Документ взят из кэша поисковой машины. Адрес оригинального документа : http://curl.sai.msu.ru/mass/download/doc/descript_eso.pdf Дата изменения: Sun Jan 18 20:44:44 2004 Дата индексирования: Mon Oct 1 20:15:44 2012 Кодировка: Поисковые слова: п п п п п п п п п п п п п п п п п п п п п п п п п п

Optimized MASS device for synchronous measurements with Paranal DIMM. Optical and mechanical design. Alignment.
Kornilov V., Potanin S., Shatsky N., Shugarov A., Voziakova O. January 15, 2004


Contents
1 Optics design 1.1 Basic principles . . . . . . . . . . . . . . 1.2 Principal geometry of MASS device . . . 1.2.1 Entrance and exit pupils. System 1.2.2 Fabry lens . . . . . . . . . . . . . 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 Field ap erture, centering mechanism and 7 7 8 8 10 11 11 11 14 15 17 17 17 17 20 21 22 22 23 24 24 24 25 25 26 26 26 28 28 29

........ ........ magnification ........ ........ ........ ........ ........ viewer . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

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 Fabry lens and viewer optics . . . . . . . . . . . 2.2.2 MASS channels 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 . . 3.2 Device alignments at the telescop 3.2.1 Fabry lens p osition . . . . 3.2.2 Viewer alignment . . . . . ... ... e .. ... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.. .. .. .. .. .. .. .. .. .. or .. .. . . . . . . . . . .

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

. . . . . . . . . . . .. .. . . . . . . . . . .

1


4 Critical parameters determination 4.1 System magnification . . . . . . . . . . . . . . . . . . . . . . . . 4.2 MASS detectors parameters . . . . . . . . . . . . . . . . . . . . 4.2.1 PMT optimal voltage and discrimination determination 4.2.2 Non-linearity and Non-p oissonity determination . . . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

30 30 31 31 32 34 34 35 36

A Optical parts sp ecifications A.1 The sp ecifications for MASS purchased optical elements . . . . . . . . . . . . . . A.2 The sp ecifications for MASS sp ecial optical elements manufactured by the contractor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B List of mechanical parts

2


List of Figures
1.1 1.2 1.3 1.4 1.5 1.6 2.1 2.2 2.3 2.4 2.5 3.1 3.2 4.1 4.2 Principal optical layout used for calculation . . Optical layout of MASS device in ZY plane . . Optical layout of the MASS device in ZX plane MASS segmentator . . . . . . . . . . . . . . . . Sp ectral resp onse of the MASS device . . . . . Image scans pro duced by centering mechanism View of the device . . . . . . . . . . Main dimensions of the MASS device View of the device without cover . . Optical b ench unit . . . . . . . . . . View of the electronic mo dule . . . . ... with ... ... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 12 13 13 15 16 18 19 20 21 23 27 28 32 33

.... optical .... .... ....

...... interface. ...... ...... ......

.. . .. .. ..

Photo catho des p osition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . System magnification adjust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Counting functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dep endence of non-Poisson parameter p on flux F . . . . . . . . . . . . . . . . .

3


List of Tables
1.1 1.2 1.3 1.4 The telescop e parameters adopted for MASS optical design. . The MASS main optical parameters . . . . . . . . . . . . . . PSU and entrance segment dimensions . . . . . . . . . . . . . MASS sp ectral resp onse in relative photon units. Wavelengths .. .. .. in ....... ....... ....... nanometers . . . . . . . . 8 10 14 15

4


Intro duction
This do cument describ es the optical and mechanical design of a low-resolution turbulence profiler (MASS) optimized for synchronous observation with Paranal DIMM, according with a Prop osal to ESO [1]. The pro ject was implemented in frame of the ESO contract No. 69255/ODG/02/9124/GWI. The electronics of the device and details related to it are presented in a separate do cument [7]. Also, separate do cuments contain Turbina Software reference guide, Turbina user guide [11], and Sup ervisor user guide [8], which complete the full description of the MASS instrument and its control software. The MASS optical scheme was sp ecially calculated for the use with a short refractive feeding telescop e C102 from Celestron company or similar. The principles of the work of MASS are describ ed in [4]and [2]. 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 sequence of assembly/disassembly is presented as well. 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), 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 software. App endices which follow give technical parameters of the optical and mechanical device comp onents.

5


Bibliography
[1] Kornilov V., The optimization of MASS device for synchronous measurement with Paranal DIMM. A Prop osal to Europ ean Southern Observatory (ESO). Decemb er 11, 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] Sarazin M., Ro ddier F., The E.S.O Differential Image Motion Monitor Astron. Astrophys. 227, 294-300 (1990). [7] Kornilov V., Shatsky N., Shugarov A., Voziakova O. Optimized MASS device for synchronous measurements with Paranal DIMM. Electronics and Device control. Novemb er 2003. [8] Kornilov V., Shatsky N., Voziakova O. Supervisor program User Guide. SV version 0.22, January 2004. [9] Tokovinin A. Polychromatic scintil lation. JOSA(A), 2003, V. 20 P. 686-689 [10] Kornilov V., Potanin S., Shatsky N., Voziakova O. Optimized MASS device for synchronous measurements with Paranal DIMM. 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.

6


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 [6]. So, the weighting function for DIMM is un-dep ending on altitude. The idea to use two different turbulence measuring devices synchronously p ointed at the same star using one mount, grew from the following: ­ Multiap erture scintillation sensor (MASS) do esn't sense 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. ­ Numerical simulations show [5] that reducing the largest MASS annular entrance ap erture (the segment D) down to 8.5 cm optimizes the metho d sensitivity for middle altitudes. This means that a non-exp ensive refractive telescop e with diameter 10 cm can b e used to feed the MASS device. Such a refractor can b e installed in parallel of the Paranal DIMM as a piggy-back device. The following general solution was chosen and implemented in optimized MASS instrument for turbulence measurements: ­ in order to enlarge short fo cal length of the refractor, the sp ecial optical interface is included in optical scheme. 7


­ to re-image the plane of entrance pupil of feeding telescop e to the exit pupil plane; ­ 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 photo catho des of MASS detectors. These general ideas follow the principles of the original MASS design. The following Sections describ e this scheme in details.

1.2

Principal geometry of MASS device

The MASS device was designed and calculated for usage with a compact telescop e ­ refractor. For the calculation of the design parameters, the data listed in the Table 1.1 were used. The schematic drawing of the feeding refractor with principal part of the mo dified MASS device is shown in Fig. 1.1 where the measures used in further calculations are shown as well. The high-p ower negative lens is placed b efore the fo cal plane to reduce the b eam convergence and obtain thus the equivalent fo cal length ab out 2 m. This combination of the ob jective PR and the negative lens (transfo cator) TF is called the "telescop e" throughout this do cument. Position x of TF lens from the fo cal plane F P R defines the magnification m at the lens, the new effective fo cal length of the telescop e, and lo cation of the new fo cal plane F (similarly to the role of a secondary mirror in Cassegrain telescop e). In our case, the fo cusing of the instrument is done by change of the distance b etween the ob jective and TF lens. Comparing to Cassegrain telescop e, we have more freedom to parameters optimization, b ecause the fo cal length of the TF can b e varied, to o. Table 1.1: The telescop e parameters adopted for MASS optical design. All dimensions are in millimeters Parameter Nominal fo cal length FP R Diameter of ob jective DP R C102 telescop e 500 102

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 optical axis of the instrument. Exit pupil will b e lo cated at a distance af (according to Fig. 1.1) from the lens. Really, the Fabry lens re-builds not original entrance pupil but its image pro duced sequentially by ob jective of a telescop e and transfo cator TF lens. The dimension and lo cation of this image dep end on the exact geometry of the telescop e (recall that telescop e includes TF lens) 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 is defined by the telescop e system as well as by the Fabry lens fo cal length and p osition. The entrance pupil lo cation is not 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 8


9 Figure 1.1: General optical layout. S -- pupil segmentation unit, F P R -- feeding refractor fo cal plane, F -- refractor+transfo cator fo cal plane, F' -- refractor+transfo cator+Fabry lens fo cal plane, PR -- refractor ob jective, TF -- transfo cator lens, LF -- Fabry lens, S' -- image of S pro duced by Fabry lens, S -- image of S' built by TF lens, E -- entrance pupil of the system, which is the image of S pro duced by refractor ob jective. All values designed by right arrow are p ositive, otherwise -- negative. Other values are explained in text.


working entrance pupils for the MASS 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 the telescop e and device geometry as well. More exactly: the lengths a, c, Fabry lens fo cal length Fabry and its lo cation da define b oth the magnification K of the instrument and the lo cation of the entrance pupil for a given telescop e with parameters F P R and FT F . For the design, the lo cation of the entrance pupil plane was chosen to b e 50 mm b efore a p ole of a feeding refractive ob jective near the front edge of the telescop e tub e. Such a p osition is very convenient for instrument check and alignment. The nominal value of K = 15.00 was chosen (K 20 in the original MASS). The exact value of system magnification influences final results of the MASS, b ecause the geometry of entrance pupil is used in computation turbulence intensity from directly measured values. Table 1.2: The MASS main optical parameters, transfo cator including. All dimensions are in millimeters Optical parameter Transfo cator lens fo cal length Fabry lens fo cal length Fabry Fabry lens diameter Dabry Distance b etween FP and EP Distance b etween FP and TF LF p osition with resp ect fo cal Instrument magnification K Entrance pupil plane p osition Scale in the fo cal plane scaleF F
TF

Nominal value -18 60 25 123 116.7 -38 16.6 -50 101

planes a lens c plane da b
E

1.2.2

Fabry lens

The optical layout of the MASS 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 adjust the 10


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 device are listed. Catalog of Edmund Optics company was used to select the Fabry and transfo cator lens. The exact formulae for computations are presented in [10]. The consideration showed that transfo cator and Fabry lenses fo cal lengths are mutually constrained. Note, that short TF fo cal length providing more compact optical interface b etween refractor and MASS, requires shorter LF fo cal length. Optimal pair was chosen as is shown in Table 1.2.

1.3

MASS optics

The optical scheme of the MASS device itself is shown in Fig. 1.2 as ZY plane view, and in Fig. 1.3 as ZX plane view (from viewer side). The main optical element of the MASS device is the pupil segmentation unit (PSU). PSU forms four reflected b eams and reflects them in different directions. The chosen magnification as well as telescop e ob jective diameter, force to adopt the maximal outer diameter of the MASS pupil segmentation unit as 5.5 mm.

1.3.1

Pupil segmentation unit

The PSU is lo cated on the instrument optical axis (see Fig. 1.3). The exact p osition of the exit pupil at PSU is provided by shifting the Fabry lens in Y and X directions. 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 1.65 around X axis. 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] the dimensions of the PSU segments were chosen as listed in Table 1.3. 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. 1.4. 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 1.3.

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. 11


FP LF MV FA RD(RA) CC PMTs SF

ExP

PSU

Z Y
V1

RC(RB)

V2

EP

Figure 1.2: Optical layout of MASS device in ZY plane. LF -- Fabry lens, FP -- instrument fo cal plane, FA -- field ap erture, ExP -- plane of exit pupil, PSU -- pupil segmentation unit, RA, RB, RC, RD -- re-imaging mirrors, SF -- sp ectral filter, PMTs -- MASS detectors, MV -- viewer removable mirror, CC ­ glass plate with central hole, V1, V2 -- transmoving ob jective, EP -- eye-piece.

12


RA LF FP RB FA TK RC SF

PMT A ExP PMT B PMT C PMT D PSU

X
RD

Z
Figure 1.3: Optical layout of the MASS device in ZX plane (corresp onds in Fig 1.2). Viewer is not shown. LF -- Fabry lens, FP -- instrument field ap erture, TK -- triangle knife of centering unit, ExP -- plane of exit segmentation unit, RA, RB, RC, RD -- re-imaging mirrors of the A-, B-, SF -- sp ectral filter, PMT -- four detectors. to the b ottom view fo cal plane, FA -- pupil, PSU -- pupil C-, and D-channels,

Figure 1.4: On the left: Top view of one of the MASS segmentator, illuminated by scattered light. On the right: Segmentator mounted in its holder.

13


Table 1.3: PSU segment dimensions and entrance segment dimensions. 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 Entrance diameter 91.3 64.7 63.9 36.5 35.7 21.6 21.1

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 12.3 for outer channels A,D and 9.7 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 4.6 around Y-axis and by -2.3 around X-axis. The inner mirrors -- by 2.0 and 2.7 , resp ectively. The resulting tilt of the outer mirrors (normal to Z-axis angle) is 5.12 . The normal pro jection onto the XY plane has a p osition angle ±117 with the axis Y. For the inner mirrors, this angle is 3.31 while their p osition angle is ±36 . 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 filter FS are placed b etween the PSU and field ap erture into central blind. The filter defines the short-wave cutoff of the MASS sp ectral resp onse.

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 filter SF and the reflectance of the re-imaging mirrors. The final sp ectral resp onse is shown in Fig. 1.5 and numerical data are presented in the Table 1.4. Such sp ectral resp onse pro duces a dep endence of MASS magnitude on star color. In Fig. 1.5 (right) the dep endence is

14


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 1.5: On left: Sp ectral resp onse of the MASS device. On right: Color equation b etween MASS magnitude and star color index B-V. plotted. Transformation from standard V magnitude is describ ed as follows: M AS S = V + 0.347(B - V )

Table 1.4: 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

Field ap erture, centering mechanism 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. The ap erture also limits the field of view of the centering mechanism: for this reason, a compromise 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. 15


Figure 1.6: Image scans pro duced by centering mechanism. On left -- scan of a p oint-like image, on right -- scan of uniformly illuminated ap erture. The size of a wide field of view for star finding is defined by the viewer design and 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 MASS detectors. Further star centering can b e done by the centering mechanism. Star centering unit is based on the triangle non-transparent knife moving across field ap erture. Edges of the knife are tilted at 45 to direction of the motion along X axis. When the knife is in the center, it fully closes the ap erture. Metho d calibration is based on a following fact: if the field ap erture is uniformly illuminated (by bright sky, for example), the level 0.5 of a maximal signal is achieved when one of the knife edges crosses exactly the ap erture center (see Fig. 1.6). There are two such p oints, obviously. Comparing a star scan slop es p osition with these reference p oints, the star image shift can b e obtained. The viewer is not used in a normal work since the parallel DIMM provides the telescop e p ointing at the star, star searching if necessary and guiding during measurements. In practice, the viewer serves as an auxiliary to ol for coalignment of DIMM and MASS feeding optics and in extraordinary cases. The optical layout of the viewer is presented in Fig. 1.2 where the main viewer parts are shown. Removable mirror MV has dimensions 12 в 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 V1 and V2 ACH18x50 (Edmund optics; fo cal length 50 mm, diameter 18 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 (or telescop e) to provide easy access for the observer. The viewer is placed in plane ZY.

16


Chapter 2

Mechanical design
2.1
2.1.1

General description
General characteristics

As it was shown ab ove, the optimized MASS device is installed at feeding telescop e­refractor using mechanical and optical interface. The interface can b e considered as the part of the device, to o. The general view of the MASS device for synchronous atmospheric turbulence measurement is presented on Fig. 2.1. In the Fig. 2.2 overall device dimensions are shown. Note, that the main b ox of the device is practically the same to one designed for CTIO/TMT pro jects. Practically all parts of the device are fabricated from hard aluminum alloy, black-ano dized. Only few critical parts are made from steel. Total weight of the MASS instrument is ab out 1.5 Kg. The device is mounted at refracting telescop e Celestron C102 with the help of a inner metric thread M 56 в 1 6G which is pro duced in the interface tub e. The interface tub e with device is tighten up the corresp onding thread on the fo cuser tub e of the refractor and fixed by lo cking nut. The device electronics is enclosed in a separate case which can b e removed and attached again easily. The connection to interior electronic elements, such as illumination LED, is provided via a sp ecial plug connector. The mechanical parts are designated on the drawings as "MDnnS", where MD ­ prefix for MASS device, 'nn' ­ assembly unit numb er, 'S' -- suffix for the sp ecific part. The prefix is omitted when a designation is mentioned b elow.

2.1.2

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. One can see it in the Fig. 2.3, where photo of the device without cover 01B is presented. This structure b ears all other units and a