Re: [APML] Another intro

["bert katzung" <katzung1@home.com>]

Hi Jason:
I think someone misread the specs on the Celestron 9.25. The secondary has a
LOWER magnification than "standard" SCTs. Correspondingly, the primary of
the 9.25 has a larger magnification.  So the supposition is that the field
will be flatter. I think this is true, from my reading of the Rutten &
vanVenrooij book. On page 68, they give a comparison of Cassegrains with two
different secondary mags, 5 (like a standard SCT) or 2 (closer to the
Celestron 9.25). Both are f10 systems, so the primary f ratios are the
reverse. The radius of curvature for the mag 5 secondary system is -160 mm
and that for  the mag 2 secondary system is -3287 mm. Much larger radius of
curvature = much flatter field. (As I understand it <G>.)
Bert

Bert Katzung
katzung1@home.com
www.astronomy-images.com

----- Original Message -----
From: "Jason Lewis" <jlewis@nomoon.org>
To: <astro-photo@seds.org>
Sent: Wednesday, 22 August, 2001 11:03 AM
Subject: RE: [APML] Another intro


> I thought that a secondary with a higher magnification would
> produce more field curvature.  I would also think that the
> higher secondary magnification would also make the OTA more
> susceptible to thermal expansion problems.  Also, wouldn't an
> OTA with a larger primary with the same OTA f/ratio produce a
> flatter field?    Can someone confirm this?  On the other
> hand, I have heard nothing but good things about the  C9.25.
>
> Jason
>
> -----Original Message-----
> I recall reading somewhere that the C9.25 has an f/3.3 primary
> and the
> secondary magnification is about 3, whereas the "normal" SCTs
> have an f/2
> primary and a secondary magnification of 5.  I may have the
> specific numbers
> wrong, but the C9.25 supposedly has a slower primary.  That
> could be the
> reason for the flatter field.
>
>
> --  APML Archives at <http://astro.umsystem.edu/apml/>  ---
>              Unsubscribe at <majordomo@seds.org>


--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>