Re: [M]: Question on spheric abberation...
| Subject: | Re: [M]: Question on spheric abberation... |
| From: | Email address hidden |
| Date: | Mon Apr 30 13:44:54 2001 |
In a message dated 04/30/2001 4:56:50 PM Eastern Daylight Time,
spectrumsoft@televiso.com writes:
<< Ive got a 10" LX-200 thats maybe 9 months old. I was out collimating the
other day, and I noticed that I never get the same kind of diffraction
pattern on one side of out-of-focus as I do on the other side of
out-of-focus. In one case, the diffraction rings look quite pronounced and
clear. When I rack the focusser (either the standard one, or my JMI
focusser) through focus, and on to the other side a bit, the diffraction
pattern looks very different - fuzzy, no clear rings, and kinda sparkly and
fuzzy. The overall pattern is nice and round on both sides of focus.
My question is this: I was reading one of Dickensons book, and it had a
section on spheric abberation. It noted that undercorrection and
overcorrection are considered defects of a scope, and are related to the
accuracy with which the glass was ground. His pics showing this problem
were very similar to what I see in my eyepiece. Ive not noticed that my
optics are particulary bad per se, but when I read this, I of course had
questions. Id like to hear from anyone who might be able to shed some
light on this. In particular, does this situation invalidate Meades claims
of "diffraction-limited" optics in these scopes? I kinda figure that it
might... >>
The good news is that you have diffraction rings on one side of focus. The
bad news is that for small amounts of spherical aberration, you should have
fuzzier but still-existing rings on the other side.
Keep looking on different nights, part of it might be the seeing. Maybe
"pronounced and clear" will become "razor sharp" and "no clear rings" will
become "pronounced and clear".
A star test can make a good scope look bad sometimes, and will always make a
bad scope look bad. So keep on looking until youre sure of which it is. It
would also help to have it side by side with a scope of known quality and
quantified defects, but those are hard to come by!
John
spectrumsoft@televiso.com writes:
<< Ive got a 10" LX-200 thats maybe 9 months old. I was out collimating the
other day, and I noticed that I never get the same kind of diffraction
pattern on one side of out-of-focus as I do on the other side of
out-of-focus. In one case, the diffraction rings look quite pronounced and
clear. When I rack the focusser (either the standard one, or my JMI
focusser) through focus, and on to the other side a bit, the diffraction
pattern looks very different - fuzzy, no clear rings, and kinda sparkly and
fuzzy. The overall pattern is nice and round on both sides of focus.
My question is this: I was reading one of Dickensons book, and it had a
section on spheric abberation. It noted that undercorrection and
overcorrection are considered defects of a scope, and are related to the
accuracy with which the glass was ground. His pics showing this problem
were very similar to what I see in my eyepiece. Ive not noticed that my
optics are particulary bad per se, but when I read this, I of course had
questions. Id like to hear from anyone who might be able to shed some
light on this. In particular, does this situation invalidate Meades claims
of "diffraction-limited" optics in these scopes? I kinda figure that it
might... >>
The good news is that you have diffraction rings on one side of focus. The
bad news is that for small amounts of spherical aberration, you should have
fuzzier but still-existing rings on the other side.
Keep looking on different nights, part of it might be the seeing. Maybe
"pronounced and clear" will become "razor sharp" and "no clear rings" will
become "pronounced and clear".
A star test can make a good scope look bad sometimes, and will always make a
bad scope look bad. So keep on looking until youre sure of which it is. It
would also help to have it side by side with a scope of known quality and
quantified defects, but those are hard to come by!
John
