RE: [APML] Meade/Celestron focal reducers

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

Hi George,

Ah..I misunderstood about the corrector.  One of perks of email, eh?  Anyways, check out this prescription that I came up with in Zemax based on R&V's.  The spot diagrams look pretty good, though it could use some tweaking for film sizes larger than 35mm.  I put the stop at R&V's location, but I think it could go pretty much anywhere within reason and still get good results...I originally optimized for it at 677mm behind the corrector.

http://www1.iwvisp.com/opticman/astroimages/slevogt.TXT

and the spot diagram:

http://www1.iwvisp.com/opticman/astroimages/slevogt.JPG

I found Dave Rowe's site...very, very nice scope that he built.  I couldn't look at it for too long because I didn't want to start drooling. :)

Cheers,
Jason


-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of George Anderson
Sent: Tuesday, October 19, 2004 7:24 PM
To: Discussion of Film Astrophotography
Subject: Re: [APML] Meade/Celestron focal reducers


Hi Jason

I didn't mean to imply that the schmidt corrector was the field
flattener element. The corrector is a zero power lens with a toroidal
curve(s) to correct for the spherical primary. In the Slevogt the field
curvature is corrected to a fair extent by having a secondary with an
almost identical ROC in the opposite sign. And you are correct in that
the corrector is about 15% outside the ROC of the primary.
If you track down Dave Rowe's home page he has some nice shots of the 8"
(IIRC) Slevogt which he built.
I have not yet been able to get satisfactory results with Oslo for a
Slevogt, even using R&V's perscription.

George Anderson
Montreal Canada

Clear skies and good health

"Lane, Jason R" wrote:
> 
> 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
> _______________________________________________
> Astro-Photo mailing list
> Astro-Photo@seds.org
> http://seds.org/mailman/listinfo/astro-photo
> _______________________________________________
> Astro-Photo mailing list
> Astro-Photo@seds.org
> http://seds.org/mailman/listinfo/astro-photo
_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo
_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo