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Super Star Tracker. Optical Design & Analysis Dennis Charles Evans 8 February 2002. Design Baseline. 125 mm diameter aperture (small size) Long focal length (4848mm = f/38 beam) Modified Schmidt Cassegrain Spherical primary Rigid secondary & beam quadrature
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Super Star Tracker Optical Design & Analysis Dennis Charles Evans 8 February 2002
Design Baseline • 125 mm diameter aperture (small size) • Long focal length (4848mm = f/38 beam) • Modified Schmidt Cassegrain • Spherical primary • Rigid secondary & beam quadrature • No spider diffraction (not certain if advantage or disadvantage) • Optical block base for alignment reference and mounting • Optically contacted or fused optics for structural stability • Approximately 5 x 109 photons into aperture (25% obscuration) • Approximately equivalent to mv=0 magnitude star • Input flux of 109 photo electrons/sec per “quadrant” • 10mm cathode diameter PMTs • Self contained, flight quality pulse counting units available • Present count rate is 500 MHz
50:50 Beam Splitter A Sharp Corner Roof Prism (2 places) Path 1 B Path 2 (OUT) C D (IN) Rotated 90 deg WRT AB Error Signal Generation
Error Signal Generation Optical Path 1 Optical Path 2 C A B D Yaw = (A-B)/(A+B) Pitch = (C-D)/(C+D)
Mass of Optical Components • F_Silica=2.2 g/cm*3 Vol(cm*3) Mass(grams) • VCor: 169.3868219 × F_Silica = 372.65100818 • VPri: 348.0089929 × F_Silica = 765.61978438 • VSec: 14.5525599 × F_Silica = 32.015631780000008 • VTer: 12.2558027 × F_Silica = 26.962765940000004 • VCyl: 3036.2764147000004 × F_Silica = 6679.8081123400008 • VBas: 2000 × F_Silica = 4400 • Total Mass 12277.05730262 grams
Huygens Point Spread Function5.34 arc-sec square; Strehl Ratio = 0.999
Huygens Point Spread Function5.34 arc-sec square; Strehl Ratio = 0.999
Huygens Point Spread Function5.34 arc-sec square; Strehl Ratio = 0.999
Huygens Point Spread Function5.34 arc-sec square; Strehl Ratio = 0.999
Huygens Point Spread Function5.34 arc-sec square; Strehl Ratio = 0.999
Huygens Point Spread Function16.04 arc-sec square; Strehl Ratio = 0.999
Huygens Point Spread Function16.04 arc-sec square; Strehl Ratio = 0.999
Huygens Point Spread Function16.04 arc-sec square; Strehl Ratio = 0.999
Huygens Point Spread Function16.04 arc-sec square; Strehl Ratio = 0.999
Error Analysis • Error: s2A±B= s2A+ s2B± 2ABs2AB • etc. • Integration Time • Increasing integration time from seconds to hours would result in nanoarcsecond error signals.
Concerns • At micro arc-second resolution everything effects everything else. • Cryo environment for structural stability • Zerodur may be more stable than Fused Silica • There may be some cost and performance advantages of using diamond roof prisms. • Error signal analysis is not easily understood without detailed model • One second integration period used for present modeling • Time integration can improve error signal by many orders of magnitude • Smoother/Broader Point-Spread-Function might improve error signal range and accuracy • Broadband beacon (Incandescent) • “Spider Masks”
History Slides Various considerations in developing the design
Alternate Quadrature Detection Schemes • Interferometry offers 5 -10 x signal processing advantage • Need 100 to 10,000 x increase • Koesters Prisms – a la HST Fine Guidance • Scale makes exit pupil on order of 0.2 mm • Small size fabrication and sensitivity are problematic • Four Tracker “long” baseline Interferometry • Baseline deployment, alignment, and stability concerns • 4 to 10x mass increases (equivalent cost increases)
GPB Design Reference C. W. F. Everitt, D. E. Davidson, and R. A. Van Patten (1986) “Cryogenic star-tracking telescope for Gravity Probe B”, SPIE Proceedings, Vol. 619, Cryogenic Optical Systems and Instruments II, Ramsey K. Melugin, Chairman/Editor.
