Re: ATM Star Test

[Bruce Swayze <swayze@europa.com>]

At 08:53 AM 7/7/97 -0400, you wrote:
>Hello,
>
>Cannot find a 'star' in my scope.  on terrestrial objects at 1000 feet
>or so, i find that i have to pull the eyepiece out of the focuser almost
>2" beyond it's farthest adjustment to focus an image.  But, when viewing
>a star, all i see is a huge white round image with what looks like a
>black hole in the middle (the secondary?).  It will not focus in.
>
>Any ideas?  the Foucault showed the ROC right at 120" so i used the
>formula in Berry's book and used 60" for the fl.
>
>Glen WB4ZNW

Glen,

You are indeed observing a star, but it's out of focus.  You are seeing the
shadow of the secondary mirror, as you suspect.  The first thing to do is to
observe the image as you rack the focuser in and out.  As you rack the
focuser all the way *in*, does the image get larger or smaller?  There are
two possible scenarios:

If the image gets larger as you go *in* all the way, then rack the focuser
out all the way until it gets as small as possible.  Then pull the eyepiece
out of the focuser carefully until it does focus to a sharp star, and you
will find out how much you must move the mirror BACK toward the rear of the
tube to get the focal plane to fall in the right place.  If the mirror is
already back as far as it will go, you have no choice but to move the
focuser farther toward the front of the tube.

If the image gets smaller as you rack all the way in, but you bottom out
before you get a sharp focus, you must move the mirror FORWARD toward the
front of the tube until the focal plane falls in the right place.

I don't know where you went wrong in following Richard Berry's formula, but
one of these scenarios will work for you.  

Bruce Swayze
swayze@europa.com
http://www.europa.com/~swayze    Swayze Optical home page
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