Re: ATM collimation?

["James Lerch" <jlerch1@tampabay.rr.com>]

----- Original Message ----- 
From: "hermit" <hermit@outofoptions.com>

> I was looking at Mars tonight.  Looked good.  Then I noticed when I racked my
> focuser in, every thing looked good and centered.  When I racked it out, I
> noticed that the secondary shawdow was off.  I don't think I ever noticed that
> before.  Off center in ONE direction only.  How can that be?

Hi Hermit,

I've seen something similar before, but in my case as I racked the focuser
in/out I could watch a Star move U/D on my CCD display.

The cause is pretty simple, the optical axis doesn't travel parallel with the
focuser travel.

In My case, the secondary was a little over-sized, and hung down a little too
far from the spider assembly.  Since it was a little over-sized, I didn't notice
that it wasn't centered in the sight tube since I couldn't see the edge's of the
secondary.

With the secondary hanging low, I had to tip the secondary to get the view of
the primary centered.  Consequently the secondary didn't bend the optical axis
at 90 degree's and out the side of the scope, but at some other angle slightly
off from 90 degrees.

In your case, the problem most likely has something to do with the focuser /
secondary relationship.  Either the secondary is askew in one, or more, of its
six degree's of freedom, or the focuser isn't squared up.  A long peep-hole
sight tube might help you find the problem, or a laser collimator with one of
the "grid" type displays may help also.

Best of luck!


Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)

"Anything that can happen, will happen" -Stephen Pollock from:
"Particle Physics for Non-Physicists: A Tour of the Microcosmos"