From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 



>From: "Jack Schmidling" 
>
>
>From: "Ross Sackett" 
>
>> The focuser need not be square to anything.  The
>> important thing is that it points directly at the
>> optical center of the secondary (that is, where the
>> optical axis hits the secondary, not its physical
>> center).  You can check this using the sight tube you
>> use to collimate your scope.  Some focusers have four
>> screws on the base for making this adjustment,
>> otherwise just shim the attachment bolts with bits of
>> sheet metal or scrap plastic.....
>
>Looking through the sight tube, how do I know where the optical center of
>the secondary is?

 This is one of the tricky parts. In order to be at the right
part of the light cone, the peep hole needs to be at the location that the
eyepieces focus at. This means that you need to be able to see the edges of
the secondary from the peep hole without moving the focuser. Most
commercial sight tubes are usually too long and you have to rack the
focuser out more to see the edges. When you do this, you end up using too
little offset towards the primary. If you have a site tube with cross
hairs, you can make a target that can be mounted over the secondary to give
a center location. This isn't an enormously critical adjustment. If done
slightly off and you do the rest of the adjustments correctly, it will
cause the 100% illuminated spot to not be centered in the field of view.
 I prefer to modify the tool to do what I need. I cut the sight
tube short enough that I could put a brass ring baffle into it that just
framed the secondary when the focuser was at the normal position.
 This effect of changing the focuser position is not as critical
for things like F/8's but becomes more of an issue for faster telescopes,
like your f/4.5.

>
>Wouldn't moving the secondary have the same effect as shimming the focuser
>for this purpose?

 Yes, but as I mentioned before, if the secondary is too far from
the mechanical center, you may experience some vignetting towards the edge
of the field of view, caused by the telescope's tube. It takes quite a bit
to be comparable to typical secondary sizes, though. Dwight

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>js
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