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Re: [ATM] Cassegrain vs. Gregorian (secondary testing)
----- Original Message -----
From: "Richard F.L.R. Snashall" <rflrs@rcn.com>
To: "atmlist" <atm@atmlist.net>
Sent: Sunday, August 29, 2004 9:18 AM
Subject: Re: [ATM] Cassegrain vs. Gregorian (secondary testing)
>
>
> Bob May wrote:
> > What are we discussing here? I thought that we were discussing the
> > differences between making a Gregorian secondary compared to a Cass
> > secondary for the same primary!
> > Bob May
> > http://nav.to/bobmay
> > bobmay@nethere.com
> > NEW! http://bobmay.astronomy.net
>
> Not quite. The discussion started comparing the Gregorian and
> Cassegrain scopes with equal characteristics. The original
> designs I presented had the same instrument length, back focus,
> aperture and focal length. The primaries were quite different.
>
Yes Richard, I think that there is more than the secondary to consider. A
Greg and a Cas of the same apature are very different instruments. You
should consider what the end use will be.
I have recently made two Gregs, One 20cm (8") f2.8 and the other 50cm (20")
f4 and both were a dismal failure, though I may revisit them at a later
date. I sort of made them in tandem, each one to solve the problem in the
other. Colimation was the greatest problem for me. Solving a Rhubek Cube is
childs play compared to these cursed things.
It only took one look through each scope to see that neither was suitable
for my purpose. It took hours to lock onto the Orion neb using the 20". Then
it moved out of the picture before I could get it in focus.The field of view
is so small and the magnification so great you just have to breath on the
scope and the image disappears. The small one was no better. I finally
located Jupiter, it was extremely faint and appeared about the size of a
basket ball. It almost filled the EP. It was so faint that I could barely
see the two fuzzy bands on it. As a comparison, our club has a Celestron C8,
and I think that if one could get a short enough EP to gain the same
magnification the results would be similar. Having said that, it is not
really a fair assesment as there may be considerable vingetting and other
abberations, they both have Nasmyth focus and no doubt both would benifite
from a little tweeking.
This is just my experience so far and not ment as a judgement of either type
of scope. I just think that all the rhetoric concerning the secondery is
inclined to overshadow the more importent issue---which of these very
different instruments will suit your needs.
Hope this helps a little.
Malcolm.
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