FOBOS Acquisition System
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Description: The FOBOS acquisition system is housed in the focal
plane module at the telescope's Cassegrain focus. It provides the observer
with a video feed of 6x4 arcminutes of the telescope's field of view
so that rough alignment of the target star may be accomplished. The system
path within the focal plane module is highlighted in green in the figure
below.
To minimize expense, 2 off-the-shelf Linos Photonics achromatic doublets were used for this design. The layout can be viewed here. A spot diagram is available here. A ZEMAX file is also available.
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System/Prescription Data
Title: FOBOS Acquisition system - 2 achromat design
Date : WED AUG 27 2003
LENS NOTES:
Uses Spindler & Hoyer achromats, yields ~5X demagnification
GENERAL LENS DATA:
Surfaces : 10
Stop : 4
System Aperture : Object Cone Angle = 2.1164
Glass Catalogs : SCHOTT
Ray Aiming : Off
Apodization : Uniform, factor = 0.00000E+000
Effective Focal Length : 70.98869 (in air at system temperature and pressure)
Effective Focal Length : 70.98869 (in image space)
Back Focal Length : 82.17712
Total Track : 436.1147
Image Space F/# : 2.429748
Paraxial Working F/# : 2.711449
Working F/# : 2.688821
Image Space NA : 0.1813457
Object Space NA : 0.03692975
Stop Radius : -14.60824
Paraxial Image Height : 3.006045
Paraxial Magnification : -0.200403
Entrance Pupil Diameter : 29.21649
Entrance Pupil Position : 312.0882
Exit Pupil Diameter : 50.50152
Exit Pupil Position : 136.3566
Field Type : Object height in Millimeters
Maximum Field : 15
Primary Wave : 0.5875618
Lens Units : Millimeters
Angular Magnification : -0.5785268
Fields : 3
Field Type: Object height in Millimeters
# X-Value Y-Value Weight
1 0.000000 0.000000 1.000000
2 0.000000 7.500000 1.000000
3 0.000000 15.000000 1.000000
Wavelengths : 3
Units: Microns
# Value Weight
1 0.486133 1.000000
2 0.587562 1.000000
3 0.656273 1.000000
SURFACE DATA SUMMARY:
Surf Type Comment Radius Thickness Glass Diameter Conic
OBJ STANDARD Infinity 83.2104 30 0
1 COORDBRK - 0 - -
2 STANDARD Infinity 0 MIRROR 42.23529 0
3 COORDBRK - -312.0882 - -
STO STANDARD (-)LINOS 322312 -538.503 -4.5 SF5 50.8 0
5 STANDARD -177.841 -6 BK7 50.8 0
6 STANDARD 242.3068 -25 50.8 0
7 STANDARD LINOS 322301 -52.3314 -16.8 SK2 50 0
8 STANDARD 41.8673 -3.2 SF10 50 0
9 STANDARD 184.3318 -68.52646 50 0
IMA STANDARD Infinity 6.091091 0
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The telescope focal plane has a 15''/mm image scale. In order to provide a
wide enough field of view for the observer, several arcminutes of the
telescope image plane must be reimaged onto the acquisition camera (SBIG
STV) CCD, which is fairly small. To accomplish this, a simple system has
been designed to provide a demagnification of approximately 5X.
