A Cassegrain telescope's exit pupil is an annulus: a clear disc of outer radius with…
PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Fourier optics and image formation · Fourier optics and image formation
What an Instrument Can See, Ch The transfer function of an imaging system
Problem
A Cassegrain telescope's exit pupil is an annulus: a clear disc of outer radius R with a central obstruction of radius εR (from the secondary mirror and its supports), 0<ε<1. Compare this with a clear circular pupil of the same outer radius R, both used for incoherent imaging at the same wavelength.
((a)) Before computing anything, predict how the annular pupil's incoherent cutoff frequency compares with that of the clear pupil of the same outer radius.
((b)) Predict, in qualitative terms only, whether the annular pupil's MTF lies above, below or equal to the clear pupil's MTF at low-to-middle spatial frequencies, and justify the prediction using the picture of the OTF as the overlap area of two shifted copies of the pupil.