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


>From: "John Sherman" 
---snip---
>
>I don't know why it appeared different inside of focus than it did outside
>of focus. But I would do a complete rundown of all the little things. Like,
>make sure the tube is all squared up. And the primary is centered in it.
>These things are not really essential (Hi Nils Olof!), however, they make it
>a lot easier to measure the offset of the secondary. Very important to make
>sure the focuser is properly aligned. I'd check the focuser when it is
>racked in and when it is racked out. Make sure the focuser holds the
>eyepiece square. Try different eyepieces, and/or rotate your eyepiece to be
>sure that is not the problem. And so forth.

Hi
 I thought I'd mention. Here is a simple way to check your secondary's
offset. Sometimes measuring things in a tube can be hard so first start out
with the secondary close to center. Now do an alignment as I described in
my previous post. If you did it right, the telescope will be properly
aligned optically but not mechanically because you haven't set the offset
correctly. While looking through a peep site, take a ruler and slide it in
to the aperture from the side of the focuser. When you can just see the
end, make a note of the distance. Now do the same sliding it across the
aperture from the other side from the focuser. Subtract the two and divide
the difference by 2. This is how much you need to fine tune the secondary(
you can ratio this by the ratio of the distance from the primary to
secondary over the primary to the aperture if you want to be really
accurate ).
 Before doing this, pay special attention to steps 1 & 2. Slight
miss aiming of the secondary has large effects on this. Things don't have
to be all that square for this method since these are all compensated for.
What you get at the end is the centered beam traveling right down the
center of the OTA. Dwight

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