[ATM] Collimating, the eccentric way

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Mike Byorick wrote to Dave Harbour:
>I revisited the basics of collimation, and noted that the fundamental
>issue is one of ensuring that the focuser is aligned relative to the
>OPTICAL PATH, of course, and not necessarily relative to the tube/truss,
>since, in many cases, the tube/truss itself cannot be used as a reliable
>frame of reference (except in a case like your yoke-mounted scope, where
>the truss is very carefully constructed!).  So the problem definition
>now becomes something like 'given the <optical axis> of the primary
>mirror, how does one then align the other components in the optical path
>with respect thereto?'.

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?' Might
look much the same, but there is a world of difference in practice. For one
thing, the focuser is usually not adjustable (if it is, so much the
better) - for another, the mirrors and the (future) optical axis (and
reflections) are readily visible through the focuser.

But first a word of tolerances. To get good performance out of a Newtonian,
you should know for sure that your method will make your collimation meet
the tolerances required - no guesswork allowed. And just relying on
everything looking concentric won't do the job - not if you have a
reasonably fast and large telescope. No way - you really need some tool for
the job. A simple, home-made one from a piece of plastic drain tubing, and a
piece of cardboard is great, but you could buy one for less than a simple
eyepiece - and it will give you orders of magnitude more precise results
than trying eye-balling to center things inside the fanciest of optical
tubes. But if you like a laser, by all means get one.

So how do you do it? You don't need three instalments, and you don't need a
strait-jacket before (or after) you're through - and the tube can be as
crooked as you please as long as it does its job of holding things in place.

Just put your sight-tube combination tool in the focuser, and make sure the
secondary appears in line with the sight-tube (it will appear as a circle or
as near as makes no difference - and you get the offset right, too, with no
extra effort).

Next, adjust the tilt of the secondary to make the primary appear centered
in the sight-tube (its periphery inside the opening, or its center spot
behind the crosshairs). This done, adjust the primary's tilt to make its
center spot appear centered in the bright circle of the combination tool -
and you're ready. And what's more - you have full control of tolerances at
each stage - you can see how far off the secondary is from the line of
sight, you can see how far the center of the primary is, and you can see how
far the center spot appears displaced from the center of the collimation
tool (typically, this should be less than 2 mm for a fast Newt - easily
checked!)

As simple as that. Forget about the tube/truss -  as long as it doesn't
intrude into the optical path (but if it does, you will notice) and supports
adequately what it should support. And don't worry if the optical axis is
reflected *exactly* 90 deg or a bit more or less - the secondary is flat and
this won't affect the image the slightest. But unless you have some kind of
suitable tool, you'll never know when you're close enough.

Nils Olof

for some helpful details, you can try
http://skyandtelescope.com/howto/scopes/article_787_1.asp
and
http://w1.411.telia.com/~u41105032/kolli/kolli.html





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