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

[Herm <hermperez@worldnet.att.net>]

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.

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.

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.

I use a modified laser to collimate my newt.. I made a pinhole aperture to
reduce the diameter of the laser beam down to 0.5mm (makes it difficult to
see in the daytime).. previously the laser spot was about 3mm wide and I
could not get consistent results.

Obviously the center spot on the primary mirror must be marked to a high
level of accuracy.

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.

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.

The use of a Vixen R200SS focuser (or the several clones made in china)
would allow the use the Vixen 60mm direct-photo adapter (and camera angle
adjuster) and most likely solve the vignetting issue.. flares would have to
solved by using flat black paint in the appropriate spots.

My R200SS came from the factory with flocking paper lining the focuser, but
the ota still benefited from an extended dew shield.


Walter Koprolin <a9125657@unet.univie.ac.at>  wrote:

>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.

Herm
Astropics http://home.att.net/~hermperez

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