Re: [ATM] strange (collimation?) problem

["David Harbour" <scarab2@cox.net>]

Gentlemen:

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.

Dave

----- Original Message ----- 
From: "Koehler, Steve" <Steve_Koehler@securecomputing.com>
To: "ATM List" <atm@atmlist.net>
Sent: Monday, June 20, 2005 9:39 AM
Subject: RE: [ATM] strange (collimation?) problem


Jan,

> BIG DIFFERENCE as compared to inside focus: secondary shadow not centered
> but very clearly offset to one side. When I centered the secondary shadow
> during the startest in the outside focus pattern, it became decentered
> inside focus and was then also clearly off collimation as seen in the
> cheshire.

I think this simply means that your optical axis is not parallel to the axis
of
your OTA.  However, I would expect the secondary shadow to be off center on
each
side of focus, but in opposite directions.

I have been puzzled by the instructions to do star collimation by centering
the
secondary shadow.  It seems to me that this will not work very well with an
offset secondary, for example.  I have been meaning to run some simulations
to
play around with this, but haven't gotten around to it, yet.

-- Steve Koehler
   steve_koehler@securecomputing.com

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