Quote:
Originally Posted by Peter Ward
Hence I'm not sure what you mean by "aperture angle" as the wavefront from the sky is perfectly parallel to the telescope aperture (or "cutter" )
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The field of view displayed on your image has a diameter also on the sky.
The angle between the outer bounds seen from the point of observation is what I called "aperture angle".
Photons from (a few diameters) aside of this field still hit the mirror, but not the sensor. Adding a reducer (between mirror and sensor) redirects photons from outside the original field to your sensor. The total count of photons you earn is now those from the original field plus the ones from the additional part of the sky your image shows.
(Btw.: Making this angle too wide will lead to shadowing by your tube, but more serious is the aberration introduced by the reducer.)