Re: [ATM] Rating mirrors,understanding seeing

[Mike Lockwood <melockwo@uiuc.edu>]

Jeff,

jeff newsom wrote:
> After looking at the pages of several different optical
> companies(Royce,Obsidian, Pegasus and so forth to name a few) and
> the writings of Mel Bartel, Bob May and others, and articles on
> optics and texts on optics(Texerau, Kriege Berry) , I'm still
> trying to get  a handle on what magnifications , on average, one
> should expect from say, a 12" , f/7 or so on any given night. Mel
> seems to be able to push his mirrors consistently to magnifications
> well beyond anything else I've seen(1000x)(Where the hell do you
> use that thing Mel? In a vacuum?  ;) ). Most "companies "seem to
> suggest that 350 to 400x is more the norm as a top end , with
> magnifications routinely in the 180,275 range.

You ask a question whose answer depends on at least all of these 
factors, and probably more:

1) Mirror temperature
2) Ground temperature
3) Wind (shaking the scope)
4) Airflow around the mirror
5) Optical quality (including secondary and eyepieces)
6) Collimation accuracy
7) Seeing conditions
8) Careful centering on the sweet spot for fast scopes (or else coma 
dominates)
9) Telescope vibration

For me, on fairly calm nights with good seeing, after the mirrors in 
my scopes have cooled, with a tracking platform, with good 
collimation, set up in a grassy area, I have pushed my 12.5" 
Cassegrain (~150" FL) to 1140x when viewing one component of the 
double double.  The view was of the airy disks of one of the doubles 
surrounded by diffraction rings of changing structure, and separated 
by lots of space.  It is one of the most memorable views I have had, 
and it came from a scope that has some residual undercorrection, the 
cause of which I have not yet determined.  The optics are smooth and 
free of astigmatism, though - the view was evidence of that.

On Saturday night (Sunday morning, 3am) at Astrofest I was using my 
new 10" F/8.75 scope on a tracking platform at 445x on Mars with 
excellent results.  The edge of the Martian disk was crisp.  Contrast 
was excellent.  I could have pushed it higher.  (I'll write an article 
on this scope soon and post it on my web site.)

With all the above factors in your favor, I would say a good 12" F/7 
should hold up at 600X on a good night, and should provide similar 
high power views of the double double (Epsilon Lyra) to those through 
my Cassegrain.  Tracking is greatly beneficial.

If Mel's 1000x was with his 20", that is 50 power per inch.  A good 
mirror, amateur or professionally made, should do that.  Mine do that 
and more.  Careful figuring will pay huge dividends.

	Mike Lockwood


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