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
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