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Combined MASS/DIMM instrument for atmospheric turbulence measurements. Optical and mechanical design. Alignment.
Kornilov V., Potanin S., Shatsky N., Shugarov A., Voziakova O. January 16, 2004


Contents
1 Optics design 1.1 Basic principles . . . . . . . . . . . . . . . . . . . . . . 1.2 Principal geometry of MASS/DIMM 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 12 12 14 14 15 16 18 19 19 19 22 22 24 25 26 26 27 27 28 28 28 29 30 30 30 31 32 32

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2 Mechanical design 2.1 General description . . . . . . . . . . . . . . . . . . . . . 2.1.1 General characteristics . . . . . . . . . . . . . . . 2.1.2 Device skeleton . . . . . . . . . . . . . . . . . . . 2.1.3 Optical b ench . . . . . . . . . . . . . . . . . . . . 2.1.4 Other assembly units . . . . . . . . . . . . . . . . 2.1.5 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 . 3.2.1 Fabry lens p osition . . . . . . ...... ...... alignment ...... ...... . . . . . . . . . . . . . . . . . . . . . . . . .

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... ... ... ... ... ... ... ... ... ... ... repair ... ... . . . . . . . . . . . . . . .

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3.2.2

Viewer alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33 34 34 35 35 35 37 39 39 40 41

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 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 3.1 3.2 4.1 4.2 4.3 Principal optical layout used for calculation . . . . Optical layout of MASS/DIMM device in ZY plane Geometry of exit pupil for two feeding telescop es . Optical layout of the MASS sub-device in ZX plane MASS segmentator . . . . . . . . . . . . . . . . . . Sp ectral resp onse of the MASS device . . . . . . . Optical layout of DIMM sub-device in ZX plane . . View of the device . . . . . . . . . Main dimensions of the device with Main dimensions of the device with View of the device without cover . Optical b ench unit . . . . . . . . . Optical plate views . . . . . . . . . View of the electronic mo dule . . . Alignment of the optical plate . . . ... ST5 ST7 ... ... ... ... ... ... CCD CCD ... ... ... ... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 11 13 13 15 16 17 20 21 21 22 23 24 25 27 31 33 35 36 37

.... camera camera .... .... .... .... ....

.. for for .. .. .. .. ..

.... CTIO TMT .... .... .... .... ....

...... programs. programs. ...... ...... ...... ...... ......

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Photo catho des p osition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . System magnification adjust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 1.4 1.5 1.6 The telescop e parameters adopted for MASS/DIMM optical design. . . . . . The MASS/DIMM main optical parameters . . . . . . . . . . . . . . . . . . . Telescop e exit pupil geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . PSU and entrance segment dimensions for feeding telescop es . . . . . . . . . MASS sp ectral resp onse in relative photon units. Wavelengths in nanometers DIMM sub-device characteristics for feeding telescop es . . . . . . . . . . . . . . . . . . . . . . . . . 8 12 12 14 16 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, according with a Prop osal to CTIO [1]. The pro ject was implemented in frame of the AURA contract No. C10389A. The electronics of the device and detailes related to it are presented in a separate do cument [6]. Also, separate do cuments contain Turbina Software reference guide, Turbina user guide [11], and Sup ervisor user guide [7], which complete the full description of the MASS/DIMM instrument and its control software. The MASS/DIMM optical scheme was sp ecially calculated for the use with two feeding telescop es: standard 10-inch Meade and 35 cm telescop e for the TMT (Thirty Meter Telescop e) DIMM custom-made by the German company Halfmann. The principles of the work of MASS and DIMM comp onents of the combined instrument are describ ed in [4], [2], and [8]. Meanwhile, according to the exp erience obtained in a year-long exploitation of the original MASS device [5], some changes have b een intro duced in the geometry of the main optical comp onent of MASS ­ 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 b oth the Meade- and TMT-fed devices which differ in typ es of CCD camera used and in the construction of the mechanical interfaces for attachment of the instrument to the feeding telescop e. 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.

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Bibliography
[1] Kornilov V., Combined MASS/DIMM instrument for atmospheric turbulence measurements. A Proposal to Cerro Tololo Inter-American Observatory. Septemb er 27, 2002 [2] Kornilov V., Potanin S., Shatsky N., Voziakova O., Zaitsev A. Multi-Aperture Scintil lation Sensor (MASS). Final design report. February 2002. [3] Kornilov V., Potanin S., Shatsky N., Voziakova O., Shugarov A. Multi-Aperture Scintil lation Sensor (MASS) Upgrade. Final report. January 2003. [4] 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 [5] A.Tokovinin, V.Kornilov, N.Shatsky, O.Voziakova, Restoration of turbulence profile from scintil lation indices, MNRAS 2003, V. 343, P. 891 [6] Kornilov V., Shatsky N., Shugarov A., Voziakova O. Combined MASS/DIMM instrument for atmospheric turbulence measurements. Electronics and Device control. Novemb er 2003. [7] Kornilov V., Shatsky N., Voziakova O. Supervisor program User Guide. SV version 0.22, January 2004. [8] Sarazin M., Ro ddier F., The E.S.O Differential Image Motion Monitor Astron. Astrophys. 227, 294-300 (1990). [9] Tokovinin A. Polychromatic scintil lation. JOSA(A), 2003, V. 20 P. 686-689 [10] Kornilov V., Potanin S., Shatsky N., Voziakova O. Combined MASS/DIMM instrument for atmospheric turbulence measurements. Optical parameters and general design. July 2003. [11] 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

We shortly remind here 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 the turbulence intensity, on the ap erture geometry and on the sp ectral range (so called weighting function W (h) [4, 5, 9]). Using these 10 measured values, a calculation of some integral characteristics of the atmospheric turbulence and a restoration of turbulence vertical profile with low-resolution (5 ­ 6 fixed layers) are p ossible. All the weighting functions drop to zero at zero altitude. 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 over full atmosphere. So, the weighting function for DIMM is un-dep ending on altitude. The idea to combine two different turbulence measuring devices in one, grew from the facts, that: ­ Multiap erture scintillation sensor (MASS) do esn't sense a turbulence lo cated in b oundary layer (first 1 km ab ove ground). Possible solution -- a generalized mo de of MASS measurement, was tested with original MASS device. It was found, that this metho d will b e able give a reasonable results with large ap erture feeding telescop e only. On the other hand, DIMM measures b oth low and high turbulence. ­ When a parts of entrance requires Cassegrain-typ e small telescop e is used to feed DIMM device, only two circular entrance pupil (ab out 15% of area) are used. MASS device uses even less part of pupil. But original MASS set of entrance ap ertures (with 13 cm largest ap erture) 40 cm or larger Cassegrain telescop e.

­ Numerical simulations show [5] that reducing the largest MASS annular entrance ap erture (the segment D) down to 8.5cm optimizes the metho d sensitivity for middle altitudes. This means that a non-exp ensive (amateur class) telescop e with diameter 25 ­ 30 cm can b e used to feed the MASS device.

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The following general solution was chosen and implemented in 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 the help of a segmentator unit (see [2]) onto four MASS channels, and to re-image the exit pupil at a 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. The following Sections describ e this pro cess in detail.

1.2

Principal geometry of MASS/DIMM device

The MASS/DIMM device was designed and calculated for usage with two different feeding telescop es: a standard 10-inch Meade amateur telescop e (Cassegrain-Schmidt optical system) and a 35 cm telescop e for the TMT (Thirty Meter Telescop e) DIMM custom-made by the German company Halfmann. For the calculation of the design parameters, the data listed in the Table 1.1 were used. Table 1.1: The telescop e parameters adopted for MASS/DIMM optical design. All dimensions are in millimeters Parameter Nominal equivalent fo cal length F0 Fo cal length of primary mirror F1 Diameter of primary mirror D1 Telescop e fo cal plane p osition 0 b ehind primary Magnification on secondary mirror m Fo cal length of secondary mirror F2 Separation of primary mirror from secondary one d Central obscuration parameter q Diameter of secondary mirror D2 TMT telescop e 2800 875 350 200 3.2 -372 619 0.292 102 Meade 10 telescop e 2500 570 254 200 4.4 -185 427 0.251 64

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1.2.1

Entrance and exit pupils. System magnification
a Fabry lens with some fo cal length F abry must b e In this case exit pupil will b e lo cated at a distance Really, the Fabry lens re-builds not original entrance by primary and secondary mirrors of a telescop e. The 8

To make an image of the entrance pupil, placed on optical axis of the instrument. af (according to Fig. 1.1 ) from the lens. pupil, but its image pro duced sequentially


9 Figure 1.1: Principal optical layout used for calculation. S -- pupil segmentation unit, F -- telescop e fo cal plane, F' -- Fabry lens+telescop e fo cal plane, LF -- Fabry lens, M1 -- p ole of the primary mirror, M2 -- p ole of the secondary mirror, S' -- image of S pro duced by Fabry lens, S -- image of S' built by mirror M2, E -- entrance pupil of the system, which is the image of S pro duced by primary mirror. All values designed by right arrow are p ositive, otherwise -- negative. dH -- distance b etween principal p oints of Fabry lens. Other values are explained in Section 1.4.


dimension and lo cation of this image dep end on the exact geometry of telescop e, and change slowly when telescop e is refo cused. Physically, the p osition of the exit pupil is fixed, and the lo cation of the entrance pupil plane are defined by the telescop e system as well as by the Fabry lens fo cal length and p osition. The entrance pupil lo cation isn't imp ortant for the results of measurements. Since the elements splitting light b etween channels are placed in the exit plane and work as ap erture stop, the working entrance pupils for b oth MASS and DIMM sub-devices, can b e defined in reverse light path as the images of these pupil stops pro duced by the Fabry lens + telescop e optical system. The ratio of the diameter of an ap erture in the entrance pupil plane to the diameter of resp ective physical element placed in the exit plane, is the magnification K of the instrument. It dep ends on 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 telescop e tub e). It is common for Cassegrain-Schmidt system, but not for pure Cassegrain. Such p osition is very convenient for instrument check and alignment. The nominal value of K = 15.00 was chosen (K 20 in the original MASS). 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 exit plane must b e chosen correctly for b oth telescop es. The exact value of system magnification influences final results of b oth MASS and DIMM b ecause the geometry of entrance pupil is used in computation 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. 1.2. The co ordinate system which is used here and further, is as follows: ­ Z-axis 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 adjustment the system magnification, which can differ from its nominal value due to lens fo cal length tolerance and uncertainties in telescop e geometry. In the Table 1.2 the main optical parameters of the MASS/DIMM device are listed. Catalogs of Melles Griot and Edmund Optics companies were used to select the Fabry lens. The exact formulae for computations are presented in [10].

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CCD

FP LF MV FA RD(RA) CC SFs PMTs PSU MR ExP DMs

Z Y
V1

RC(RB)

V2

EP

Figure 1.2: 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, SFs -- sp ectral filters, PMTs -- MASS detectors. DIMM sub-device: DMs -- two DIMM re-imaging mirrors, MR -- folding mirror, CCD - plane of CCD detector. Viewer: MV -- viewer removable mirror, CC ­ glass plate with central hole, V1, V2 -- transmoving ob jective, EP -- eye-piece.

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Table 1.2: The MASS/DIMM main optical parameters. All dimensions are in millimeters MASS/DIMM parameter Fabry lens fo cal length Fabry Fabry lens diameter Dabry Distance b etween FP and EP planes a LF p osition with resp ect fo cal plane da Instrument magnification K Entrance pupil plane p osition bE Scale in the fo cal plane scaleF for TMT telescop e 140 25 123 -38.7 15.52 -100 108 /mm for Meade telescop e 125 25 123 -28.7 14.50 -100 115 /mm

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. The table b elow lists some characteristics of the pupil geometry for b oth feeding telescop es. The drawing of exit pupil is shown in Fig. 1.3 for b oth feeding telescop e. The further light separation (segmentation) b etween four channels of MASS subdevice and two channels of DIMM sub-device is pro duced with help of MASS pupil segmentation unit (PSU) and two spherical mirrors DM1 and DM2. Evidently, these elements must b e within the exit pupil. To provide these lateral shifts of the exit pupil are foreseen in the design (see Sect. 2.2). Table 1.3: Telescop e exit pupil geometry. All values are in millimeters Pupil dimension Inner diameter Outer diameter Clear segment width TMT telescop e 7.1 22.6 7.7 Meade 10 6.5 17.5 5.5 MASS pupil segmentation unit of Meade telescop e as 5.5 mm, select size and p osition of the mirror spider must not cause

These data force to adopt the maximal outer diameter of the as 5.5 mm and maximal diameter of DIMM ap erture for the case to o. For the case of TMT telescop e, there is more freedom to DIMM ap ertures inside exit pupil, but the telescop e secondary 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. 1.2 as ZY plane view, and in Fig. 1.4 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. 12


DM1

DM1

PSU

PSU

DM2

DM2

Figure 1.3: Geometry of exit pupil for two feeding telescop es. Left -- Meade telescop e. Right -- TMT feeding telescop e. PSU is shown by yellow. DIMM masks are shown by cyan. Black -- the placement of DIMM re-imaging mirrors.

RA LF FP RB FA

SF

PMT A ExP PMT B PMT C PMT D PSU

SF

RC

SF

X
RD SF

Z
Figure 1.4: Optical layout of the MASS sub-device in ZX plane (corresp onds to the b ottom view in Fig 1.2). 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, SF -- four sp ectral filters, PMT -- four detectors.

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1.3.1

Pupil segmentation unit

The PSU is lo cated off the instrument optical axis (see Fig. 1.3 and Fig. 1.4) at distance of 6.5 mm for b oth feeding telescop es, 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 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 [5] and geometry of exit pupil (see Table 1.3 ) the dimensions of the PSU segments were chosen as listed in Table 1.4. Table 1.4: PSU segment dimensions and entrance segment dimensions for b oth feeding telescop es. All values are in millimeters Segment/ Channel Segment Segment Segment Segment Segment Segment Segment D D C C B B A outer inner outer inner outer inner outer Physical diameter 5.50 3.90 3.85 2.20 2.15 1.30 1.27 TMT telescop e 85.4 60.5 59.8 34.1 33.4 20.2 19.7 Meade telescop e 79.8 56.6 55.8 31.9 31.2 18.8 18.4

The PSU is fabricated from hard bronze, its mirrors are p olished as a whole with optical quality. The reflecting coating is made by evap oration under vacuum and consists of 3 layers: a Chromium layer dep osited on the bronze, an Aluminum layer, and a protective S iO overcoating. The microphotograph of the PSU is shown in Fig. 1.5. Final diameters of PSU segments, measured with the help of such microphotographies for all pro duced segmentator, agree well with the nominal diameters of Table 1.4.

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. 14


Figure 1.5: On the left: Top view of one of the MASS segmentator, illuminated by scattered light. On the right: Segmentator mounted in its holder. 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. Usage of simple spherical mirrors under such angles pro duces significant astigmatism. Re-imaging mirrors are tilted slightly to direct light to PMT photo catho des. The outer mirrors are symmetrically tilted by 4.5 around Y-axis and by -3.9 around X-axis. The inner mirrors -- by 1.9 and 1.0 , resp ectively. The resulting tilt of the outer mirrors (normal to Z-axis angle) is 5.96 . The normal pro jection onto the XY plane has a p osition angle ±131 with the axis Y. For the inner mirrors, this angle is 2.19 while their p osition angle is ±62 . Reimaging mirrors are coated by multi-layer dielectric film, reflecting up to 99% in the blue-green region of the sp ectrum. 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 diameter ab out 4.0 mm. The glass sp ectral filters FS are placed b etween the re-imaging mirrors and the PMT photocatho des. The filters define the short-wave cutoff of the MASS sp ectral resp onse. Additionally, the filters close the holes in MASS/DIMM b ox lead to PMTs, thus protecting the instrument from dust.

1.3.3

MASS sp ectral resp onse

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 bi-alkali photo catho des. The sp ectral resp onse of MASS is shap ed by the PMT sp ectral sensitivity, the transmittance of the sp ectral filters SF and the reflectance of the re-imaging mirrors. The final sp ectral resp onse 15


S()
1,0

MASS-V 0,6

0,8

0,4

0,6

0,2

0,4

0,0
0,2

-0,2
400 450 500 550 600

, nm

0,0

0,5

1,0

(B-V)

0

Figure 1.6: On left: Sp ectral resp onse of the MASS device. On right: Color equation b etween MASS magnitude and star color index B-V. is shown in Fig. 1.6 and numerical data are presented in the Table 1.5. Such sp ectral resp onse pro duces a dep endence of MASS magnitude on star color. In Fig. 1.6 (right) the dep endence is plotted. Transformation from standard V magnitude is describ ed as follows: M AS S = V + 0.347(B - V )

Table 1.5: MASS sp ectral resp onse in relative photon units. Wavelengths in nanometers 420 430 440 450 460 470 480 490 S () 0.000 0.004 0.206 0.720 0.963 1.000 0.956 0.891 500 510 520 530 540 550 560 570 S () 0.823 0.729 0.636 0.552 0.467 0.391 0.317 0.255 580 590 600 610 620 630 640 650 S () 0.203 0.160 0.061 0.025 0.029 0.005 0.004 0.000

The integral parameters of the MASS sp ectral resp onse are: effective wavelength for A0 star 496 nm, effective sp ectral bandwidth ab out 85 nm.

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 16


Z X
LF FP MR FA ExP DM1

DM2

Figure 1.7: Optical layout of DIMM sub-device in ZX plane (corresp onds to the top Fig 1.2). The MASS sub-device and viewer are not shown. Common part: LF -- Fabry -- instrument fo cal plane, FA -- field ap erture, ExP -- plane of exit pupil and mask. DM1 and DM2 -- DIMM re-imaging mirrors, MR -- folding mirror, CCD -- plane detector.

view in lens, FP DIMM: of CCD

CCD detector surface. The distance b etween the mask holes defines the DIMM base (see Fig. 1.2 and Fig. 1.7). The diameter of 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 select the DIMM base with the 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. During the alignment of the DIMM sub-device we found that fo cal length probably is 1 mm less than nominal. 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 1.6. Table 1.6: DIMM sub-device characteristics for b oth feeding telescop es. All values are in millimeters Segment/Channel Physical diameter of ap erture Diameter of entrance ap erture Physical DIMM base Entrance DIMM base Scale on CCD TMT telescop e 6.4 100 15.5 230 88 ± 3 /mm Meade telescop e 5.5 80 12.0 170 93 ± 3 /mm

The distance b etween the DIMM mirrors and the folding mirror MR is 100 mm. Fig. 1.7 is drawn for the case of TMT feeding telescop e, where the b eam cross-sections are maximal. In this case the reflected b eam cross-sections at the MR mirror are equal to 2.5 mm and the 17


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 changes the abb erations as well. In the pl