Re: ATM forwarded msg on collimation

[Bruce Swayze <swayze@europa.com>]

At 04:15 PM 7/17/97 -0700, you wrote:
>Art Russell wrote:
>> 
>> In thinking about confirming the orientation of my focuser, a 2 inch
>> Tectron low profile focuser, would it be appropriate to consider center
>> spotting the diagonal? Is there a loss of transmission as a result? Somehow
>> the idea of spotting one of the holy of holies (although I did centerspot
>> the primary) doesn't click for me. Any problems with this approach?
>> 
>A small black spot at either the geometric center (if you don't
>implement the offset) or the optical center (if you do implement the
>offset) is quite useful for seeing how your secondary is being held by
>the spider, especially if you use a laser for collimating. And the small
>black spot won't hurt anything - some people have even stated that the
>center of the secondary is not used, just like (and because of) the
>center of the primary is not used. Sounds logical, but I would have to
>think about that some more before I agree. Anyway, just think of it as
>just one more speck of dust.
>
>Steve Bell

I tried a different approach over the weekend with my 15" truss-tube dob,
and it works so well that I'm recommending others to try it.  I took the
diagonal mirror and carefully laid it face down on a clean sheet of paper.
Draw around the perimeter with a sharp pencil and cut out this elliptical
piece of paper.  Fold it in half both lengthwise and across it's minor
axis, thus locating the exact center.  Cut a very small hole in the center
and use this piece of paper as a temporary "mask" during collimation.  I
taped mine onto the diagonal holder with 2 small pieces of tape, and it was
easily removed after collimating.  Keep it in your eyepiece box for later use!

Now you can use your laser in the focuser to position your secondary mirror
directly under the focuser (assuming your focuser is collimated squarely to
the tube).  Not only that, but the reflected beam will bounce off the
primary and the return beam can be used to collimate the primary mirror.
It's very easy to see the return beam since it strikes white paper.  

Bruce Swayze