On the performance of Planck-like telescopes versus mirror aperture

Future space mission like MAP and PLANCK will be able to shade new light on our knowledge of the Universe thanks to their unprecedented angular resolu tion and sensitivity. The far sub-degree angular resolution is obtained cou pling usual detectors, radiometers and/or bolometers, to an optical system, namely a telescope. The wealth of cosmological information is encoded at h igh l values (similar to 1000) which can be reached with resolution of abou t 10'. Distortions of the main beam resulting from the current focal plane arrangement and the optical design of the PLANCK satellite will degrade ang ular resolution and sensitivity per resolution element possibly compromisin g the final results. The detailed design of the PLANCK telescope is continu ously changing with the aim of optimizing its performance. In the present w ork we present a methodological study on the relation between telescope opt ical design, focal plane arrangement and optical performances, focussing on the dependence of angular resolution on primary mirror aperture. Different independent approaches have been developed to quantify the impact of main bt am distortions on cosmic microwave background (CMB) science yielding nea rly the same results. The so-called PHASE-A telescope is unacceptable with respect mission main goals. Larger telescopes (namely with effective agm ap erture greater than or similar to 1.5 m) are therefore preferable. This pap er is based on the PLANCK LFI activities.