Short, long or doesn't matter? Collimating eyepiece

[hms_furious]

I have a 150i so only need to check the secondary, does it matter whether you have a short one or a long one? I have trawled this forum and there seems nothing conclusive at all!

Thanks again

[andrew s]

You want the size of the end of the sight tube to match the diameter of the secondary as closely as possible when viewed via the peep hole. This would lead me to conclude longer is better.

The eye is not good at matching concentric circles so you want them as close as possible.

Regards Andrew

Edited March 12, 2021 by andrew s

[AlexK]

The long answer: http://web.telia.com/~u41105032/kolli/kolli.html

[Captain Scarlet]

Ideally you need a tube that 1. allows the peephole/your eye to be placed at the primary mirror's focal point, and from that viewing position, 2. the open end as viewed from the peephole needs to appear close enough to the edge of the secondary so you can accurately judge its "concentric-ness". That would suggest "long-ish" to me.

(It's only from the focal point that a properly-set secondary appears perfectly circular.)

But also it actually doesn't matter too much if the secondary is not in the perfect position, which is what the circularity-test determines. What's much more important is that the central axes of the eyepiece/focuser and primary mirror coincide: i.e. the tilt of the secondary rather than its position in space. IE that which you might use a Cheshire to do.

 

Cheers, Magnus

Edited March 12, 2021 by Captain Magenta

[Pixies]

Aside from the above reasons, I find that I can't use a short Cheshire as the cross hairs are so close to my eyes that I can't focus on them and the  doughnut. If you wear specs, I think that's a consideration.

[ScouseSpaceCadet]

I used a short cheshire in both a 130p and 150p just fine... and a collimation cap worked too.

[Spile]

A simple cap with washer works for me.

[MrFreeze]

I can't focus on the cross hairs of a short cheshire at all, with or without spectacles. Annoying as I overpaid for one from a UK company (not FLO) who advertised it as 'with instructions', and then provided it without instructions. When I queried this I was told this was the new 'improved' version. They did then go on to provide some pdf instructions in an email. Replaced it with a cheapo chinese long cheshire, and had no problems at all with that.

David