|
Most of the diffraction arises from the aperture stop, which is the primary mirror (reflectors), corrector (Maks and SCT's) or lens (reflectors) - not the tube.
In practice very few astronomical scopes have an aperture stop in front of the first element - about the only one that does is the lensless Schmidt.
Rob - The issue of the limiting magnitude is still bothering me greatly actually as the more I look into it I think its influenced by a combination of two effects - increasing the magnitude pushes the sky background down, and the narrower field of view typical of extreme high power eyepieces (ortho or monocentric eyepieces) reduces the total illumination entering the eye still further.
Over the past few weeks I've been trying to devise a more serious test that I can try when the opportunity arises to get out under some seriously dark skies in steady seeing (I mean 9/10 skies). The weather here unfortunately has been most uncooperative.
|