Re: [ATM] Ghosts in the Coulter

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Stephen Koehler wrote:
> I made a quick simulation of planetary ghost images, then reverse
> engineered it to get a mirror surface map that would cause that
> distortion.  Here's the result: 
> http://www.atmlist.net/contrib/s-dot-c-dot-koehler-at-gmail-dot-com/misc/davis.png
>  ......... Assuming this is all valid, I conclude that primary
> mirror distortion is highly unlikely to cause the ghost images.
> This argument does not say anything about what the secondary mirror
> could do, since that case is a bit different.

Now you've got me thinking.

Having seen (rapidly moving) ghost images due to a (very) warm mirror, 
that must have been caused by "wedges" of air of different temperature
that simply refracted some of the light from the primary off to the
side.  The object being viewed at the time was a moderately-sized disk
of Mars (not nearly as large in angular size as Venus is now).

Based on David's sketch of Venus, assuming it is fairly accurate, it 
seems that distortion caused by a secondary would have to be quite 
severe to cause the image the shift nearly the diameter of Venus, and 
I would be very surprised if the secondary was to blame, unless the 
coating has been damaged in some weird way.

So, assuming the ghosts happen with a variety of eyepieces, it is my
suspicion that these ghosts are caused by something farther away from 
the eyepiece than the secondary, like an issue with the primary or its 
mounting or some really horrible, stable tube currents.  (The latter 
may have been ruled out already if the scope was cooled before 
viewing, but it is possible given the fact that Venus is not at the 
zenith where tube currents would change in form.  Forgive me for not 
recalling all the details of the thread.)

	Mike Lockwood

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