Re: [APML]: Lumicon CEG

[Wil Milan <wmilan@airdigital.com>]

Jim Pennington wrote:

> Last night I was experimenting with my new CEG.  It was not a good night to
> shoot astrophotos but a good night for familiarization.  I made a 30 min.
> shot of B33 with unhypered SG800+ and got the film developed today.  I
> noticed a very pronounced two pointed "spike" coming off Zeta Orionis in
> addition to the halo normally seen with my scope on bright stars.  Upon
> closer examination of the reducer lens in the CEG, I noticed that it does
> not have the "color" of a coated lens and was wondering whether or not
> anyone knows for sure that it is in fact coated.  To my eyes, it looks
> absolutely clear.
>
> I have never seen this when using my old Meade OAG with Celestron f/6.3
> reducer.  Could I be getting some kind of internal reflection?

Either a reflection or a diffraction spike, I think. I've had exactly the same
experience with a Lumicon CEG. See

http://www.psn.net/~airdigital/Horsehead_with_diffraction_spike_Ultima11_Lumicon_EasyGuider.JPG

Note that in that image there are really *two* spikes, one reddish and one
bluish. My guess is that this is an artifact of light being refracted through
the prism or the edge of the prism, thereby causing the chromatic separation.

The spikes and vignetting were the reasons I got rid of the CEG and went to a
Giant EasyGuider for my 11" SCT. The GEG cured both problems. Unlike with the
CEG, the GEG's pick-off prism is mostly out of the light path, which I think is
the reason the diffraction spikes have largely disappeared.

Wil Milan
Astrophoto web site: http://www.psn.net/~airdigital/astrophoto.html
--
"When I consider your heavens, the work of your fingers, the moon and the stars
which You have set in place, what is Man that you are
mindful of him, or the son of Man that you care for him?"  -- Psalm 8