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GBI TMA CONCEPTS Rev B

GBI TMA CONCEPTS Rev B. M. Sholl 26 July 2007. Previous Work (UM and M. Lampton). Much good work has been done on RCC RC requires transmissive Corrector for wide-field applications (see Lampton presentations) Key advantages of RCC: Simple mechanical design (tube)

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GBI TMA CONCEPTS Rev B

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  1. GBI TMA CONCEPTSRev B M. Sholl 26 July 2007

  2. Previous Work (UM and M. Lampton) • Much good work has been done on RCC • RC requires transmissive Corrector for wide-field applications (see Lampton presentations) • Key advantages of RCC: • Simple mechanical design (tube) • Loose tolerances (thermal stability) • Major design tradeoffs between chromatic aberration, image sharpness and field distortion • Mapping imagers tend to be TMAs (Ikonos, Quickbird, Worldview, Nextview, Topsat, ROCSAT II, HiRISE)

  3. TMA: Three Mirror Anastigmat • Dietrich Korsch, 1972, proposed a class of telescopes with three curved mirrors • Anastigmatic Three Mirror Telescopes, Applied Optics, vol. 16, No. 8, August 1977 • Wide FOV, easy stray light baffling, zero chromatic aberrations • Advanced at time for LST (aspheric mirrors, electronic focal planes could not take advantage of FOV) • Aspheric mirror manufacturing now routine (Tinsley, Reosc, Kodak/ITT, Goodrich, SESO) with numerically controlled polishing machines • Large arrays and TDI may utilize large FOV • Real TMAs optimize for small blur across field, and strictly speaking are not anastigmatic

  4. TMA classes • On-axis (axisymmetric PM) • Annular field • Full field • See Lampton/Sholl, 2007 SPIE 6687 #23

  5. Annular Field TMAs • On-axis (axisymmetric PM) • Annular field • Blind spot in center of focal plane • Large FOV • Front end (PM/SM) can be optically slow (easier tolerances) • High distortion (~1-2%), bad for cartesian TDI • Adopted for step and stare 2-D focal plane array on SNAP • Example: Hirise, Pleiades (only uses eccentric slice, ref. Fappani et al SPIE 6687 #24 2007)

  6. HiRISE: Annular Field TMAGallagher et. Al., SPIE 5874, 2005

  7. Full Field TMA • On-axis (axisymmetric PM) • No blind spot • Large FOV • Very low distortion (~0.01-0.02%), good for TDI • Often used with only one half of FP, and incomplete (non-circular) mirror sections on aft end to prevent vignetting

  8. On-axis designs have central obscuration, which degrades MTF

  9. Off-axis, full-field • No obscuration in FOV or pupil plane • Improved MTF • MTI, Worldview, TOPSAT • Space inefficient (requires optics to side of PM) • PM is “cookie cut” from larger mirror • PM/SM spacing must be relatively long to keep geometric aberrations low • Not easy to package in Pegasus fairing

  10. Off-axis telescopes • Topsat • MTI

  11. Pegasus Fairing

  12. Full-field off-axis

  13. Pleiades • Will not fit in Pegasus fairing • High distortion (pincushion) • Requires CCDs at various angles to accommodate distortion • Pranyies et al SPIE 5570 P.568, 2004

  14. Full-Field Tertiary-Pickoff(FFTP, recommended GBI baseline) • 9km swath • 480km • 7m focal length

  15. FFTP Optical Prescription 6 surfaces FFTP _1.OPT Diam diam X Z pitch mir? Curv Aspher note ---------:---.------------:--------------:------:----.---------:---.-------:-------:-----:------- 0 : 0.0 : 0 : mir : -0.756230778: -0.9502878: 0.66 : 0.19: PM 0 : -0.484000 : 0 : mir : -1.883182489: -3.4676032: 0.25 : : SM 0 : 0.66 : 0 : mir : -1.147305396: -0.4417273: : : TM 0 : 0.045 : -20 : mir : 0 : : : : EM -0.25 : 0.36 : -65 : mir : 0 : : : : FM 0.25 : 0.368483933357: 90 : film : 0 : : : : FP : : : : : : : :

  16. FFTP Performance • Residual phase error: • 18nm RMS • 0.78um geometric blur • Sensitivity: • 46mn RMS @ ±1um SM/PM despace • Touchy! Diffraction limited OPD at 0.633um is 45nm RMS • 2um @ ±1um SM/PM despace • Desire: slow front end • Packaging difficulty: extraction mirror close to PM

  17. Conclusions • Annular Field (AF) configurations • High distortion (unacceptable for TDI) • FFTP configuration • Sensitive to mechanical misalignment • Packaging of extraction mirror difficult due to close proximity to PM • Low distortion • Easily delivers requisite 1.1º FOV • Fully illuminates circular focal plane (S. Harris FP) • Telecentric focus greatly simplifies bandpass filters • TBD • Packaging work with Paul Turin • Slow down front end? • Detailed vignetting analysis

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