RE: [APML] Mewlon 250

[Radu Corlan <rcorlan@pcnet.ro>]

On Tue, 3 Jun 2003, Gene Horr wrote:

> -----Original Message-----
> From: astro-photo-bounces@seds.org
> [mailto:astro-photo-bounces@seds.org]On Behalf Of Radu Corlan
> Sent: Monday, June 02, 2003 7:20 PM
> To: Discussion of Film Astrophotography
> Subject: Re: [APML] Mewlon 250
> 
> On Mon, 2 Jun 2003, Frank Barrett wrote:
> 
> >> I'm starting to understand (and hit) the limits of my C8 and am starting
> >> to consider what my next scope should be (drooling really<g>).  What I
> >> want is an RC, but that's kinda outta reach right now.  I've read great
> >> reviews and I'm considering a Mewlon 250 for the following reasons:
> 
> > The Mewlon is a Dall-Kirkham - which has even more coma than the sct.
> 
> Ah, nope.  In fact the star images are far better than _any_ of the non
> RC Cassegrain variants within most of the field.  Only at the edges do
> the stars get as bad (but not worse) than a SCT.  

Well, that's not exactly what my book says ;-). Does anybody know the 
secondary magnification (or the primary f/number) for this scope? It would 
be nice to do some spot diagrams. 

Another thing to think about is that the Dall-Kirkham is a lot easier to 
build than a classical Cassergrain - the ellipsoidal primary can be 
null-tested, and the secondary is spherical; We'd see a lot more of these 
scopes around if they didn;t have the coma problem. 

> What's even better 
> about this design is that the coma is very easily corrected.  The
> downside of the corrector/reducer is that it does introduce noticeable 
> vignetting across a 35mm frame.  There is a f/6 reducer/corrector in
> the works but it is most likely only for CCD work.
> 
> The advantage of the R-C design is that it can be made "faster" and that
> the stars are even across the field which makes it a great tool for 
> scientific measurements.  

The R-C is coma-free; so the stars are always ellipses, not little comets 
- which is indeed great if you want to get a star's position, since the 
centroid of the star's image remains at the center.

> Also most designs have much less field curvature than other production 
Cassegrain variants.

While this is true, it's not a merit of the R-C design. The R-C has more 
field curvature for a given optical configuration. However, nobody builds
R-C scopes with a small secondary (and high magnification), because then 
it would be impossible to illuminate a wide field, and most R-C scopes are 
optimised for photography.

But if you build for instance a classical cassergrain with the same 
optical configuration like an RC (same primary f number and secondary 
magnification), the field curvature would be lower.

> 
> All in all I would not recommend the M250 for emulsion imaging, but 
> optical quality has nothing to do with it.  It is an outstanding CCD 
> platform.  The main problem with film is the system's "speed".
> 
> I'll try and do some test shots for you.

Test shots would be great. There's nothing i like more than being proven 
wrong.

Radu

> 
> Gene Horr
> genehorr@houston.rr.com
> 
> 
> 
> 
> 
> 
> 
> 
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-- 

-------------
Radu Corlan       Snail Mail: Bucuresti sect. 1, 
rcorlan@pcnet.ro  str. Argentina nr. 28, Romania

   You can still escape the "Gates" of Hell!   
                 Use Linux!                       


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