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Subject: Re: [M]: Culmination Question
From: Howard Pinsky
Reply To: mapug@shore.net
Date: Wed Jan 15 00:29:28 1997

Howard Pinsky wrote:
>
> Chris_Vedeler@egghead.com wrote:
> >
> > Last night I was doing some culmination on my 10" LX200. I discovered an
> > effect that I was wondering was normal, or a sign of an optical problem.
> >
> > At about 100x an out of focus star in the center of the field of view shows
> > near perfect culmination, but stars toward the edges show what appears to
> > be either coma or astigmatism. The central obstruction is skewed towards
> > the center of the field, with the defraction rings stretching around it.
> > These out of focus images at the edge still remains very nearly circular,
> > but the relative location of the central obstruction in the star is moved
> > towards the center of the field of view.
> >
> > I realize that this may be too low of power to do good culmination, and the
> > effect seems much less dramatic at higher power, but I have never read
> > anything regarding this effect.
> >
> > Is it normal to have the stars towards the edge of the field of view at low
> > power to give the appearance of the scope being poorly collimated? May
> > this be an effect from the eyepiece? I'm using the 26mm Meade 4000.
> > Should all the stars in the field, regardless of power show the same
> > pattern if properly collimated? If this is indeed an optical defect, what
> > is it called?
> >
> > Thanks,
> >
> > Chris Vedeler
> > cvedeler@egghead.com
>
> Chris,
>
> The images of stars should be the same throughout the field of your
> telescope. If you are collimated corectly, you should see a small airy
> disk with a dimly lit diffraction ring for each star everywhere in the
> eyepiece field even at the edge. What you're looking at appears to be
> coma. It only occurs off axis and would get worse at larger field
> angles ie: at the edge of the eyepiece field of view. This lens
> aberration could be coming from the primary or secondary mirror, the
> diagonal or the eyepiece. Bad eyepieces with coma will also usually
> show lateral color aberrations as well resulting in red and blue ghost
> images adjacent to the central white image. Try to isolate it by just
> using the Meade 26mm eyepiece without the diagonal. If the condition
> remains, try a different eyepiece of about the same power. Also try
> using a lower power eyepiece as well, such as a 40mm Plossl. If the
> condition gets worse at the edge of the field, the primary or secondary
> miror is probably to blame. This would likely require a factory
> replacement by Meade. -By the way, going to a much higher power should
> reduce the effect of coma since the off-axis field angle is reduced.

----------
Chris,

I just researched a little more and found that coma could be caused by a
tilted or de-collimated mirror. You might try tuning up your
collimation using a high power eyepiece on stars located both on and
off-axis.

Sorry if I misled you into thinking you had a bad primary mirror - it
may simply be a matter of fixing up your collimation.

Regards,
Howard