Avoiding the shadow of the secodary?

When I view brigh objects such as Jupiter, Saturn, or the moon, in my 11 inch f4.55 Dob, I see, even at high powers such as 161, 181 or 300, a fuzzy spot in the center of the image; the rest of the image is very sharp. If I back my head awsy from the eyepiece, the fuzzy spot gets sharper and sharper and it finally becomes a sharp black image of the secondary.
Has anyone else been bothered by this phenomenon?
Bill Meyeers

Hi Bill,
First, what size is the secondary? Sounds like you have a extra large on or the exit pupil is too large with your EPs which I doubt is the case at 300x.

As Floyd mentioned, this phenomenon is a common problem related to exit pupil. It has to do with the exit pupil of the light coming from the eyepiece being larger than the entrance pupil of your eye. It is much more noticeable during daytime viewing when your eye's pupil is much smaller due to all the light entering your eye. An 11" scope can gather quite a bit of light on brighter objects and as we all know, the moon and even Juipter can be uncomfortable to look at through a large newt. This causes the eyes pupil to consrict (losing dark adaptation, bummer) and making the exit pupil of the ep, that may normally be fine for dark adapted eyes to be too large for the constricted pupils.

I've noticed this in my F4.3 dob, but find that it is not a problem when I use a polarizing filter or even one of those cheap moon filters. Frankly, I have not noticed it on Saturn and have to wonder if your eyes are particularly light sensitive, meaning they would constrict more than some.

Best wishes,

Randy