Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://curl.sai.msu.ru/mass/download/doc/elt_description.pdf Äàòà èçìåíåíèÿ: Mon Sep 11 23:05:42 2006 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 20:16:25 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: ï ï ï ï ï ï ï ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ï ï ï ï ï ï ï ï ï

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 A