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 MirrorsLenses 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  Will need different telescopes (maybe 2/3 bands per telescope) 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 CATR shows very low xpol and aberrations across large FP More compact  but needs to be cooled

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 Horn beam pattern through a 30mm machined slab of UHMW polypropylene with various incidence angles Example: investigation of the effect of several slabs on co and cross-pol beams

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 Primary Sec 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 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 Projected aperture

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