What Length of Cheshire Collimator Do I Need?

[Pugnut]

I'm considering these 2 cheshire collimators from skies the limit on ebay.

Cheshire Collimator for Newtonian Telescopes - long | eBay

Cheshire Collimator for Newtonian Telescopes - short | eBay

I have read somewhere that you need to choose the length of your cheshire according to the focal length of your telescope, something to do with using its sight tube facility.

My scope is a skywatcher explorer 150p F5.

Can anyone help?

[Probablynot]

And is any old collimator OK?

The ones from FLO and other reputable suppliers cost a bit more than the ebay ones. Does that mean they're better? More reliable? More accurate?

I'm tempted by the ebay ones, I have to admit. But if they can't be trusted to be accurate, is it false economy to buy one?

(cor, that sounds as though I'm in the pocket of FLO and/or the other top suppliers! I'm not, I promise! I'm after the best option like everyone else!)

[johninderby]

Alan at Skies The Limit is well known to most SGL members and is known as one of the good guys and very highly regarded. Great to deal with.

It's a small one man operation and hence doesn't have to be VAT registered which does help a lot with his prices.

John

[Probablynot]

Thanks, John, but sorry - I didn't want to divert attention away from Pugnut's enquiry re short/long collimators...

[Probablynot]

Actually, from what I can see, the short collimating ep is only important if you have a fragile focuser that can't stand the weight of a long one.

But that only comes from info available on ebay. I'm still hoping someone with better knowledge than me will come up with a reply.

[Pugnut]

Can anyone help?

[pajr777]

There is an old post here which discusses this issue: http://stargazerslounge.com/beginners-help-advice/89430-cheshire-eyepiece-colli-cap.html#post1305896

It suggests that a short cheshire would suit you as you have an f5 scope - with a longer one you might struggle to see the very edge and clips of the primary mirror, which is the easiest way to judge if the secondary mirror is correctly aligned.