B-pol optical configurations

1. B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

2. Instrumental requirements • Resolution goal: TBC • Many pixels with several spectral bands • Large focal plane • Optical system with no or very low Focal Plane curvature • Low systematic effects • distortion, ellipticity, cross-polarisation • beam homogeneity across FP • Similar beam for both polar. Orientation

3. Heritage and knowledge • From Planck and Herschel • Very good predictions of beam performances • A bit more difficult for bolometers • Very good beam characteristics • Technology for large reflective telescope available • But: in need of improvements for a B-Pol mission: • Better surface accuracy (ie Planck mirrors) • Mirrors might need to be actively cooled (depends on the frequency coverage needed) • Lenses • In mm range only a few Balloon borne / Ground based experiments have used them. • So far only A/R coated lenses of about 20/30cm diameter have been made • In principle larger lenses could be made but with unknown results so far. Typical measurement /model comparison of Planck telescope (ESA/Thales/TICRA) Coated Polyethylene Lens for QUAD(100-150GHz)

4. B-Pol 2007 – Previous proposal Reminder of optical configuration

5. B-Pol 2007 – Optical configuration 6 spectral bands: 45 to 350 GHz Each telescope system consists of three lenses

6. Optical performances • Not an optimised system • But: • This is assuming ideal components • Main beam asymmetry • Not as performant as reflectors • Unknown (not computed) far sidelobes characteristics T. Peacocke

7. Critical review of the present config • Good points • One telescope system for each spectral channel • Better spectral isolation • Potential lower aberrations for edge pixels • Spatial resolution is the same at all frequencies • Fairly compact and fits in a medium size mission • Bad points • Technology not ready • Low TRL but ESA funding available for studies • Unknown or limited characterised / modelled characteristics • Larger losses than mirrors and larger emissivity. • Will need to be actively cooled (below ~10K? TBC) • Chromatic aberration • Need work on Anti-reflection coating

8. Comparison

9. Some developments after proposal • ESA has released an Invitation to Tender for preparatory work • But some work has already been performed • Investigation of software packages that could accurately model lens systems. • Some experimental developments to test these models Example: investigation of the effect of several slabs on co and cross-pol beams Horn beam pattern through a 30mm machined slab of UHMW polypropylene with various incidence angles

10. Other points to take into account for any configuration • Size of pixels  size of focal plane • Standing waves with other components • Feedhorn / bare bolometers • Feedhons have a low return loss (-20dB typically) • This is not the case for bare pixels • Half Wave Plates / Filters • Between lenses Return loss and cross-talk due to QO components

11. B-Pol 2010-2011 proposal New optical configuration?

12. What’s next? • Everything will depend on: • The required spatial resolution • The size of the mission (Medium or Large) • For a resolution of few arcmins a telescope aperture size of a few metres is required • Basically, forget lenses! Even if we could make these • These would be too voluminous and heavy • Chromaticity aberrations would be too large to fit several bands • Even if multi-layer A/R coating could be made • How to cool these? Uniformity?......etc • The solution would have to be a single reflective telescope

13. 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

14. Possible for large arrays Focal plane Sec Primary Compact test range configuration Used in many CATR + Clover and Quiet Large Focal plane possible • Example: Clover design • FP diameter = 250mm diameter • Size limited by filter diameter not by aberrations • Flat Focal Plane • Edge pixel eccentricity ~ 0.02 • Optical configuration allows good baffling Projected aperture BUT: secondary nearly as large as primary mirror An Herschel-like mirror size (3.5m) with this configuration would lead to a much larger mission

15. Conclusion drawn on present technology • For a space mission • If a spatial resolution of about 1 to half a degree is enough • We can think of 2 solutions potentially each having pros and cons • As we have seen with the previous proposal, a lens-based system might be more suitable but with a lot of work to be done still to bring a suitable system to flight readiness level • Other reflective configurations are investigated • But it is very unlikely to get a single compact/small system with many pixels and several spectral bands (excepted with large improvement in detector technology) • If a higher spatial resolution is needed (less than ~ 10 arcmins) • Then only a mirror-based imager or an interferometer should be considered

16. Other potential technologies • Lenses: use of negative refractive index • Potentially reduces lens thickness and size • Not really developed so far, just an idea • We do not know what additional systematic effects could be associated with this. • Mirrors • Lighter, stronger material? • Surface accuracy? • Cooling system? • Interferometry ? • See J.C. Hamilton and P. Timbie talks