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Subject: Re: [M]: Halo fixing
From: Howard Pinsky
Reply To: mapug@shore.net
Date: Mon Dec 09 00:20:34 1996

At 01:38 AM 12/8/96 EST, you wrote:
>Seems to me that the corrector and/or the mirror need cleaning. I get the same
>halo problem when I get dewed up. I have heard of Meade sending out dirty
>scopes. Maybe the "film" is the result of some type of outgassing, or even oil
>from a pneumatic tool.
>

I think the halo problem comes from bad coatings on the corrector plate. As
light enters the scope some of it is being reflected by the inside coating
of the corrector. These rays then hit the outside surface of the corrector
and some are reflected back into the main mirror. The corrector plate has a
zonal aspherical correction on each surface so that at least some of the
"second surface" reflected light will come to a focus near the main focal
plane. Since this energy is slightly out of focus it appears as a halo or
ring of light - just like when you defocus a bright star by maybe .050 or
0.100". This halo is considerably dimmer since it is a second surface
reflection and has about 0.2% of the original energy.

"The game is [still] afoot" - Sherlock Holmes

By the way -- I think this is a wonderful way to learn about our scope and
solve problems at the same time. Of course anybody who has a serious halo
problem should contact Meade regardless of the outcome of this thread.

Regards,

Howard Pinsky

Electro-Optical Engineer and Amateur Astronomer