RE: [ATM] Collimating, the eccentric way

["mbyorick" <mbyorick@starband.net>]

Since my eccentric musing (concerning collimating a scope with a
'crunch-oidal' tube) has elicited commentary from some heavy-duty ATMs,
I thought I should try to explain my pretzel logic in ONE installment...

Nils Olof Carlin noted:
>You could look at the other side of the coin - and rephrase the problem
as:
>'given the focuser, how do you align the secondary and primary to it?'

I have typically employed the 'focuser-centric' technique you described
in your previous post (and on your web page), using a homemade sight
tube with double-crosshairs.  However, my focuser is easily adjusted,
and my mirror is not yet mounted in its new mirror cell, so, once I
determined that my scope's tube was irregularly-shaped, I started
thinking about initial alignment/collimation from the mirror cell
forward.  

First, run a string from the center of the mirror cell to the opposite
end of the tube, 'quasi-parallel' to the edge of the tube on which the
focuser is mounted, i.e., so that the distance between the string and
tube I.D. at the focuser is equal to the distance between the tube I.D.
and the center of the mirror cell.  

The string should pass directly under the center of the focuser (as
determined with a sight tube) through the bolt hole in the center of the
spider (of course, the spider will probably have to be re-positioned),
thus simulating the mirror's optical axis.  

At this point, you have a properly positioned spider (relative to the
mirror's optical axis and to the focuser), and a focuser that is
'automatically' aligned in one plane relative to the optical axis, since
the string/line has established the 'left/right' focuser alignment by
default, leaving only the 'up/down' alignment (with respect to the
longitudinal axis of the OTA).  The 'up/down' focuser alignment can be
carried out simply by putting a square to the string and aligning the
focuser accordingly. You are now 'in the ballpark', as David Harbour
would say.  

Put the diagonal back in the spider, and -NOW- align the diagonal in
accordance with your straightforward method.

I think the above 'mirror-centric' technique may avoid a result where
you have a perfectly collimated system, but where the 'focuser-centric'
method has allowed the focuser to remain 'tilted' relative to the
tube/truss, and subsequent collimation relative to the focuser has
caused the mirror's optical axis to be skewed relative to the
tube/truss, such that vignetting (etc.?) may occur.  While this 'method'
may be neither particularly beneficial nor necessary for most scopes, it
at least made me realize that there is more than one valid initial frame
of reference for scope collimation.

My thanks to Nils Olof Carlin and David Harbour for indulging me in
these 'eccentric' collimation discussions!

Mike Byorick
SW New Mexico

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