Re: [ATM] mirror box baffle question

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

Dave,

There are a few key things about a baffle just above a primary 
mirror.  Keep the baffle centered with respect to the optical axis of the 
mirror.  If the baffle is NOT centered, it acts just like a de-centered 
primary mirror.  This is not good for collimation, especially if you 
collimated the telescope, then added a baffle that was not centered.

If you are adding such a baffle just above the primary, I would suggest you 
cut the ID smaller than the physical OD of your primary mirror.  Turned 
outer edge is a common characteristic on mirrors.  By sizing the baffle in 
this way you can eliminate the beveled edge and any turned edge.  Blocking 
the outer 1/8-1/16" of the clear aperture of a 16" diameter mirror is 
inconsequential to the viewing experience in terms of light loss.  However, 
it can dramatically increase the star's quality because there is much less 
scatter.

I think the farther away you are from the surface of M1 with this baffle, 
the harder it will be to get it "centered."  Mechanical axis and optical 
axis are rarely (exactly) coincident...  My suggestion would be to put a 
primary baffle just above the mirror's face.  This baffle should have an ID 
as I've outlined above.  Any other baffles should have an ID that allows 
for mechanical to optical axis errors and is chosen to fit your required 
field and focal ratio of your system, for any given location in the length 
of the telescope.

A primary baffle can be made so it is centered but is easily 
removable.  Don't let it touch the front surface of the primary mirror.  It 
doesn't take much force to locally disturb the figure of even a full 
thickness mirror.

Thank you.

Sincerely,
Shane Santi - President
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833


At 11:57 AM 11/18/2006, you wrote:
>Can anyone comment on the diameter of a baffle in the
>mirror box. Should it be the approximate diameter of
>the mirror itself? If so, are there issues with having
>such tight tolerances between the optical path and the
>baffle? I designed my baffle for my 16" mirror with a
>16" baffle hole. Now I'm having second thoughts.
>
>Kriege & Berry have a short description in "The
>Dobsonian Telescope", but I found it confusing and
>there are no images to go from. In Fig. 11.4 (later in
>the book) it would appear that the light baffle
>diameter is several inches larger than the mirror, but
>it could be an illusion.
>
>Recommendations? Suggestions?
>
>Dave
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