Re: ATM trouble setting up focuser

["Bob May" <bobmay@nethere.com>]

You should be able to go through a shrinking of that image when you go from
one end of the focuser travel and the other and if it keeps going
smaller/larger when you hit the end of travel, you indeed have a bad
measurement for focal length.  The best way to correct this is to change the
position of the primary in the tube, hopefully towards the secondary rather
than away from it unless you have a lot of tube in back of the mirror.
If you are getting a point where the fuzzy is minimum and then it gets
bigger while you are focusing, you have found the correct focal length but
you have a bad optical system or that's just as good as it gets.  A good
optical system should have a very small distance in the focuser where the
image of a star just seems to "snap" into focus.
Don't feel to ashamed of the lack of focus on your first scope.  Most people
really don't measure the FL correctly in the tube and thus have to go and
redo something.  I tell people to build the top end first and make sure that
the tube is long at the bottom end and then move the mirror and support up
and down the tube until they have the correct position so that they can
focus about any EP without problems.  After you have found the correct
position, you can then attach the support to the tube and then cut the tube
to the best length and add on the trunnion bearings at the balance point -
don't forget to do the balance with your EP in place!
Bob May
http://nav.to/bobmay
bobmay@nethere.com
----- Original Message -----
From: Vincent, Mark <Psvincent@augustana.edu>
To: <atm@shore.net>
Sent: Tuesday, June 19, 2001 7:51 AM
Subject: ATM trouble setting up focuser


> I just read a post by jep about not being able to focus and I think I've
got
> the same problem.  Last night I finally finished my first scope, an 8" f6.
> I can't seem to focus on anything, so I assume that I've incorrectly
> measured distance from back of tube to focuser.  I did find Mars (at least
> I'm fairly certain that's what I found), which appeared as a large disk...
> but no detail, except for the crisp outline of my spider superimposed on
it.
> Otherwise, I can't see any stars (or anything else).  The sky was clear,
> lots visible to naked eye.  There was a fair amount of wind though.  The
> scope had been in my garage during construction, so I'm sure it was at
> ambient temperature.  Here's my setup:
>
> true focal length: 45.5"
> focuser height (made of PVC): 1 3/4"  (this is measured without eyepiece,
> right?)
> distance from back of tube to surface of mirror: 1 7/8"
>
> (I had incorrectly measured the focuser height to be 2"-- once installed
> through inside of tube, it was only 1 3/4".)  So I calculated:
>
> 45.5-2-5= 37.5"
>
> Tacking on the distance from mirror to back of tube, I arrived at 39 3/8"
> from back of tube to center of focuser hole.
>
> The focuser is one I saw described on a web page (don't have the URL
handy)
> made of PVC drain pipe and PVC electrical conduit... it only has roughly
> 3/4" of travel.
>
> So my questions are: 1) does my description above lead you to suspect that
> I've incorrectly placed the focuser on the tube?  and 2) if so, how can I
> correctly determine where to put it?  I must say that this thing *looks*
> impressive, just wish I could see something.
>
> Thanks!
>
> Mark Vincent
>