Mirror mask mounting point

[MarkInSpace (Mark)]

for a Newtonian reflector, can I mount a mirror mask at the top of the tube, or does it need to be directly in front of the mirror?

[Joshua Bunn (Joshua)]

Hi Mark,
It needs to go in the mirror, two reasons I can think of. One, so your effective aperture is the same for off axis light as for on axis light, and 2 so that no off axis light get sunder your aperture mask, which is what would happen if it was mounted on the front of the tube.


Josh