GoBears Telescope Ideas M. Lampton UCB Space Sciences Lab 15 March 2007

M. Lampton, UCB SSL Mar20072 Overview of Telescopes TypeTelephoto ratio > 3? (see below) Flat field? Astigmatism? Baffling?Diffraction Limited Field of View? Schmidt camera0.5: NGcurved NG zero astigmatism MarginalVast OK Classical Cassegrain Any: OKcurved; has astigmatism NG OK0.2 degrees NG Ritchey ChretienAny: OKcurved, has astigmatism NG OK0.4 degrees NG Ritchey Chretien plus refractive corrector Any: OKMarginal; needs lenses, small astigmatism Maybe OK2 degrees but chroma Maybe Three mirror anastigmat AnyFlat field No astigmatism OK4 degrees Telephoto ratio = effective focal length / telescope package length; EFL is constrained by pixel size / angular resolution Telescope package length is constrained by spacecraft envelope.

M. Lampton, UCB SSL Mar20073 Zurbuchen et al 2007 Ritchey-Chretien cassegrain with refractive corrector/flattener

M. Lampton, UCB SSL Mar20074 But.... Lenses are not usually first choice in space environment; radiation damage can degrade transparency Field correction is marginal if 2.3 degree field full-width is sought *and* if diffraction limited images are required Better is to adopt a telescope design whose theoretical performance allows significant margins --- so that the in-process degradations still allow full on-orbit performance

M. Lampton, UCB SSL Mar20075 However.... Ritchey-Chretien + corrector may well be cheapest alternative! Important! Two mirrors, five lenses, one barrel. Ritchey-Chretien + corrector may well be quickest alternative! Also important! Ritchey-Chretien + corrector may well be the least fussy! Tolerant of misalignments, easiest focussing. It may well prove best to deliver some telescope that is quick & cheap rather than something that is the best optically!

M. Lampton, UCB SSL Mar20076 What is usual: mountaintops? Wide field mountaintop observatories have all gone to Ritchey- Chretien + corrector configuration Sloan Digital Sky Survey: 2.3 deg field, five wavebands vis-NIR, flat field Not diffraction limited! Seeing limited, ~ one arcsecond. OK when viewing upward through atmosphere

M. Lampton, UCB SSL Mar20077 What is usual: space? Hubble is a Ritchey-Chretien no corrector not widefield: narrow field observatory strongly curved focal surface strong astigmatism

M. Lampton, UCB SSL Mar20078 What is usual: spaceborne remote sensing? Three-mirror anastigmats are the rule –“anastigmat” = no spherical, no coma, no astig, no curvature –Three powered (curved) mirrors + one flat mirror –no lenses, hence no chromatic aberration –since no chroma, can align/focus with a laser at one wavelength and be *certain* optimum at other wavelengths –waveband filters can however complicate this! Easily diffraction limited throughout visible and NIR Widely adopted for contemporary space astronomy: –narrow field telescopes e.g. JWST –wide field telescopes e.g. SNAP

M. Lampton, UCB SSL Mar20079 Example: Korsch 1972 style TMA Lampton & Sholl SPIE 2007 Fig 1 60 cm aperture EFL=3.33m, f/ mirrors, no lenses Baffling not shown –requires SM fez –requires outer tube –requires inner tube –50% central obscur Delivers 2 micron rms geometrical over 2.2 degree field diam Flat field No spherical, astig, chroma, coma, surfaces GoBears3.OPT MLL Mar 2007 EFL=3.33m f/5.55 X Z pitch mir? Curv Aspher Diam dia : :------:------: : :------:-----:--- 0 : 0 : 0 : mir : ? ? 0.62 : 0.2 : 0 : : 0 : mir : ? ? 0.28 : : 0 : 0.40 : 0 : mir : ? ? 0.36 : : 0 : : -20 : mir : 0 : : 0.19 : 0.05: -0.22: ? -65 : mir : 0 : : 0.15 : : 0.22: e 90 : film : 0 : : 0.15 : : : : : : : : : :

M. Lampton, UCB SSL Mar Resolution: MTF, PSF, EE

M. Lampton, UCB SSL Mar Resolution Spreadsheet OPTICAL CONSTANTS Optical Aperture = 0.6meters Focal Length =3.33meters f/number or "speed" =5.55 VIEWING SITUATION Orbit Altitude =460km Orbit Speed = Desired Ground resol =1.2m Angular resolution =2.61microrad or angular resolution =0.52arc sec Required Linear resol = 8.69microns Detector Pixel size=8.8microns RMS effect of pixels =2.5microns RMS blur silicon diffusion= 1.0microns RMS blur attitude control =0.0microns RMS blur aberrations =3.0microns Total RMS Gauss=4.0microns Total FWHM Gauss=9.6microns BLUR SUM OF SQUARES INCLUDING DIFFRACTION Live linked to above data set GaussiansBessel diffractionTotal wavel FWHMFWHMFWHM micronsmicronsmicronsw/diffract microns microns microns microns microns microns microns microns

M. Lampton, UCB SSL Mar Signal/Noise Spreadsheet In work.....

M. Lampton, UCB SSL Mar Conclusions so far Ritchey-Chretien+Corrector versus TMA FeatureRCCTMAAdvantage Fewer parts2 mirrors 5 lenses Simple frame 5 mirrors No lenses Angled frame Slight RCC AlignmentEasy secondaryTricky secondary RCC Cheaper primary f/3 primary hyperboloid f/1 primary ellipsoid RCC AberrationsSome but perhaps minor NoneTMA