Re: [APML] Reducing Coma

["William R. Mattil" <wrmattil@ix.netcom.com>]

Roland Roberts wrote:

> Well, I don't either (other than the pain in the neck the next day
> from bending over the eyepiece to manually guide and the longer
> exposure time from stopping the lens down).  I figure I'm going to try
> it.  I was just hoping someone could chime in with personal experience
> or a knowledge of optics that said "yes, it does/sometimes helps."

Let me also add that pretty much every text that I have ever read on
this subject all seem to agree that Astrophotography is the harshest
test of any optical system. And they also all agree that often a *very*
good normal, daylight use etc :^) lens may actually be slightly worse.
Most of them (books etc) indicate that trial and error seems to be
required. My personal experience (such as it is) indicates that my old
50mm Pentax lens is quite usable. I will close this lens up one f-stop.
My telephoto lenses are real cheap and don't produce very good
results.... I have heard that often a marginal lens can benefit from a
minus violet filter.

> I have never touched a lens that didn't need to be stopped down at
> least two stops to get a good focus.  But this is the first time I've
> used this particular 135mm lens and I was surprised that the only
> aberration visible was coma.  My 50mm lenses show some very strange
> distortions when wide open at f/1.4.

Its reassuring that we must be shopping in the same place for lenses :^)

Perhaps some of the much more knowlegable will weigh in here, but if you
havn't read them three of the more illuminating texts that I have are:

1) Wide-Field Astrophotography by [APML's own] Robert Reeves

Excellent source of good information but beware... after reading this
you will have to have a Schmidt Camera... and that can get expensive :^)

2) Astrophotography for the Amateur by Michael Covington

This one covers prime focus, eyepiece projection and a multitude of
other subjects

3) A Manual of advanced celestial photography by Wallis and Provin

Out of print, and some will say that the information contained is dated
but I really like this one too. If I were limited to two books it would
be this one and Roberts. I find myself rereading them from time to time.

Regards

Bill


-- 
William R. Mattil	| Fred Astaire wasn't so great.
wrmattil@ix.netcom.com	| Ginger had to do it all backwards
(972) 399-4106		| and... in high heels.

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