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

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

Hi Alen!

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.

It might be possible to reduce or eliminate all the reflections with a
tube extension up front and a new flat black paint on the inside of the
tube. 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. 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.

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

You are right, the "diffraction spikes" around Alnitak could be caused
by the mirror clamps. I think I remember there were four instead of the
usual three, and they certainly did protude into the light path.

Greetings from Vienna/Austria/Europe,

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


On Fri, 23 May 2003, Alen Koebel wrote:

> Interesting report. I've been using one of Meade's
> older (early 1990's) Schmidt-Newtonians for A-P and
> so far have achieved some satisfying results, without
> any of the obvious artifacts you saw. But mine is a
> 6" f/5 model and, as I said, of the older generation,
> which is not the same mechanically and could be a
> little different optically (e.g., size of secondary)
> compared to the LXD55 equivalent Meade sells now.
>
> I have previously posted URLs to a couple of shots
> with the scope (repeated below), including one of the
> California nebula that has a fairly bright star in
> the field. I have also taken a Horsehead shot, not
> quite 30 minutes on Elite Chrome 200. It is
> predictably somewhat less "dense" than your 30 minute
> shot at f/4, but there are no Schmidt ghosts, far
> less obvious refraction spike effects, much less
> vignetting and less star distortion due to field
> curvature and residual coma. I haven't image-processed
> it yet (I'm still on the learning curve for that!)
> but when I do I'll post the image here for comment.
>
> http://www3.sympatico.ca/the.koebels2/Astronomy/m13.htm
> http://www3.sympatico.ca/the.koebels2/Astronomy/california.htm
>
> Although the LXD55 6" f/5 model is not identical to
> my OTA, I suspect it would be a better choice than
> the 8" f/4 for A-P. It has about the same focal
> length,
> yet it is significantly lighter at 13 lbs. (a GP-DX on
> a good wooden tripod should handle it well). At f/5
> there is less residual coma, less field curvature,
> and potentially less vignetting (provided the
> secondary on this model is not overly small - it
> may well be too small on the 8" f/4). Exposures are
> a bit longer, but f/5 is nothing to complain about
> IMO.
>
> I intend to pick up a used 6" f/5 OTA and try it on a
> GP-DX mounted on a surveyor's tripod (much better than
> the stock tripod) to see if it performs better or
> worse than what I'm using now (which is not detachable
> from its crude fork mount). Given the low prices of
> these OTAs on the used market, it's worth the
> experiment for me. But only with the 6" - I did not
> even consider getting the 8" model.
>
> BTW, the diffraction effects you see may be caused by
> the mirror clips. Paul LeFevre, who uses an 8" f/4 for
> CCD (HX916) and previously used a 6", has posted some
> modifications to the LXD55 S-N mirror cell:
>
> http://www.lefevre.darkhorizons.org/lxd55/clipectomy.htm
>
> The focuser drawtube may also contribute. Apprarently,
> it protrudes into the light path when racked in enough
> for a camera. The same thing was true on my older OTA
> - I had to have the drawtube cut down on a lathe.
>
> -----Original Message-----
> From: Walter Koprolin
> [mailto:a9125657@unet.univie.ac.at]
> Sent: Friday, May 23, 2003 8:16 AM
> To: astro-photo@seds.org
> Subject: [APML] Photographic test of Meade 8" f/4
> Schmidt-Newton
>
> Hi everybody!
>
> This might be of interest to the group:
>
> I recently had the opportunity to shoot two 35mm
> astrophotos with a Meade 8" f/4 Schmidt-Newton. I
> have written a short test report about it:
>
> http://www.astro.univie.ac.at/~koprolin/Art/SN8/
>
> As you can see, the telescope is problematic in
> astrophotographic use. The images suffer from
> vignetting and distorted stars near the edges. If
> bright stars are present near or within the
> photographed field, multiple reflections and Schmidt
> ghosts also become a major hassle.
>
> All these artifacts except the radial distortion of
> stars can be eliminated or nearly eliminated during
> image processing, but some of them only with
> considerable effort.
>
>
>
>
>
>
>
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