collamation question

[manny]

Hi all
How do you check for offset in a12"L'bridge? the manual says that it is set in the factory but if you move the secondary while collimating or jarred while being shipped would'nt that move the offset?and if so how do you get it back to specifications? Iam still learning the science of collamation.
Thanks
Manny

[Blue Skies (Jacquie)]

If you use a sight tube/cheshire for collimation you will get the down offset automatically as part of the process.

I think I'd better try to dig out the formula for calculating it again so you can work it out for your scope, to be sure.

[Blue Skies (Jacquie)]

Try this article: http://www.fpi-protostar.com/ftp/techp2.pdf

[manny]

Thanks Blue Skies
You are correct.If using a sight tube the offset should automatically be set.
Found an article by Nils Olof Carlin "Ofsetting Your Secondary Mirror) in a Sky & Telescope article at this site.
www.skyandtelescope.com/howto/diy/3306996.html?showAll=y&c=y
This Nils Olof must be the High Priest of collamation he explains things
in away a newbie like myself can understand.
Thanks Manny

[Blue Skies (Jacquie)]

Yeah, Nils is a bit of an international legend. He has written quite a few web pages over the years to help beginners out. I ran across his stuff years ago and it helped me out when I was starting too, and he's on the ATM list, where I'm a lurker.

[circumpolar (Matt)]

I have been using 'Classical Offset' as opposed to 'Partial Offset' on the 12" LB. Well I think that's what it's called.

2.90"(2.91") / f5 = 0.145"
=74mm / f5 = 3.68mm Offset (vertical) away from the focuser.

The 74mm is the minor axis of the secondary.

This seems to be working very well for me as I know my focuser axis is perpendicular to the tube axis, after many hours of tweeking.

[Geoff45 (Geoff)]

The point of the offset is to ensure even illumination of the field of view in the eyepiece. This is necessary when you are trying to minimise the size of the diagonal. However, with the GSO dobs, the secondary is usually over generous in size, so that anything beyond the "partial offset" which happens automatically with a collimating eyepiece is probably unnecessary. Note that the offset doesn't do anything for or against the quality of the image in the centre of the field.
Geoff