GBI TMA CONCEPTS Rev B M Sholl 26
GBI TMA CONCEPTS Rev B M. Sholl 26 July 2007 1
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, Hi. RISE) 2
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 3
TMA classes • On-axis (axisymmetric PM) – Annular field – Full field – See Lampton/Sholl, 2007 SPIE 6687 #23 4
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) 5
Hi. RISE: Annular Field TMA Gallagher et. Al. , SPIE 5874, 2005 6
Full Field TMA • On-axis (axisymmetric PM) – No blind spot – Large FOV – Very low distortion (~0. 010. 02%), good for TDI – Often used with only one half of FP, and incomplete (non-circular) mirror sections on aft end to prevent vignetting 7
On-axis designs have central obscuration, which degrades MTF 8
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 9
Off-axis telescopes • Topsat • MTI 10
Pegasus Fairing 11
Full-field off-axis 12
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 13
Full-Field Tertiary-Pickoff (FFTP, recommended GBI baseline) • 9 km swath • 480 km • 7 m focal length 14
FFTP Optical Prescription 6 surfaces X FFTP _1. OPT Z Diam pitch mir? Curv diam Aspher note -----: ------------: -----: -------: ------0 : mir : -0. 756230778: -0. 9502878: 0. 66 : 0. 19: PM : -0. 484000 : 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. 36 : -65 : mir : 0 : : FM 0. 25 : 0. 368483933357: 90 : film : 0 : : FP : : : -0. 25 : 0. 0 : : 15
FFTP Performance • Residual phase error: – 18 nm RMS – 0. 78 um geometric blur • Sensitivity: – 46 mn RMS @ ± 1 um SM/PM despace • Touchy! Diffraction limited OPD at 0. 633 um is 45 nm RMS – 2 um @ ± 1 um SM/PM despace • Desire: slow front end • Packaging difficulty: extraction mirror close to PM 16
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 17
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