Re: ATM Fast vs Slow scopes

["Andy Steere" <andy@www.system.missouri.edu>]

On Thu, 30 Jan 1997, Robert  C. Pfaff wrote:

> >35mm Panoptic and you had to move your head off-axis to see it easily. 
>
> Now the point is if you have a full size secondary that is needed
> for a f4 telescope it may have great images in a narrow field for high
> power ( the aberations out of the field) but central obstruction
> will cut contrast somewhat unlike an unobstructed refractor.

I contend that unless the scope will be used for photography (the scope in
question was designed for visual only), you don't need a full-sized
secondary. The secondary in the scope in question was a 3.1" combined with
a very low profile focuser. Yes, there is some light loss at low
magnification, but it was not noticeable at 36x in the Panoptic.  There
seemed to be no light loss at all at the powers we were using most
(60x-400x).  You could see the mirror clips in the out-of-focus star
images at 100x and up (didn't try it at a lower power).

I feel secondary size and it's effect on contrast is somewhat overrated. 
When you take a primary that already sits at the edge of the "Rayleigh
cliff" (say 1/4 wave) and you add a central obstruction... you push that
telescope right off the cliff (where quality differences are much more
obvious).  I think this may well explain why the contrast seems to degrade
visually on most fast f-ratio scopes (IMHO few f/4 scopes are better than
1/4 wave, and most are worse).

The telescope in question PASSES THE STAR TEST!  Exactly the same pattern
inside and outside of focus. The surface of the mirror is very smooth as
well.  Even with a larger secondary... I would feel most confident in
comparing this telescope with any 6"-7" refractor.

For those interested, the primary mirror of the 12.5" f/4 scope was once a
Cave cassegrain, with a 50" focal length, and of ocean-wave quality.  :-) 
It was left at f/4 because that was easier than trying to change it.  The
refiguring was done by Dr. Mark Russell.  The rest of the telescope was
put together out of parts we had laying around (the mount belongs to a
club member, the focuser was swiped from another telescope we were not
taking, and the fiberglass tube is an old Parks that I personally sanded
and repainted). 

Again, if anyone is interested in seeing the scope I'm talking about, go
to the URL at the end of this message and choose "STAR PARTYS" and then
1995 Astrofest.

> I own a 12.5 f4 telescope. ( did not make this mirror). I find
> that only my most expensive EP's  ( Nagler's) will work.

Interesting.  None of my Meade Series 4000 Plossls (I have a nearly
complete set), with and without a couple different barlows, had any
problem with the light cone. 

Andy Steere - andy@www.system.missouri.edu
http://www.system.missouri.edu/andy/