Re: Odd star test

Hi, Ed.
Your description -- center bright vs. center dark (on either side of focus) -- indicates either over- or undercorrection (depending on which side of focus showed bright & which side showed dark).

How many fresnel rings did you see when defocused; do you recall? And what magnification did you use? Prism diagonal or mirror? While we can't estimate the degree of spherical aberration based on those parameters alone, it may provide some insight as to the "relative" correction of the scope.

I'm going on very little, obviously, but I'm an optimist: assuming you *are* seeing spherical aberration, I suspect your refractor is still doing fine. A typical good-quality refractor sample of recent/current vintage (i.e. ~1990+) will likely display s.a. in the realm of ~1/5 wavelength. This is still quite good. And at that level, the bright center/dark center difference can be easily visible.

Couple other aspects of the fresnel pattern you might look for &/or ask yourself:
-Are the rings uniform and evenly illuminated?
-Any ring(s) stand out as being odd or "out of place" in terms of brightness (relative to a neighboring ring)?

And the IN-focus diffraction image:
-How many diffraction rings visible -- one? maybe two? lots?
-How bright is the airy disc, relative to the first diffraction ring -- *much* brighter? about the same?

You're welcome to keep picking our brains, and I'll be happy to relay any hunches I may have (based on your descriptions).

Best wishes.
-Dan

Can you some how use a mask, either make a central obstruction OR mask off the outer to just leave the center and get a better analysis that way? If the center is indeed bad, then by masking out most of the outer "good" area, you should see the star test show more of the real abberation the center is displaying.

I'm with you, then -- a small axial zone. If I can dig up any more specifics, I'll pass them along here.

Hope I didn't sound patronizing in my previous post -- didn't mean to. I obviously didn't read your original description as well as I should have.

Whether or not the aberration is worrisome, I guess that might depend on the "role" the scope plays in your own astronomy "realm." If my Pentax displayed the same aberration while still performing as it does (which is very well), I wouldn't worry about it at all. In other words, let the scope's history of performance (and its in-focus views) do the talking, so to speak.

Cheers,
-Dan