Re: [ATM] Secondary Offset

["Mitchell R" <funnybone101@embarqmail.com>]

-----Original Message-----
From: Stephen Koehler [mailto:s.c.koehler@gmail.com] 
Sent: Thursday, January 24, 2008 1:14 PM
To: Mitchell R
Cc: atm@atmlist.net
Subject: Re: [ATM] Secondary Offset

Mitchell,

>      Hm Okay. Normally I sort of just eye ball it, there is no way for me
to
> truly center it under the focuser so I just adjust it till it looks
centered
> under the focuser, hopefully that is offsetting it enough.

A typical site tube consists of a 4 to 5 inch tube inserted in the
focuser with a cap containing a centered peep hole of about 1/8 inch
diameter.  The peep hole keeps your eye centered in the focuser when
you look through it.  You compare the inside edge of the far end of
the site tube with the circular outline of the secondary.  Once they
are concentric, the secondary is centered.  This is "eye balling" the
centering of the secondary, but it's plenty accurate.  It's much
harder to judge the placement of the secondary without a site tube to
keep your eye on the center line.

As far as offsetting goes, if you have centered the secondary as
described, you have also offsetted it toward the primary.  If,
instead, you were to perfectly align the center of the secondary with
the optical axis of the focuser, the secondary would not look centered
in the site tube.

I apologize if I'm repeating things you already know.  It sounded like
you had not used a site tube.

-- 
Steve Koehler


Thanks, I just don't have a site tube. I used my collimation cap, which only
centers your eye. Then by racking the focuser all the way out you can get an
idea of how centered you are. A site tube is probably better, but it's what
I got. The laser collimator seems to do a good job, I get crystal clear
images from my 6", though they were pretty darn good to begin with just by
eyeballing it (relatively slow, F/6.5).  

I was wondering why when I used my new laser collimator, it didn't look like
it was hitting the center of the secondary when looking through the side,
but through the focuser I'm sure it would look as though hitting dead
center.

Thanks again!

Mitchell

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