[ATM] Classical Cassegrain Design Tradeoff Help Needed

[mpeck1 at ix.netcom.com (Michael Peck)]

At 11:33 30-01-04 +1000, Frank wrote:

>We should compare a F/14.7 Cass with a F/3.9 Newt in which case
>the Newt ( I believe) would perform better.
>
>Cheers
>Frank Q

I guess what you're wondering is, how does an f/3.9 Newt compare to an 
f/14.7 cass at the same angular distance off axis, right?

Instead of comparing my cass, lets take a look at the configuration that 
Gerald is thinking about, namely a 350mm diameter f/14.5 cass with f/4 
primary. In his case the maximum field of view with a 2" eyepiece would be 
about 0.55? (that's a linear field of about 48.8 mm which is a little 
large, but never mind).

Plugging the configuration into OSLO LT and setting the field size to 
0.275? I get a total RMS wavefront error (550 nm) at the edge of the field 
of about 0.26 waves. Breaking that down into Zernike components the 
contributors to the wavefront error are:

CASS:

defocus 0.236
astig           0.081
coma            0.054

Those are RMS contributions. By the pythagorean theorem the sum of the 
squared contributions equals the square of the total wavefront error.

Now I do the same thing after removing the secondary, looking at the 
Newtonian prime focus. Again according to OSLO, at 0.275? the total RMS 
wavefront error is about 0.72 waves, consisting of the following components:

NEWT:

defocus 0.067
astig           0.095
coma            0.702

Here <http://home.netcom.com/~mpeck1/astro/temppix/cassvsnewt.png> is a 
side by side wave optical comparison of the PSFs of these two wavefronts, 
at the same image scale (the cass is on the left). This looks more 
realistic if push back away from your monitor a couple feet and squint. The 
left hand image gives a pretty good impression of what I see in my cass: 
there's a bright nearly pointlike core with an elliptical splash of light 
pointing away from the optical axis.

This is one of the reasons I sold my flat after using it once to star test 
what was originally intended to be a convertible cass/newt.

Mike Peck

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Michael Peck
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