8" SCT Vignetting with a 50mm EP? HELP!

Considering purchasing a 50mm Plossl for my 8" f/10 SCT Telescope. However.. I've heard there might be Vignetting or possbily a black circle in the FOV?? This is all very confusing to me, if anyone knows about this I would appreciate. I would like a nice wide FOV... but I don't want to waste my monety on a 50mm eypiece if it won't work right in my SCT. Thanks!

Alan,

The dark circle sometimes seen is the shadow of the secondary mirror in the exit pupil of the scope. Under the right conditions it can be seen with any obstructed scope, Maks, Newts, SCTs etc. Naturally, it isn't there with unonstructed scopes such as refractors or off axis reflectors.

The exit pupil from the eyepeice is actually a small image of the objective of the scope and includes the diagonal or secondary mirror shadow and the spider assembly etc. The size of the exit pupil is dependant on the power the eyepiece gives in the scope and for the combination you are considering, (50mm eyepiece, 2000mmFL = 40X), the exit pupil is about 200mm/40 =5mm. (size of the obj / mag).

In that 5mm circle of light of the exit pupil is the central dark circle which is the image of the secondary mirror. If the central obstruction in the scope is about 30% by diameter, then the dark spot in the centre of the exit pupil is about 1.5mm. At night when your eye is well adapted to the dark, your large eye pupil receives all the light from the objective and you will probably have no problems. In daytime, when your eye pupil is much smaller (maybe as small as the dark spot) you will see black outs if you position your eye right on the dark shadow in the exit pupil.

So yes, in your scope, it's a pretty good bet any low power eyepiece is going to give you blackouts if you want to do daytime observing. No, you shouldn't have trouble with star gazing at night when your eyes are dark adapted and your eye pupil is much larger.

There is another problem with some eyepieces that creates blackouts appearing from the sides of the view,(called kidney beaning because of the shape of the dark shadows). This can occur when the eye is not placed at exactly the correct distance from the eyepiece. Some eyepiece designs are worse than others in this respect and rubber eyecups help to alleviate the problem by assisting with the correct placement of the eye.

hope this helps,
dan