. The b ench with the optics may b e detached from the device if needed. No additional adjustment is exp ected after back mounting of the b ench on the device. In particular, the removal of the optical b ench is needed for the fo cusing of the Fabry lens to allow the easy access to its rotating holder and to reduce the large depth of sharpness caused by the small field diaphragm size. After b ench removal, the segmentator may b e illuminated with a small lamp and its image may b e investigated and measured in the entrance pupil plane. The pupil segmentation unit is mounted in the so cket of the b ottom tie 01D of the device b ox. PSU may b e removed and fixed back for checking or cleaning. The sp ecial cover 01F allows an access to the fixing and adjustment screws of the PSU. In the MASS channel (sub-device), the light reflected from PSU and then from re-imaging

18


Figure 2.1: Side view of the MASS/DIMM device without CCD camera. See designations in text.

19


Figure 2.2: Cross-section view of the MASS/DIMM device. mirrors falls onto photomultipliers PMT. In the DIMM channel, the light b eam is reflected by mirror MR and arrives onto the CCD detector plane. In order to remove the side cover of the device, the electronic and photometric unit 08 must b e first unscrewed and removed as one b ox. The viewer 03 with Kellner typ e eyepiece can b e removed for rob otic observations. The mechanical parts are designated on the drawings as "MEnnS", where ME ­ prefix for ELT MASS/DIMM device, 'nn' ­ assembly unit numb er, 'S' -- suffix for the sp ecific part. The prefix is omitted when a designation is mentioned b elow. In Fig. 2.1 the general view of the device is shown. The cross-section of the device is presented in Fig. 2.2 to explain its interior design.

2.1.1

Device skeleton

The force structure of the device consists of 3 elements: device base 01A, U-profile main b eam 01C and b ottom tie 01D. These parts are screwed together and form rigid through-like frame. This structure b ears all other units and assemblies. Do not disassemble the device skeleton 20


unless there is no other solution! Device base holds the Fabry lens unit 02 and is screwed to mount ring 02A. The transversal b eam 01E (called b elow ­ optical b ench) is attached to the main b eam. The optical b ench b ears most parts of the device optics. On the outside of the main b eam, a CCD camera interface 01G is fastened. Also, the switching knob of the viewer mirror, 05E, is placed at one side of the b eam. The optical plate with PSU and DIMM re-imaging mirrors DM1 and DM2 is mounted on the b ottom tie and covered by 01F. Also, the electronic b ox is set on the tie and fixed to the tie. Inside the device, PMTs blind 07C is screwed to the upp er plane of the tie. The cover 01B (the second half of the device b ox) is fastened to the base and to the tie.

2.1.2

Optical b ench

The optical b ench 01E is a central assembly unit of the device. On the top plane of the b ench three functional units are mounted: ­ unit of the re-imaging mirrors RA, RB, RC, and RD; ­ viewer removable mirror unit; ­ fo cal plane unit. On the b ottom planes the folding DIMM mirror MR and the central blind are fastened. The top and b ottom views of the optical b ench are shown in the Fig. 2.3. In this pictures, the central blind is removed. The re-imaging unit consists of the mirror supp ort 07A with so ckets for mirrors, where the mirrors lie free, and the cover plate 07B, which fixes the mirrors. The supp ort is fastened to the optical b ench with help of four M2.5 screws. Two push screws PP5 p ermit to adjust a little the total tilt of the mirrors holder.

Figure 2.3: The top (left) and b ottom (right) views of the optical b ench. The viewer removable mirror is a more complex unit. It contains the supp ort 05A, the clamping cramp 05C which limits the mirror rotation and b ears the Hall sensor plate, the mirror 21


holder 05B with two half-axes and the switching -like spring. The mirror MV is cemented to its holder. Also, the cover plate 05D which holds the glass plate CC with central hole is screwed to the supp ort. Illuminating FOV LEDs are mounted at the cover plate. The plate CC is glued to the holder. The fo cal plane unit includes the field ap erture 06A pressed into the so cket of ap erture supp ort 06B. The folder mirror MR of the DIMM sub-device is supp orted by the sp ecial supp ort 06D to which it is pressed by the spring cover plate 06E. The supp ort itself can b e aligned with help of 6 screws PP6 which fasten it to the optical b ench. A blind 06C is utilized to prevent direct light passing from the field ap erture to the PMT photo catho des. It also reduces the scattered light from the exit pupil elements: DIMM mask, mirrors, holders. All electronic parts placed in the main case of the device are lo cated on the optical b ench. The b ench b ears a connector for this electronics that matches the connector in the electronic b ox. More information ab out electronic elements which are inside the main b ox can b e found in the Do cument [8]. In addition to the ab ove-mentioned electronics, two control light LED PCB is mounted to the b ench directly.

Figure 2.4: The view of the complex Fabry lens unit

2.1.3

Fabry lens unit

The Fabry lens unit is sup erimp osed with mechanism, providing MASS/DIMM device tilt with resp ect to optical axis of the feeding telescop e. The Fabry lens unit itself includes the tub e 02B with a thread for the p ositive lens holder 02C. The thread serves to fo cus the Fabry lens. The negative lens is installed in the holder 02D which is able to move resp ectively 02C by thread again. This p ermits to change distance b etween lenses and varies the equivalent fo cal length of the complex Fabry lens. Optical elements are mounted in their holders with using a thin lo cking nut 02E. After fo cus alignment, to fix the lenses p osition the lo ck nuts 02G and 02F are used.

22


2.1.4

Viewer

The viewer consists of three parts: the eye-piece so cket 03A, the viewer tub e 03b. and the viewer flange 03C. The latter is p ermanently screwed to the b ox cover. The re-imaging lenses V1 and V2 are installed in the so ckets of the eye-piece part and of the viewer flange with the help of lo cking nuts. After viewer removing, the viewer flange must b e close by sp ecial cap 03E to prevent light and dust p ollution.

2.1.5

Optical plate

Figure 2.5: The view of the optical plate with PSU holder and two DIMM mirrors alignment plane. The optical plate 04A and other parts related to this unit are fabricated from stainless steel to provide stability of the alignments for PSU and DIMM re-imaging mirrors. The external views of the plate are shown in Fig. 2.5. In the center of the plate a low central pad CP (height 1 mm) is placed. All three adjustable parts are pressed against this pad on one end. The PSU holder 04C is fastened by 3 screws -- 2 pulled and 1 pushed. The PSU itself is installed in thread hole of the holder by PSU fo ot and lo cked by 2 screws with help of lo cking plate 04E. When unlo cking, PSU has a freedom to rotate around its axis. The DIMM mirror plates (right and left) 04B are mounted by 4 screws, 2 pulled and 2 pushed. The DIMM mirrors are cemented in the so ckets of the mirror plates. From the opp osite side of optical plate, a MASS/DIMM mask 04D that defines the exact geometry of the exit pupil is inserted. Right angular p osition is provided by sp ecial pin pressed in the optical plate. This p ermits the optic plate to b e removed from the MASS/DIMM device either with the mask or without it. In the latter case, the mask must b e fixed to the b ottom tie with help of 2 screws. For these screws, the four holes in the plate are foreseen. The optical plate is mounted in the so cket of the b ottom tie of the device b ox. The sp ecial cover protects the optical plate and its fixing and alignment screws.

23


2.1.6

Electronics mo dule design

The electronics mo dule (see Fig. 2.6) consists of two parts: the PMTs housing and the electronics case. The parts are screwed together and are not detachable from each other. The PMTs housing 08A contains 4 PMTs, 3 PCBs of the photon counting electronics, the Teflon spacer 08C, which prevents PMT photo catho des from contact with housing, and the shutter mechanism.

Figure 2.6: View of the detached electronics mo dule. The shutter mechanism (inside PMTs housing) consists of two steel blades with holes, a cramp, a lever, and the axis 08G ­ Shutter knob. The axis passes through the hole in the housing, its rotation closes (clo ckwise) or op ens (counterclo ckwise) the shutter. The shutter do es not provide full darkness when the electronics is detached from the device, but protects the PMTs from direct daylight. The housing must b e always closed with the cover 08B when p owered. The PCB which b ears auxiliary electronics, two external connectors, and the connector to the main-case electronics, is mounted on the frame 08H of the electronics case. The cover 08I protects the electronics from the outside. In the cover, a window for the LED indicators is made. When electronics is p owered, the green LED shines. A presence of HV is indicated by the red LED and data exchange ­ by the yellow LED.

2.2

Alignment p ossibilities

Some alignment features are provided. Most of them are intended for assembly pro cess only. Other alignments are done when the device is attached to the feeding telescop e. All the alignments must b e p erformed when the telescop e is fo cused at infinity. The alignments are: ­ fo cusing of the Fabry lens; ­ lateral shifts of the exit pupil with resp ect to segmentator; 24


­ ­ ­ ­ ­ ­ ­

tilt of the viewer mirror; centering of the CC plate; rotation a PSU segments around their axis; tilts of the PSU in XZ and YZ planes; tilts of the MASS re-imaging mirror assembly in two directions; tilts of the DIMM mirrors DM1 and DM2 in XZ and YZ planes; tilt of the folding DIMM mirror MR in YZ plane;

2.2.1

Common optics

The fo cusing of the Fabry lens is done by rotating the LF holder 02C in the thread of the supp ort tub e 02B. The error of 0.5 mm in the Fabry lens p osition pro duces the magnification error less than 0.5% and the shift of the entrance pupil plane along optical axis ab out ±100 mm. So, an accuracy of the Fabry lens fo cusing ab out 0.5 mm (half a turn) is more than sufficient. The full range of fo cusing of ±10 mm around the nominal p osition is provided. The nominal Fabry lens p osition dep ends on the particular feeding telescop e. Do not forget to fix the final p osition with help of the lo ck ring 02F. To set the correct magnification factor (usually, the maximal p ossible for current telescop e) the fo cal length of the LF can b e adjusted. For this, remove lo ck nut 02G and rotate the negative lens holder 02D with resp ect to p ositive lens holder. Rememb er, that if you twist out holder, the distance b etween lenses is enlarged and equivalent fo cal length is decreased (see Fig. 1.2). As result, the magnification K increases. In this case, the size of exit pupil, insp ected with additional fo cusing to ol, will decrease. Also, fix the holder by lo ck nut 02G and rep eat the fo cus pro cedure. The coincidence of the image of instrument exit pupil and complex segmentator is achieved by tilt of the whole device with resp ect of the mount ring 02A. Three pair p op-pushing screws PP2 p ermit to provide needed p osition with of accuracy of ab out 0.1 mm. Forth pair is used for final fixing. The inclination of the viewer mirror is fixed during the device assembly and should provide b eam axis parallel to the viewer mechanical axis. The tilts are regulated with help of set screw in the viewer supp ort cramp 05C. The residual offset of the on-axis star image from the viewer center may b e eliminated by shifting manually the glass plate CC, up to ±1 mm in b oth directions.

2.2.2

MASS sub-device optics

After the MASS segmentator is fixed in its place, the PSU p osition angle should b e tuned to correct value and the segmentator should b e inclined as a whole as well. The aim of these alignments is to direct the reflected b eams precisely into the centers of the resp ective MASS re-imaging mirrors. These alignments are provided by the push-and-fix screw pairs having the full range of ab out ±1.5 (this is enough, given the roughly correct initial segmentator setting under these angles). Finally, to center the PSU images on the PMT photo catho des, the re-imaging mirror assembly is aligned with an accuracy not worse than ±0.2 (corresp onds to the centering errors of ab out 1 mm on the PMTs). Given that the mirror supp orts are already made with correct angles, the alignment range of ±1 is sufficient. Use PP7 screws for this. 25


Figure 2.7: The optical plate with PSU in its holder and two DIMM mirrors alignment plates. 1 -- screws M2.5 fixed plate to device, 2 -- pull alignment screws M2 of the PSU supp ort, 3 -- push screw M2 for PSU tilt alignment, 4 -- PSU lo ck screws, 5, 6 -- push and pull screws M2 for alignment of the DIMM mirrors tilt in X direction, 7, 8 -- push and pull screws M2 for alignment of the DIMM mirrors tilt in Y direction, 9 -- windows for mask fixing screws.

2.2.3

DIMM sub-device optics

Similarly to the MASS channels optics, the initial setting of DM1 and DM2 mirrors is also made with the roughly correct angles. Precise alignment of them is aimed to comp ensate for the manufacturing and assembly imp erfections and, more imp ortant, ­ to provide the two stellar images ab out 0.2 ­ 0.5 mm apart from each other in the CCD fo cal plane. This is the most fine tuning of the device optics since setting of the star images separation to within ±5 corresp onds to the 5 µm shifts of the DM mirror supp orts. The DM mirrors are adjustable within the ±1 range which covers ±3 mm range on the CCD (see Fig. 2.7). The tilt of the folding mirror MR should make the image plane parallel to the CCD surface. This tilt is not that critical, b ecause the star is put always in the same place in the field of view during measurement. More imp ortant, that the star images must b e lo cated near CCD detector center. Note, that the tilt of re-imaging DIMM mirrors in YZ plane, pro ducing the similar alignment, is strongly limited: when the tilt is small, the vignetting at folding mirror edge app ears, otherwise ­ optical abb erations are significant.

2.3
2.3.1

Disassembling and assembling
Disassembly sequence for alignment, maintenance or repair

Do not forget to close the PMTs shutter b efore disassembly of the device! Disassemble the parts only to the state needed for the device maintenance or optics alignment or cleaning. Some parts of the device can b e removed without device op ening, in arbitrary order: ­ The Fabry lens can b e removed with its holder only. Before, mark the p osition of the holder inside the supp ort tub e to re-establish the fo cusing at assembly. ­ The viewer can b e detached if the instrument is aligned and further work is planned in automatic mo de. For this, unscrew the viewer tub e with eye-piece together from viewer flange. Protect the first viewer lens V1 by a sp ecial cup (provided in the accessories). 26


­ To check or clean a V2 lens, unscrew the viewer tub e from an eye-piece so cket. ­ The electronics mo dule can b e detached to do some checks or alignments. Turn off the device, b e sure that PMTs shutter is closed. Remove 4 M3 screws (2 near the viewer and 2 from the b ottom tie) completely, then pull the electronics mo dule away from device b ox, to unplug it from the internal connector. ­ Optical plate with MASS PSU and DIMM re-imaging mirrors DMs can b e removed to check the optics or the p osition of the exit pupil. First, slacken 2 screws M2.5 and remove the optical plate cover. When unscrewing 3 M2.5 screws (see Fig. 2.7) completely, supp ort the plate by hand. If you wish to remove the mask to o, b efore unscrew 2 M2 screws which hold the mask in b ottom tie so cket. To provide access to the optics inside of the main device b ox, the cover 01B must b e removed. To do this: detach the electronics mo dule first, unscrew 4 M3 screws -- 2 which fasten the cover to the device base (near the viewer) and 2 which fasten the cover to the b ottom tie. With some effort remove cover away in the Y-direction. Then, the optical b ench where most of the optics is installed, can b e removed from the main b eam. To do this, from internal side of the main b eam, unscrew completely 4 screws which are around the folding mirror supp ort 06D. Flip the mirror in the viewer-on p osition to detach the mirror semi-axis from the gro ove in the mirror knob. Pull gently the optical b ench out in the Y-direction. Further disassembly is not recommended. If it is really needed, consult the designers for additional recommendations.

2.3.2

Disassembly of the electronics mo dule

Disassembling the electronics mo dule includes several steps, which must b e done sequentially. To remove the PCB of p ower and auxiliary electronics, one must: ­ ­ ­ ­ unscrew 3 M2.5 screws which fasten the electronics cover 08I and remove this cover; unscrew completely 2 M2.5 screws from the plate 08J of DB9 line connector; unscrew 3 M2 screws that fasten the PCB itself; if it is necessary to remove the PCB completely, unsolder the HV yellow cable and disconnect the blue cable.

To change the PMTs or repair the counting electronics, do the following: ­ unscrew 4 M2 screws from the PMT housing cover 08B and remove this cover; ­ unscrew completely 2 M2 screws from the counters PCB (connector side) and unscrew from the base the long M2 screw with Teflon tub e; ­ disconnect this PCB and turn it by 180 ; ­ unscrew 3 M2 and 2 M1.6 screws from the amplifiers PCB; ­ with the help of a thin screwdriver (< 1.5 mm), b egin to unscrew 2 M1.6 screws through the holes that are nearly opp osite to the connectors edge of the PCB, simultaneously pulling up the PCB itself; ­ when these screws are detached from the PMT housing, fold the PCB very carefully, pulling the PMTs out of the housing. 27


2.3.3

Assembly

Assembly is done in reversed order. A few recommendations may b e useful for this pro cess. ­ When mounting the optical b ench back to the main b eam, pay attention to the p osition of the gro ove in the axis of the viewer knob. The mirror half-axis must ho ok into this gro ove. Be sure that the b ench lies correctly in the b eam b efore tightening finally the 4 screws. ­ When installing the cover b ox back to the device, do not damage the rubb er cord which is glued in the gro oves of the b ottom tie and the device base. Also, check that the connector on the optical b ench is correctly inserted in the corresp onding hole of the cover. Be sure not to leave a slot b etween the upp er edge of the cover and the device base. ­ When fastening the PCB, b e sure that the PCB is laid correctly and tightly. ­ When attaching the electronics mo dule, b e careful to insert the connector pins correctly into the matching connector on the optical b ench. ­ When installing the holder with Fabry lens, do not reverse it. The negative lens of the complex Fabry lens must face the telescop e.

28


Chapter 3

Alignments
3.1 Preliminary alignments

Preliminary MASS/DIMM optics alignments are p erformed during device assembly. These alignments include a correct placement and tilt of the optical elements to provide light pass through MASS and DIMM channels. Alignment p ossibilities were describ ed ab ove in Sec. 2.2. To align the optics, one will need to prepare some additional to ols: a kind of the optical test b ench, the laser light source, and a telescop e mo del (see b elow). The optics test b ench may b e arbitrary but providing enough rigidity and the source-toMASS distance of the order of 0.5 ­ 1 meter. The attachment of the device to the b ench must provide the p ossibility to adjust the p osition of the light source (laser b eam) with resp ect to the device in two directions. The semiconductor laser of no more than 3 mW p ower is set on the opp osite end of the b ench. The variable resistor of a few KOhm is recommended to b e connected sequentially with the laser to adjust laser b eam intensity. The laser supp ort must also allow the slight corrections by angle. In addition, one needs the weak negative lens to attach to the laser to make the slightly divergent b eam. It is needed to illuminate homogeneously the entrance pupil of the mo del telescop e. The latter is attached to the device instead of the Fabry lens holder and consists of the go o d-quality ob jective lens (fo cal length ab out 50 mm) and the pupil diaphragm of the size ab out 5 mm set in front of the ob jective at the distance equal to the lens fo cal distance. The telescop e fo cusing should b e p ossible.

3.1.1

MASS PSU alignment

Since the MASS PSU in ELT MASS/DIMM is a monolithic element, the alignment of the separate segments is not required. PSU alignment includes tuning p osition angle of segmentator around its rotation axis and tilts in X and Y direction. Switch on the laser and direct its b eam into the field ap erture. Incline the device by ab out 3 in YZ plane or shift the laser to provide that laser b eam falls on the PSU installed. If the laser b eam is wide enough (but no lens is installed in front of the laser), all four segments of the PSU will b e illuminated. Otherwise, firstly p oint the b eam on the largest D­segment. One can see the reflected b eam sp ots near the re-imaging mirrors.

29


Correct PSU orientation will direct the b eams onto the mirror centers. After finishing the tuning of rotation angle, tighten the PSU lo ck plate. In principle, it is p ossible to use a sp ecial mask with the marked mirror centers put atop of the mirrors, but normally the laser b eam sp ots are sharply seen at mirror surfaces. After doing these rotating alignments, try to set the b eams closer to the mirror centers using tilts of the PSU supp ort with help of 3 screws (No 2 and 3 on Fig. 2.7). Normally, a combination of the prop er tilt of the PSU supp ort and appropriate rotation angles of the segments provide the reflected b eams falling close enough to the re-imaging mirror centers that no light is lost somewhere in the further path due to vignetting. There is no individual alignment for each re-imaging mirror. The supp ort of those mirrors as a whole can b e tilted slightly in two directions. This p ermits to align a little the p osition of segment images built by re-imaging mirrors RA, RB, RC, and RD on the PMT photo catho des. To check the correct p osition of the images, a sp ecial mask can b e used. The exact p ositions of the PMT photo catho des with resp ect to electronics mo dule reference plane are shown in Fig. 3.1. The manufacturing accuracy is sufficient to provide right image centering on the PMT photo catho des.

01B 01D

Figure 3.1: Photo catho des p osition with resp ect to the reference plate of the b ottom tie.
9.5

Photocathode

Pay sp ecial attention to D­segment image, b ecause it is the largest one. To check the pupil images as they will b e on photo catho des, attach the mo del telescop e to the device. With help of a negative lens, pro duce the divergent laser b eam, fo cus the mo del telescop e. In the dark ro om, the images of segments are seen on the pap er mask placed in the plane of PMTs photo catho des. Also, the images can b e observed directly using a magnifying lens when PSU is illuminated by any scattered light.

3.1.2

DIMM sub-device preliminary alignment

Before DIMM sub-device alignment, remove the main blind 06C. First approach can b e done without mo del telescop e. If the field ap erture is illuminated by scattered daylight, it is p ossible to see its image in the plane of a CCD detector. With help of alignment screws No 5, 6, 7 and 8 at the optical plate (see Fig. 2.7), put the images built by right DM1 and left DM2 re-imaging mirrors into the center of CCD detector. This images must b e separated at ab out 50 pixels symmetrically with resp ect to the frame center. The CCD frame orientation dep ends on how the CCD is installed at its interface. Put the CCD frame columns along the Z-axis of the device. In this orientation, the images must b e separated horizontally. 30

6.5


If CCD camera has its own fo cus p ossibility -- fo cus the camera to provide sharp image of field ap erture edge. The test picture of the field ap erture is shown in the Fig. 3.2 on left. The separation is bigger than nominal -- ab out 100 pixels. The ap erture images must b e illuminated uniformly. If some vignetting is observed, this means that folding mirror has an incorrect tilt. The alignment of its supp ort may b e done. As a rule, accuracy of the supp ort fabrication is sufficient to skip this alignment. Install the mo del telescop e on the optical axis of the device. Pro duce a "star" image in the fo cal plane of the instrument. In the fo cal plane of CCD detector, two "star" images will b e observed. This pro cedure must b e done with the faded laser intensity! Align the p osition of images vertically and horizontally. The images must b e p oint-like without any noticeable abb erations. During this stage you can adjust a separation b etween "star" images. Do not mix up the right and left image! If you are not sure where is the right and left image in CCD frame -- change slowly the CCD camera fo cus. When images are set correctly, the approach of the camera to device pro duces enlarging of the images separation. If you detect that images are mixed up, transp ose them with help of alignment screws No 5, 6, 7 and 8 at the optical plate. See the Fig. 3.2.

Figure 3.2: On left: The CCD frame of the illuminated field ap erture. On right: Right aligned artifical star images with the horizontal separation of 41 pixels. The describ ed alignments are normally done in the lab oratory once after the device optics assembly but it is useful to check this alignments at the telescop e, to o. The rest alignments related to the installation of the device on the feeding optics are describ ed b elow.

3.2
3.2.1

Device alignments at the telescop e
Fabry lens p osition

First alignments after the device attachment to telescop e are convenient to do with the sp ecial to ol (Fig. 3.3) delivered with MASS/DIMM instrument. This to ol represents a wide-field eyepiece designed to install on the device instead the optical plate 04A. In first, remove optical plate. To do this, unscrew the screws No 1. Then, with care, separate it from b ottom tie using thin screwdriver. Mask 04D must b e rest on the place b ecause it is fixed by 2 screws to the b ottom tie 01D. Install wide-field eye-piece, fo cus it to mask edges.

31


Figure 3.3: Wide field eye-piece for exit pupil alignments.

Then, remove the side cover 01B and optical b ench 01E. Illuminate well the entrance ap erture of the telescop e or p oint telescop e to the bright ob ject such as a white wall. The image of the telescop e entrance pupil can b e viewed directly through the mask with help of WF eyepiece. Tilting device by push-pull screws PP1 put this image symmetrically with resp ect to mask holes. Be sure that the telescop e is preliminary fo cused at infinity. Place some flat opaque ob ject with a sharp edge (e.g. a pap er strip e or a ruler) into the plane of the entrance pupil (top end of the telescop e tub e). Observe the image of this ob ject in the plane of exit pupil. If the Fabry lens is correctly fo cused, the pupil image with an ob ject shadow will b e seen sharply. Otherwise, Fabry lens fo cusing must b e p erformed. The simplest way to do this is to remove MASS/DIMM from the telescop e after marking its adopted p osition angle. Spin out LF holder at 2 revolutions. Install the device on the telescop e again. Check sharpness and rep eat fo cus pro cedure if needed. In practice, such pro cedure converges quickly. When the correct LF fo cusing will b e reached, the image of the telescop e entrance pupil may app ear less or greater than mask. One can see inner or outer vignetting of MASS or DIMM ap ertures. In this case alignment of the instrument magnification must b e done. To set the correct magnification factor (usually, the maximal p ossible for a given telescop e), the fo cal length of the LF can b e adjusted. For this, remove the lo ck nut 02G and rotate the negative lens holder 02D with resp ect to the p ositive lens holder. Rememb er, that if you twist out holder, the distance b etween lenses is enlarged and equivalent fo cal length is decreased (see Fig. 1.2). As a result, the magnification K increases. In this case, the size of exit pupil, insp ected with additional fo cusing to ol, will decrease. Also, fix the holder by lo ck nut 02G and rep eat the fo cus pro cedure. Then check the correct p osition of the exit pupil again. When Fabry lens is fo cused and laterally aligned, remove the WF eye-piece, install the optical plate and the optical b ench. To check the MASS channels, lo ok at the PSU through PMT holes with help of a lens. Segment images must b e uniformly illuminated without any vignetting. Point telescop e to some star. Doubled star image must app ear in the CCD frame. Fo cus the telescop e and correct the telescop e p ointing to provide the images in the center of the CCD frame. If needed, make alignments of images separation, not letting them to shift vertically. Close the cover of the optical plate.

32


3.2.2

Viewer alignment

When a star image is lo cated in the center of ap erture (it can b e checked again by a partial illumination of the telescop e entrance) the viewer can b e aligned, to o. Lo ok in the viewer. To see an illuminated central hole, attach the electronics mo dule. If the star image is offset from the center of the illuminated circle, one needs to detach the side cover and lo osen the CC glass holder fixing screws. Using a magnifying lens for controlling the star image in the glass hole, move the holder until star drops in the center of the hole. Very precise alignment is not needed here. When done, tighten the screws to fix the glass holder to the viewer mirror supp ort and mount the side cover 01B at the device (see Sect. 2.3.3). Lo oking in the viewer, check fo cus. Viewer fo cusing is made by eye-piece shifting in o cular tub e.

33


Chapter 4

Critical parameters determination
4.1 System magnification

As it follows from Sect. 1.2.1, a system magnification dep ends on parameters of the telescop e optics as well as parameters of the MASS/DIMM optics. The system magnification transforms the physical dimensions of the pupil segmentation elements into the sizes of the instrument annular entrance ap ertures which are included in the theoretical formulae. Therefore, exact system magnification is needed for MASS sub-device correct work as well as DIMM work. The measurement of the system magnification has to b e p erformed in a dark ro om. All the alignments and a real telescop e fo cusing must b e done b efore. It is very convenient to do this just after fo cusing Fabry lens while optical plate 04A is removed and wide-field to ol is installed at the device b ottom tie. Remove the top (o cular) part 04H of the to ol. Put some strong light source in front of the field lens CL2. The mask 04D image is built in the entrance pupil plane of the MASS/DIMM + telescop e system. Place some semi-transparent screen (pap er sheet) in the plane of the entrance pupil and, if the light source is bright enough and well collimated (like LED flash), one can see directly the images of all three holes in mask (MASS and 2 DIMM). Measure the diameter of these holes and distance b etween DIMM ap erture with help of any ruler or mark the hole edges for further measurement. While examining the edges of these mask holes, make sure that there is no vignetting in the system (edges are equally sharp, vertical size is equal to the horizontal size). The magnification of the system is obtained by division of the measured sizes by the corresp onding physical diameters of the mask 04D holes (see Table 1.1). If the image is well-fo cused, the precision of image size measurement of the order of 0.5 mm is easily achievable and is more than enough for our purp ose. Check the matching of these estimations, compute the mean magnification value and put it in the device.cfg file.

4.2

DIMM scale

DIMM scale in the CCD detector plane is critical to transform a measured rms in CCD pixels into arc-seconds. The real scale may differ from the preliminary value (see Table 1.3). Determination of DIMM CCD scale must b e done after the all device alignments are finished and feeding

34


telescop e is fo cused precisely. The b est way to obtain the scale is taking a CCD image of a known binary star with a separation b etween its comp onent in range 20 -- 60 . The magnitude difference b etween comp onents is preferably less than 3 m . In the Fig. 4.1, part of the CCD frame is presented. One can see two pairs of images. Separation b etween images of the same brightness is defined by DIMM re-imaging mirrors DMs alignment. Distance b etween bright and weak comp onents in the given example is known as 25 . To determine the scale factor, this distance must b e measured on the frame in pixels with help of any graphics program which provides pixel co ordinates output. Dep ending on DIMM software requirements -- a scale constant in arc-second/pixel or pixel/arc-second must b e the calculated. In the given example, the scale value of 0.99 /pixel was found.

Figure 4.1: CCD image of the binary star HR8895 obtained for scale determination.

4.3
4.3.1

MASS detectors parameters
PMT optimal voltage and discrimination determination

In order to cho ose the working p oint (optimal HV level common for all PMTs, and individual discrimination levels), one needs to conduct the counting characteristics registration. Counting characteristics are recorded using the Detector Counting measurement function of the Turbina program (Menu Tools) (see [13]). Since the fluxes from the control light differ much in different channels, at least two levels of the control light are recommended to set in the sequence, to have the curves with the plateau fluxes from 300 to 1000 pulse/ms. With lower signal level, the precision of the non-Poisson parameter is degraded, with bright light, the strong non-linearity is already encountered. An additional control light level equal to zero (0) must b e set in a sequence to get the dark current characteristics. The grid of high voltage levels covers normally the range 550 to 950 V with a 50 V step. While fine-tuning the settings subsequently, the step and range may b e lowered. The discrimination threshold level is tuned within a range from 0.3 to 0.9 mV with a step 0.1 mV. These input parameters for the measurement are set in turbina.cfg file in the Section Operations SubSection Detectors counting measurement. The accumulation time of each p oint should b e long enough for the reliable estimate of non-p oissonity. The estimate of the precision of its determinations is:
2 p

=

2 1 (1 + ), N F

(1)

where N is a total numb er of micro-exp osures, F is a mean count p er micro-exp osure. In practice, to achieve the relative precision of p ab out 0.5% one needs the accumulation time 35


Figure 4.2: Light counting functions. Left: Flux dep endence on the high voltage for 7 threshold levels (0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 mV), lowest curve corresp onds to the 0.9 mV level. Right: Non-Poisson parameter as a function of Voltage for 7 threshold levels, here lowest curve corresp onds to the 0.3 mV threshold. more than 100 s at high fluxes. This implies ab out 2­4 hours pro cess for the total cycle of measurements. Since the drifts and temp erature dep endences are p ossible, the rep etitive measurements for checking the working p oint stability are necessary. These measurements can b e done with a narrower range of input parameters to economize time. The dark current characteristics are aimed to determine the range of the HV level and discrimination thresholds where the dyno de or pulse amplifier noise is negligible. For making the light characteristics, one needs to measure the relations of b oth flux and non-Poisson parameter p on the HV level U . An example of such relations is given in Fig. 4.2. From this figure it follows that the high voltage must not b e lower than 800 V. The counting characteristics b ecome flat enough, fluxes dep end weakly on the discrimination threshold and the non-Poisson parameter approaches the value ab out unity only ab ove this value. Note, nevertheless, that for the threshold of 0.9 mV the HV has to b e not less than 900 V to provide a low slop e of the HV dep endence. On the other hand, the HV of 800 V is quite enough for the threshold of 0.5 mV. It is b etter to use HV as low as p ossible. Since the HV value is common for all the PMTs, joint analysis must b e done. Doing this, keep in mind that non-Poisson parameter is most critical for PMT in the channel A. Note, that the upp er limit for PMT R7400 is 950 V and this value should not b e selected for a long term usage. An additional constraint is the over-light protection. Note here, that since the relation of an average ano de current on the high voltage supply is quite steep, the safety limit of the over-light system (counted in pulses p er second) decreases significantly when the HV level grows.

36


4.3.2

Non-linearity and Non-p oissonity determination

In order to treat correctly the photon statistics and compute the correct scintillation indices, one has to know the non-linearity parameter and the non-Poisson parameter p. Correct value of the parameter is critical at high fluxes (in C and D-channels), while an exact value of the parameter p is needed at low fluxes (in A and B-channels).

Figure 4.3: Dep endence of non-Poisson parameter p on flux F for C and D channels. Line is linearly fitting the measured p oints. Both these parameters are derived from the dep endence of p on the light flux F which needs to b e sp ecially obtained. To get the p­F relation (see previous section), one can use the sp ecial function Detector Statistics measurement in the Turbina program, placed in menu item Tools. The measurements of flux F and non-Poisson p values are made with currently set values of the discrimination thresholds of counters and high voltage level. The grid of the control light relative intensities is supplied dense enough to get the needed precision of the output parameters. Some fifty values from 0.0 to 1.0 with a step 0.02 are recommended. The duration of one p oint measurement is determined by the formula (1) and may b e of the order of 40 sec or more. These input parameters for the measurement are set in turbina.cfg file in the Section Operations SubSection Detectors statistics measurement. The typical relation of the non-p oissonity p on the average flux in channels C and D is shown in Fig. 4.3. It is clear that this relation is practically linear. It should b e noticed meanwhile that the b etter fit is obtained with a quadratic approximation of the relation. Use the least-square metho d to get the linear regression co efficients (the handy graph-plotting program xmgrace provides such a p ossibility as many others). The crossing p oint of a line fit with the p-axis (constant term in regression) determines the parameter p. The line slop e in the p oint of zero flux is equal to -2 where the non-linearity is expressed in milliseconds if the flux F is counted in pulses p er milliseconds. The slop e of fitting line in the Fig. 4.3 corresp onds to a non-linearity parameter ab out 16 ns.

37


Appendix A

Optical parts specifications
A.1
Des. 1

The sp ecifications for MASS/DIMM purchased optical elements
Part and parameters Fabry lens 1 Fo cal length: -150 mm Diameter: 25.0+0.0 mm -0.2 Fabry lens 2 Fo cal length: 75 mm Diameter: 25.0+0.0 mm -0.2 Viewer lenses Fo cal length: 50 mm Diameter: 18 mm Control lenses 1 Fo cal length: 75 mm Diameter: 30 mm Control lenses 2 Fo cal length: 40 mm Diameter: 25 mm Kellner eyepiece Fo cal length: 12 mm 1 Barrel diameter: 1 4 inches Manufacturer Edmund Optics Sto ck name/numb er ACH25x-150MgF2 TS NT45-423 ACH25x75MgF2 TS NT32-325 ACH18x50MgF2 TS NT32-913 PCX30x75MgF2 TS NT32-486 PCX25x40MgF2 TS NTNT45-279 -- Total q-ty 4 Rem. 1

2

Edmund Optics

4

1

3

Edmund Optics

8

1

4

Edmund Optics

4

1

5

Edmund Optics

4

1

6

Any

4

1. See sp ecification at www.edmundoptic.com

38


A.2

The sp ecifications for MASS/DIMM sp ecial optical elements manufactured by the contractor
Part and parameters Removable mirror Size: 12 â 18 mm Substrate: BK7 glass Thickness: 2 mm Surface Accuracy: /4 DIMM folding mirror Size: 10 â 15 mm Substrate: BK7 glass Thickness: 3 mm Surface Accuracy: /10 MASS mirrors Diameter: 12.8 mm Curvature radius: 102 mm Substrate: BK7 glass Thickness: 3 mm Surface Accuracy: /4 DIMM mirrors Diameter: 10.8 mm Curvature radius: 136 mm Substrate: BK7 glass Thickness: 5 mm Surface Accuracy: /10 Segmentator Diameters: see Tab. 1.1 Curvature radius: 250 mm Substrate Material: Plastic replica Surface Accuracy: /4 Circle reticle Central hole 1.2 mm Thickness: 1.0 mm Diameter: 13.0+0.0 mm -0.2 OP6 Part numb er OP4 Ref. op4.dwg Total q-ty 8 Rem. 1

Des. MV

2 OP3 op3.dwg 8 1

MR

2 OP1 op1.dwg 32 1

R1-4

2 OP2 op2.dwg 16 1

DM1,2

2 md10a.dwg md10b.dwg md10c.dwg md10d.dwg op6.dwg 8 1

PSU

2 9 3

CC

1. Coating: Protected aluminum, R avg. > 87% 2. Surface Quality: 40 ­ 60 scratch and dig over central 95% of surface

39


Appendix B

List of mechanical parts
The table contains the list of mechanical parts which are needed for pro duction of the MASS/DIMM device. The parts are group ed in assembly units. Quantity is given for one copy. Remarks "S", "R", "C" are the assigned ranking estimations of the part work-consuming -- simple, rotationsymmetry and complex. The list contains 57 parts. Needed fasteners and standard items are not included in this table and will b e presented separately. Des. ME01 ME01A ME01B ME01C ME01D ME01E ME01F ME01G ME02 ME02A ME02B ME02C ME02D ME02E ME02F ME02G ME03 ME03A ME03B ME03C ME03D ME03E Part Box Device base Box cover Main b eam Bottom tie Optics b ench Segmentator cover CCD camera interface Fabry lens unit Mount ring Fabry lens tub e Positive lens holder Negative lens holder Lo cking nut #1 Fixing nut #1 Fixing nut #2 Viewer Eyepiece so cket Viewer tub e Viewer so cket Lo cking nut #2 Viewer cup HA HA HA HA HA 1 1 1 2 1 R R R R R HA HA HA HA HA HA HA 1 1 1 1 2 1 1 R R R R R R R HA HA HA HA HA HA HA 1 1 1 1 1 1 1 C S C C C S S Material Q-ty Rem.

40


Des. ME04 ME04A ME04B ME04C ME04D ME04E ME04F ME04G ME05 ME05A ME05B ME05C ME05D ME05E ME05F ME05G ME05H ME05I ME05J ME05J ME05K ME06 ME06A ME06B ME06C ME06D ME06E ME07 MD07A MD07B MD07C MD07D

Part Pupil segmentation unit PSU supp ort DIMM mirror plates MASS segmentator holder Ap erture mask PSU lo cking plate Pupil control to ol #1 Pupil control to ol #2 Switching mirror unit Supp ort Mirror holder Clamping cramp Illuminators plate Right axis Knob axis Switching knob Bushing Bushing nut Left axis Left axis switch spring Central unit Field ap erture Ap erture supp ort Central blind Folding mirror supp ort Spring cover plate Other parts MASS mirrors so cket Mirrors cover plater PMTs side blind Connector supp ort

Material

Q-ty

Rem.

Steel Steel Steel HA Steel HA HA HA HA HA HA Steel Steel HA Steel Steel Steel Steel Steel Steel HA HA HA Steel HA HA HA HA

1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

C C S S S R R C C S R R R R S S R R R R R R C S C S S S

41


Des. ME08 MD08A MD08B MD08C MD08D MD08E MD08F MD08G MD08H MD08I MD08J

Part Electronics b ox PMT house Detectors cover PMT separator Shutter blade Shutter cramp Shutter lever Shutter axis Electronics frame Electronics cover DB9 supp ort

Material

Q-ty

Rem.

HA HA TF Steel Steel Steel Steel HA HA HA

1 1 1 2 1 1 1 1 1 1

C S S S S S S C S S

42