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

[Walter Koprolin <a9125657@unet.univie.ac.at>]

Hi Alen!

> 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.

Well, than you are of course much better off. The 8" Schmidt-Newtonian has
only a 2" opening at the focuser and will vignet much more, also taking in
account the steeper light cone...

> 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.

I doubt that. The use of a spherical mirror makes the optical system
coma-free, one of the advantages of a Schmidt camera. And if the system
has no coma, a coma corrector will introduce some.

The problem here is the curved focal plane, so the edges of the flat film
plane will be slightly out-of-focus and stars there distorted radially. A
flatfield corrector like those used with fast refractors, which have the
same problem when used for astrophotography, would help here, but it
should be specially designed for the optical system.

"Creative focusing", i.e. focusing not at the center of the FOV but
somewhere further out will also reduce the distortion at the edges, but
will create slightly bigger stars at the center.

  Walter Koprolin
  Gallery of Astrophotography & CCD, Observation Reports,
  Telescope Testing: http://www.astro.univie.ac.at/~koprolin


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