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

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

Hi Herm!

> Walter, besides the flares and vignetting your results compare very well to
> my Vixen R200SS newt with its coma corrector/field flattener. Your
> collimation was off and this makes the stars at the edges elongated.

The collimation was off only by a small amount, it shouldn't show up on a
photograph. A Schmidt-Newtonian is not as collimation-critical as a
straight Newtonian.

I think the elongated stars at the center of the Horsehead Nebula image
are due to a small guiding error in declination, I had troubles with the
mount at the time when I made the test shots. The Seagull Nebula image got
better guiding.

> If your collimation was perfect (as it has to be with an f4 newt), then the
> stars in the center of the frame would be perfect, but coma and field
> curvature would make the stars in the corner elongated and point to the
> center of the frame.. this is not what I see in your pictures.

That is exactly what I see in my slides, look closely at the stars near
the edges in the raw scans.

> I bet that if this scope was collimated perfectly then you would be
> surprised by its performance.. I was with my R200SS when I finally figured
> out how critical collimation was.

A Schmidt-Newtonian has a factor 4 less coma than a Newtonian (if I
remember this correctly), and is therefore less critical on collimation.

> Vignetting issues are a different matter, you will get it with an f4 beam
> of light and a T-adapter.. I use a Vixen direct-photo adapter and this
> takes out 99% of the vignetting, as a result I dont do PS processing in my
> pictures to remove vignetting effects.

The Vixen R200SS has a larger focuser that the Meade SN-8, and has a
dedicated direct-photo adapter, therefore it is much less subject to
vignetting. The Meade SN-8, hoever, has a focuser with a long 2" drawtube.

> My conclusion?.. I think this may be an excellent photographic instrument,
> with several advantages over a classical newt with a Ross corrector. Coma
> correctors in newts must be used at exact distances from the focal plane,
> but the correctorless Meade design makes it possible to use an off-axis
> guider. Its impossible to use an off-axis guider in my R200SS due to the
> coma corrector.. a Lumicon newt guider with its coma corrector vignettes
> horribly at f4.

Hmm, I doubt this Schmidt-Newtonian could be an excellent photographic
instrument. Off-axis coma and field curvature can't be cured easily, and
degrade this scope's photographic performance, even if you manage to
reduce vignetting by using a larger focuser and larger photo adapter and
reduce reflections by lining the tube with flock paper and by a tube
extension up frount to reduce straylight.

However, near the center the image certainly usable, so if you crop out
the edges, the images should be ok.

Yours sincerely,

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


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