Fuzzy at the edges - but why?

[Ags]

When viewing stars through my ST80, stars are out of focus at the edge when using my 24mm hyperion - in fact they are fuzzy for most of the view and only sharp in the very middle. When I take photographs it is obvious that the edges are not in focus when the center of the image is in focus. So is the fuzziness I see due to the hyperion not coping with an F5 scope, or is it accurately showing me the field curvature of the ST80?

[todd8137]

curse of the hypes

[Ags]

Hmmm, I knew hypes were not perfect at F5 but I thought they were at least acceptable. But the view I am seeing is simply 'orrible. I'm wondering if it is a combination of poor F5 hype performance plus the field curvature of the ST80...

[hemihaggis]

well i have used a hyperion on my F4 scope , yes you get a little fuzzy toward the edges , but the main fov is very good . id say only about the outer 10%-15% was fuzzy .

[John]

I guess it depends what the "fuzzy" stars actually look like. Astigmatism produced by the eyepiece in the fast optical system tends to make stars look distorted, a bit like "seagulls" to me, and progressively more so towards the edges of the field of view. These can't be got rid of by adjusting the focus.

If it's field curvature then I think you should be able to bring the stars at the edge of the FoV to sharp focus but the ones in the centre will then be out of focus.

It could be a combination of the two of course.

[ronin]

Combination of field curvature of both cope and eyepiece, and for the ST80 spherical aberation at the outside of the objective. And you can add in some CA.

The scope will not be producing a flat field, the image it makes will be curved. The eyepiece will also have a slight object plane curvature. These 2 will go in opposite directions so the edges are unable to be in focus with anything near the centre.

The 80mm f/5 achro will give spherical aberation at the edges, so again the edges will be out of focus, actually will never be in focus.

The objective will also display CA which in effect add to the problem as the various wavelengths will not focus at the same plane. The edges again displaying the worst case.

I think that Skys the Limit do a flat field eyepiece that is designed for the field curvature of f/5 scopes. In effect the image plane of the eyepiece is designed in to handle it better. If you have the spare money it may be worth a try but how well it will work I have no idea.

Best solution is something like an f/8 - f/10 ED doublet, preferably one with FPL-53.

[Ags]

Best solution is something like an f/8 - f/10 ED doublet, preferably one with FPL-53.

:-)

One for the future maybe...