Transcript Document
B-pol optical configurations Telescopes and Focal plane options B. Maffei for the B-pol collaboration G. Pisano A. Murphy T. Peacocke Configurations Requirements Resolution goal ~ 0.5 deg at 100GHz Large focal plane no or very low FP curvature Low systematic effect • distortion, ellipticity, cross-polarisation • beam homogeneity across FP • Similar beam for both polar. orientation Let’s assume an imager Optics Needs telescope: refractive (lens) or reflective (mirrors) Focal plane options – Beam formation Feed horn arrays Bare detectors • Antenna coupled detectors • Lenslet Refractive telescope configuration Goal for any telescope: fit as many detectors possible + low sys effects SAMPAN and EPIC/Spider studies BICEP and Re-imaging optics of QUAD (working instruments) Needs 2 lenses for low FP curvature At 217GHz Aperture Stop Lenses Telecentric Focus SAMPAN study: Lenses material: HDPE, Silica or Germanium Mass: from 3 to 8Kg – up to 30Kg with mount Reflective telescope configurations QUAD: Cassegrain with secondary supported by Zotefoam Pros • on-axis • Edge pixels are similar Against • Secondary mount • Needs re-imaging optics for low FP curvature Planck + many others Gregorian off-axis with D-M condition Curved focal plane Reduced FP size Possible for large arrays Compact test range configuration Used in many CATR + Clover and Quiet Large Focal plane possible Example: Clover • FP diameter = 250mm diameter • Size limited by filter diameter not by aberrations • Flat FP • 256 horns @ 150GHz • 165 horns @ 97GHz • Edge pixel eccentricity ~ 0.02 • Optical configuration allows good baffling Design for B-pol Focal plane Sec Primary ~ 40cm First order comparison (based on simulations – Ideal telescope) Will need the same aperture size in either case ~ 30-40cm Lens based system advantages: Possibly Mass ? (maybe not an issue if telescope + mount in CF) Compactness On axis: • Symmetry of pixel beam characteristics • Advantage for scanning strategy ? Mirrors advantages Achromaticity Lower Losses Comparison Lenses need to be cooled (larger dewar) to reduce emissivity Mirrors do not have to (already low emissivity), BUT if we want to cool the telescope to lower background will need much bigger dewar Real systems Telescopes are not perfect: will produce systematic effects Even if mirrors affect the beams, the effects are very well know (predictions and measurements) Lenses imperfections knowledge is not that well advanced in the microwave range Reflective telescope systems Planck RFQM test with a high dynamic range: telescope + feed horns Alcatel-Alenia Space X-polarisation beam at 100GHz Co-polarisation beam at 100GHz Main beam Planck-RFQM – mid and far sidelobes Alcatel-Alenia Space Far sidelobes Moon rejection Earth rejection Intermediate beam cuts Raw measurements Measurements dynamic limit Predictions Sun rejection Lenses knowledge Several big lenses have been developed and used so far (QUAD, BICEP) Both are using Polyethylene Max transmission (with A/R coating) ~ 99% in ideal case (no loss) Problems seen on both experiments Non systematic variation in pointing differences for detector pair (PSB) strong suspicion on extra lens effect (bi-refringence ?) Models to predict performances lack accuracy not well simulated at the moment Performances and models will improve but current lenses are causing problems still A/R coating Coated UHMW polyethylene lens operated at 4K for QUAD Single layer works fairly well at 4K but not large band enough If multi-layer A/R could achieve a much larger BW, we do not know how it will behave when cold Comparison summary Mirrors Lenses Size due to off-axis config. Inhomogeneous for large diameter ? Different pixels might see different parts of the lenses beam variation Needs mount adds weight Bi-refringence (increased when cooled?) Not telecentric non symmetric beam variation across FP Chromatic aberration + A/R coating Band-Width Might need different telescopes (see Spider) If needs to be cooled large dewar Standing waves - even with 99% transmission Material ? : either problems or lack of data Well understood and modelled Lack of knowledge and many systematics will heavily rely on extremely good calibration Experience on manufacture and use Properties variation with T ground calibration ? Thermal gradient across diameter Clover design (CATR) shows very low xpol and aberrations across large FP More compact but needs to be cooled If all problems solved and/or understood (feasible?) potentially better system Focal Plane based on feed horns Well known and understood technology Very good performances and low systematics Low cross-pol - below 40dB typically High efficiency, better than 25dB return loss Low sidelobes and good control of straylight The Good Works with various detectors (Bolos and HEMT) The bad Mass / volume Reliability/yield for mass production Several design/manufacture possibilities are being investigated Large aperture diameter (4-8 l) Reduced number of pixel – hundreds The ugly Antenna coupled bolometer Adopted for several new instruments but poor amount of data so far The latest data right now (that I know) are from JPL Talks from J. Bock Publications by Kuo et al. Main advantages Reduced size • SAMPAN study: 20000 pixels for 165mm diameter FP Fairly easy filtering (intrinsic filters) Antenna coupled bolometer beams Measured Beam Patterns Single polarisation antenna X-polarization Single polarisation antenna + filter Dual polarisation Dual polarisation Y-polarization antenna + filter antenna Radiation pattern (meas.) of antenna coupled detectors FT of radiation pattern with cold stop Kuo et al, 2006 Ant. coupled bol. spectral response Long slot single pol. antenna Dual pol. antenna Single pol. Antenna + BP microstrip filter Dual pol. Antenna + BP microstrip filter Measured Spectral Response Low efficiency due to reflections in test system. After correction Lenset The idea is to put a small lens on top of each detector. No data available We will have the problems related to lenses (see previously) Possibly more serious problem the lens is small any small imperfection (fraction of the wavelength) will potentially impact the beam Bare detectors potential Pbs Cross-talk ? Staylight ? Use of cold stop: how each detector across focal plane will be affected ?