Re: ATM precision of secondary mirrors

[Bruce Swayze <swayze@europa.com>]

At 01:52 AM 9/13/97 PDT, you wrote:
>
>> BTW, if the primary intended use of your telescope is planetary
>> observing, why did you choose a short focal ratio Newtonian? It seems
>> to me that where long focus is demanded, a better way would be
>> to make a Gregorian with an efr of around f/100 to f/150.  Then you
>> could use very long relief eyepieces at high powers, or do photography
>> CCD or film without having to use questionable quality projection methods
>> to get a halfway decent image scale.  Anyway, with either a Cassegrainian
>> or a Gregorian, you do not have to have a reference flat for figuring.
>
>The more I learn about optical performance from people who know
>more about it than I do, the more I'm convinced that newtonian focal
>ratios in the f/6 range can deliver first rate planetary images.  At 72
inches,
>the focal length of an f/6 12.5 incher is not exactly short in my book.  Don
>Parker's 16 incher is f/6 --granted 25% longer f.l., but it doesn't
>seem to trouble him any.
> Using a 2x barlow with a 12mm eyepiece with Fabio's scope would
>give 305x and 457x with an 8mm--still what I find in the comfort range. How
>many nights per year  can you go higher than that to good effect?
>If Fabio achieves the optical standards that he's striving for I see no
>reason why a rig like this can't yield superb comfortably viewed images
>-visual, CCD, or photography.
>
>Dan Chaffee

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?  

Clear skies,


"*-.,_,.-*"`"*-.,_,.-*"`"*-.,_,.-*"`"*-.,_,.-*"`"*-.,_,.-*"`"*-.,_,.-*"`"*-
Bruce Swayze
swayze@europa.com
http://www.europa.com/~swayze    Swayze Optical Home Page 
http://www.europa.com/~swayze/spider.html  Homemade Spider page
"*-.,_,.-*"`"*-.,_,.-*"`"*-.,_,.-*"`"*-.,_,.-*"`"*-.,_,.-*"`"*-.,_,.-*"`"*-.