Focussers

[Andrew*]

A scout on eBay revealed this deal:

http://tinyurl.com/y8fodj

First, I don't really understand the significance of a focusser. Why are some much better than others? If you buy a Crayford, is it going to be good no matter what model?

And is this one suitable for a 4.5" reflector, or do you need a very specific model?

:?

Andrew

[daz]

It's all down to engineering AP. Crayfords employ a roller (or rollers) against the flat of the draw tube. A normal rack and pinion has a gear on a dowel, running against a toothed strip. Consequently, it can suffer from backlash (see another thread on this).

The crayford gives a smoother movement and therefore finer adjustment on the focus. Crayford usually have a focus lock as well as a tension adjustment.

There are different fittings for refractors and reflectors, and you need to ensure that it comes with, or you can get, the base plate to fit your OTA.

HTH

[iancandler]

I'd also check that the one you pick will match up to the curve of your tube.. most seem to be optomised for 6" and above scopes now.. but trust me the change is worth it.. with a crayford objects just seem to almost "snap" into focus.. I wouldnt go back now..so use them on both the dob and my little frac'