[Fwd: Re: ATM RE: SCTs and Difficult Optics]

[Roger Ceragioli <rogerc@as.arizona.edu>]

-------- Original Message --------
Subject: Re: ATM RE: SCTs and Difficult Optics
Date: Thu, 28 Feb 2002 13:37:21 -0700
From: Roger Ceragioli <rogerc@as.arizona.edu>
To: Dominic-Luc Webb molmed <Dominic.Luc-Webb@molmed.ki.se>
References: <Pine.GSO.4.44.0202281258420.311-100000@mbox.ki.se>



Hi, Dominic-Luc.

Yes, it is a major challenge to make a good SCT.  I haven't done one 
yet, but I have some experience making fast aspheres (under f/1 and with 
about 350 waves of asphericity).  The hardest part is getting the curves 
smooth and avoiding or fixing little errors that are confined to a small 
sector of the optic ("azimuthal errors").  Also, being able to measure 
what you've got at any stage during fabrication is crucial, but very 
very difficult.

Dave Rowe, who's on the list, has actual experience making Schmidt 
plates and (I'm sure) can clue us in to the difficulties.  Perhaps those 
difficulties are less for wide-field photography, because of the need 
for less resolution, than for a Cassegrain system which (usually) is 
meant for high-resolution?   

One problem intrinsic to the Schmidt design is spherochromatism 
(variation of spherical aberration with color).  Commercial SCTs have 
quite a bit of spherochromatism, which you can often detect but 
examining the diffraction pattern of a white star through focus.  Inside 
and outside of focus the coloration of the diffraction pattern changes 
are shows fringes.

This is a minor difficulty, of course, and what the SCT excells at is 
convenience.  If you buy an SCT, chances are that you want a light, 
compact scope.   So commercial SCTs often are a great value for a little 
cash.  But probably most people on this list are looking for high 
quality optics.

As for aspherizing of the primary, no.  The primary was spherical, and 
there's no way (even using Chinese slaves) that Celestron could afford 
to tinker with aspheres on the primary and secondary.  Also, the 
corrector plates' neutral zones were at about the 70% radius, not at the 
86%.  I stuck both plates down to a master reference flat and measured 
carefully.  Definitely not at the 86% zone, but well inside that.  And 
finally, the back surface of each plate was aspherized and not the front.

Hope this helps,
Roger Ceragioli

Dominic-Luc Webb molmed wrote:

>On Wed, 27 Feb 2002, Roger Ceragioli wrote:
>
>   > Recently, I had the opportunity to test two Celestron SCTs (a "Fastar 8"
>   > and a "Fastar 11").  Despite the fact that some of the recent vintage
>   > Celestron and Meades may be good, these two brand new ones were rather
>   > nasty.
>   > You just can't guarantee high quality for the Big Mac prices they
>   > charge, in my opinion.
>
>I am afraid I have to agree. It scares me a bit. Some of the
>commercial Newts are quite decent. It makes me think there is
>little hope of an ATM making a decent SCT. I am trying some
>of these catadioptric systems myself. We'll see what happens.
>Also, was the primary really spherical or perhaps slightly
>aspherized?
>
>
>Dominic
>
>North 59 37' 30"
>East  17 48' 10"
>
>
>