Re: [ATM] strange (collimation?) problem

[Andy Saulietis <iss@pvtnetworks.net>]

Dave writes,



>The practice of offsetting secondary holders (so that the secondary is
>offset to make the perimeter of the border of the field that is vignetted
>symmetrical) is of highly dubious (and "very slight") value: I have written
>about this extensively; you might look in the archives. Everyone experiences
>a certain degree of vignetting of field, with long focus eyepieces, or the
>film plane of a camera. Who cares whether it is symmetrical or not?
>
>No one I know.
>
>Don't offset your secondary holder. No advantage, only headaches in
>collimation. Just my STUPID 2 cents' worth. Do what the "Heavies" tell you
>what to do.

I'll bite..what's *wrong* with offsetting the sec mirror..don't see what 
the collimaion
headaches are..

Some pro's to offsetting IMHO:

-one can use the actual minimum possible size secondery if it's centered
in the imaging cone (which is not the center of the OTA)

-asymmetrical vignetting can be a problem with using CCD cameras for
differential photometry

-noticeable asymmetrical vignetting is in the eye of the beholder,
there is no 'acceptable' minimum amount

-personally, I prefer symmetry in the optical train vs in its
support structure.

-and it's not hard to do, just build the offset into the sec spider/mirror 
holder.
  Amount of offset will be dependent on size & fical length of the primary.

Re the 'collimation difficulties' caused by the asymmetrical location of the
secondary..easily fixed by adding a masking disk larger than the secondary,
centered on the OTA. This idea can be expanded to making the mask almost the
same size as the primary, so during daylight collimation one sees a thin 
annulus
of light vs most of the primary. If the eye is centered in the focuser via 
a small
aperture, any miscollimation of the primary shows up as asymmetry in the 
ring of
light. Basically does the same thing as a laser but can handle perforated 
primaries.




Andy Saulietis
ISS Enterprises
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Mayhill NM 88339


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