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

[Alen Koebel <alen_koebel@yahoo.ca>]

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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