RE: [APML] Photographic test of Meade 8" f/4 Schmidt-Newton

[Alen Koebel <alen_koebel@yahoo.ca>]

Walter Koprolin wrote: 

> Thank you for your comments and compliments to you
> for your well-made photographs. It seems that your
> older 6" f/5 Schmidt-Newton (and maybe even the new
> 6" model) is better suited for astrophotography than
> the new LXD55 SN-8 model from Meade.

Thanks. There seems to be a natural tendency to go 
for that larger aperture and ignore the smaller 
scope, but I think in this case it's counter-
productive. 

> It may even be possible to reduce vignetting by
> shortening the drawtube and using a custom-made
> adapter for the camera instead of the standard T2
> adapter. 

I should mention that the older model I'm using
(an MTS-SN6) has a helical focuser. The opening for
the drawtube is actually around 2.4" and now that
I've reduced the drawtube length, there is probably
little vignetting of the light cone into the focuser
itself. On top of that (no pun intended) I use a
Lumicon 2" Easy-guider, which does not use a standard
T-ring adapter. Rather, it uses only the outside part
of the adapter, which attaches directly to the Easy-
guider's body, resulting in a considerably wider clear
aperture than with a T-ring. As a result, I think most
of the modest and easily-corrected vignetting I am
seeing is a result of two other optical considerations
I can't do anything about: the size of the secondary
and the fact that the primary is no bigger than the
corrector. A Schmidt-corrected system will vignette
under this condition and the vignetting will increase
with shorter focal ratio (or so I interpret from what
I read in the Telescope Optics book by Rutten and van
Venrooij.)

> However, I doubt that anything can be done against
> the radial distorsion of the stars near the edges 
> of 35mm format, this is most probably caused by 
> the curved focal plane of the f/4 optics and could
> only be corrected by a flatfield corrector, but 
> none is available for the Meade Schmidt-Newtonians
> I am aware of.

I wonder if a Lumicon coma corrector would yield 
better results if used on this scope. It's a coma
corrector, but there is still residual coma in
these scopes and it might also flatten the field.
Seems silly to use a corrector on a supposedly 
already-corrected system, but it might just work. 

> At f/5, the focal plane is less curved, thus the
> distortions are bound to be less obvious.

That was my expectation, seemingly confirmed by my
photos. Mind you, even at f/5 I have to be careful 
to focus in the right place on the frame, otherwise
I do get distortion that I attribute to the field
curvature. With proper focus, coma at f/5 seems almost
nonexistent, with just a touch in the extreme corners.



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