Re: ATM Another poor man collimating

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Bob,

>On the subject of collimation, i currently use a Cheshire tube (cheapy
>from Orion). After reassembling my 10-inch Starfinder the parts seem to
>end up more or less in the right place, so it doesnt take long to get it
>all basically lined up. However i have some questions re using the
>Cheshire
>
>1) I can easily get the two cross-hairs to more or less line up, but HOW
>CLOSE is reasonable? 

I'm not familiar with the Orion device. A Cheshire has a bright face usually at 45 deg to 
the tube, lit up through a hole in the side to make a background for the mirror center 
spot, during alignment of the primary mirror. Of course you could put crosshairs at the 
"inner" end, to help with aligning the secondary, is this it?


>Problem is that the tube itself moves slightly
>within the focusser, throwing the crosses out of alignment. Furthermore
>i cant focus on both crosses at the same time, so am never totally sure
>how close the alignment is. Is it a case of "if it looks close it IS
>close" ?

Do you mean you align the cross seen directly and its distant-looking reflection? There 
is no point in doing that unless you have first centered the cross on the mirror center 
spot by adjusting the secondary - and if you have, you should center the spot in the 
reflection of the bright face of the Cheshire instead. See next question.
>
>2) I assume the Cheshire test does not ensure that the main mirror is
>centred in the field of view. Should I mark the center with a dot, then
>line this up with the center of the Cheshire?

Yes, you should. If you want to align the secondary using crosshairs, or a laser 
collimator, you need something to aim at. To align the primary using the Cheshire, it is 
also needed.
>
>3) Does a laser collimator do a better job than the Cheshire, or is it
>just another way of blowing money on astronomy :=}

A laser is really great when you adjust the secondary. To adjust the primary, however, it 
can't do the job like a good Cheshire can. And this is, after all, the critical part. So, 
for the job a laser collimator *can* do, you need no fantastic precision.

Nils Olof