From: Peter Ceravolo
Sent: Sunday, January 05, 2003 9:14 PM
To: RC Optical Systems Users List
Subject: Re: Two Questions?

I had a discussion with Peter Ward, a RCOS dealer in Oz, about a 12.5" 
system that did not give equal in and out of focus star images, even 
though the in focus images were OK. Peter spoke with Paul Jones about 
the problem as the performance of the system did not seem to jive with 
the interferometry test results. Jones advised Peter to verify the 
mirror spacing, after this was done the optics performed well as far as 
the star test is concerned.

This high level of finesse is important for visual use, or near 
diffraction limited imaging like Hubble achieves in orbit. Down here in 
the soup, and even on Kit Peak it is not a big issue, at least within 
reason because we;

1/ unlike the eye, we typically do not sample the diffraction pattern 
enough with our CCD cameras to resolve it well.

2/ exposures are very long, reminiscent of Tech Pan film to get a good 
s/n, and so the images get smeared by seeing/wind buffeting/focus and 
guiding.

3/ differing expectations. Alan French during a talk at Stellafane a 
while back said "When observers look through telescopes they see star 
images. When telescope makers look through a telescope they see 
aberrations". I nearly split a gut laughing because it is true, at least 
as far as this guy is concerned.

A 16" scope at f/8 produces an 11 micron dia Airy disk at focus or about 
0.9 arc seconds projected on the sky. In order to resolve the 
diffraction pattern clearly you really want to have more than the two 
6.8 micron ST10 pixels sampling it, more like four 3.4 micron pixels. So 
you can see that a slight modification of the diffraction pattern caused 
by less than perfect (i.e. a little worse than 1/4 wave but not 
horrible) optics will not be glaringly apparent in the imagery because 
the diffraction pattern is under sampled anyway.

If you have to push the focus out past the design spec for your RC, you 
will not have a fully corrected system. For example, using Zemax optical 
design software I set up a 16" RC. Of course the wave front aberration 
is zero with an ideal system. I then moved the secondary closer to the 
primary by a 8.5mm to shift the focus out 60mm. Well... the wave front 
aberration now balloons to 1/2 wave.

I then did a diffraction encircled energy analysis: Zemax tells me that 
a perfect system contains about 80% of the light in a 2x2 array of 6.8 
micron pixels. The 1/2 wave front contains about 40%. However a 4x4 
array of pixels gets 93% and 75% respectively. Is this bad? It sounds 
not so great, but in practice I suspect most observers, except Adam on 
Kitt Peak, would be content since a real star image (as opposed to a 
charged particle hit) will occupy a minimum of a 2x2 array of pixels. 
And those are the faintest smallest stars, not the majority which will 
cover more pixels even with a perfect system. That has been my 
experience anyway, if I am missing something *please* let me know.

It would be interesting to hear from you imagers out there; How big are 
your pixels in arc seconds and how many pixels cover the smallest 
images? One of the reasons why Adam's images are so incredible with the 
16 RCOS scope at f/8 atop Kitt Peak is the ST10's 1/2 arc second/pixel 
sampling coupled to great seeing. Obviously the set up is finessed... 
including the observer. I would personally like to see some of his more 
mediocre images to make me, as an observer, feel better. :-)

Peter Ceravolo