Re: ATM precision of secondary mirrors

[Ken Lowther <s0109366@cc.ysu.edu>]

On Mon, 13 Sep 1999, Bruce Swayze wrote:

> I agree, and would go so far as to say that the old "rules" are now being
> broken routinely.  The envelope is being pushed.  As proof of this, we've
> used both my 7-inch f/4 and my 15-inch f/5.5 at extremely high powers for
> planetary observing.  One night of superb seeing last fall allowed
> magnification of 1080x on Saturn, and we even doubled that to over 2100x
> before seeing any deterioration in the image begin to set in.  We had a
> group of witnesses observing this, all with our mouths open and our jaws
> down around our knees.  I routinely use the 7-inch f/4 at powers of up to
> 350x on the planets with outstanding views.  One experienced refractor buff
> told me he saw no difference between this and an expensive Apochromat.  We
> had another night like that at the Oregon Star Party two weeks ago, and Mel
> Bartels had his 20-inch f/5 on the Ring Nebula at over 2000x, holding crisp
> and sharp.  The tiny pinpoint central star was clearly visible.  He even
> bumped it up over 6000x at one point.  I'm not making this up.  There were
> hundreds of people there that weekend, many of whom saw this, and other
> quality scopes that performed in a similar fashion.  A favorite scope was
> Judy Dethloff's 16-inch f/4, delivering knockout views of Saturn at very
> high mags.  But there wasn't a refractor or a cass in the field that came
> anywhere close to this kind of performance.  
> 
> Over the past couple of years, I have come to the conclusion that there are
> 3 crucial things that allow this to work for you:  critical attention to
> collimation, extremely good optics, and the best possible eyepieces.  (I
> mean, that about describes your optical train, doesn't it?)  
> 
> My point is, we enjoyed this kind of performance through a variety of
> instruments with varying degrees of diagonal obstruction, and differences
> in other details that have been endlessly debated.  The instruments that
> passed the above 3 criteria performed.  No matter how everything else was
> built. 
> 
> It is indeed possible to have the best of both worlds.  You can enjoy an
> all-around instrument that will give you the best planetary views as well
> as the deep sky splendors and richer field views.  Man!  Is this hobby
> getting good or what?  
> 

When using these high powers on sort focal ratio scopes, are you using
coma correctors, or is the image still far enough from the edge that off
axis coma isn't a factor?

Ken Lowther  
s0109366@cc.ysu.edu
Youngstown, Ohio  USA