RE: [APML] Meade/Celestron focal reducers

["Lane, Jason R" <jason.lane@navy.mil>]

Hi George,

Yeah that was the one I was thinking of, although the Slevogt corrector plate is outside the center of curvature of the primary.  This isn't equivalent to the field flattener that we're talking about tho..it's equivalent to the Schmidt corrector.  I agree, though, that they might be using the same basic concept (primary and secondary having roughly the same curvature).  

-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of George Anderson
Sent: Monday, October 18, 2004 8:25 PM
To: Discussion of Film Astrophotography
Subject: Re: [APML] Meade/Celestron focal reducers



"Lane, Jason R" wrote:
> 
> Hi Ray,
> 
> Good point about the longer SCT.  I agree that a longer tube variant would typically have better field correction, but it may also be possible with the more compact scopes.  Although compactness is definitely a trade-off, one perk of designing for photography vs. visual is that your performance requirements are different and in some ways more relaxed.  For example, while you want a flat field, you can sacrifice sharpness since you don't require diffraction limited performance (ie Airy discs)..only that point sources image to grain size or less (ie. less than, say, 25um).  I don't know if you could do it with spherical mirrors and keep a short tube, but using aspheric mirrors you probably could.  I wonder if the f/6.3 has the same size secondary mirror or a larger one, since a photographic SCT doesn't need as small a secondary as a visual scope does.
> 
> Can't wait to see Duncan's results.
> 
> Cheers,
> Jason


Hi Jason

In Rutten and Venrooij Telescope Optics they describe a Slevogt which is
similar to an SCT. Although the primary and secondary mirrors have the
same radius of curvature. The main departure in the Slevogt is that the
corrector is at the radius of curvature of the primary and not at the
focal point.
But Meade may be using the same concept with the f/6.3 and achieving a
fairly flat field.
Dave Rowe built a Slevogt and his initial results looked good as he was
aiming for a flat field across a Hassey film holder without the use of a
field flatener.

George Anderson
Montral Quebec

Clear skies and good health
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