Can someone please explain how a Crayford focusser differs from a regular rack and pinion? I've searched the web and can find plenty of references to them, ie "this telescope comes with a nice shiny Crayford focusser!", but nowhere can I find any mechanical details!
A rack and pinion focuser typically has a plastic (sometimes Teflon) "bearing" which is just a flat strip against which the focuser tube slides. By turning the focus knob, you turn a small pinion gear which engages a long, straight "rack" gear, thus moving the focuser tube back and forth. Friction along the tube provides the "feel" to such focusers and is not usually very consistent -- particularly under heavier loads -- and the fine-touch to your focusing depends entirely on the way the gears are cut and how sensitive your fingertips are. In the wintertime, when you wear gloves or mittens, an R&P focuser can feel "rough" and make it hard to achieve critical focus.
A Crayford focuser usually has multiple (three or four) bearing surfaces, often provided by little rollers which "clamp" the focuser tube between them. This gives a more consistent "feel" to the focusing action and, in turn, allows you a finer control over critical focusing. Since there are multiple bearing surfaces, the Crayford design generally performs better in the cold.
Although looking at my "crayford style" focuser it seems to still have a rack....
I might have to look into this further....
Typically the focuser tube runs between four small bearings on one side, and the round focuser knob shaft against a flat machined surface on the other.
The advantages are no slop or backlash in the movement.
I got that picture from cloudynight forum. Sorry I dont have a link to the thread, but having been asked the same question the OP showed a picture of the focuser supporting the weight of a full wine bottle.
I have a DX2 crayford from JMI and it is great. If I extend the focuser out in the proper method and then try and push the drawtube back in it doesn't move unless I apply a reasonable amount of force. It will not move if you just bump it or touch it with your hand and it won't move in use if you bump your eye up against the eyepiece. I guess the friction force from the various rollers on the drawtube is enough to hold it in place.
I have found the focusing with this focuser much easier than with my other rack and pinion focusers.
If your focuser has gears (and I'm not an expert) then I would be questioning it being called a true crayford. If it is "crayford like" then it might use rollers at the top and still employ the r&p gears at the bottom. What do the focusers referred to as crayfords that come with the GS scope upgrades look like?
They are a true crayford style. As in starklers picture, but with dressing. Nice and smooth so far.
I think with rack and pinion you need a quality one.
Looks like a standard rack & pinion focuser typical of most synta manufactured newtonians. Generally ok for most low power observations but reveal a frustrating amount of shifting at real high powers even with the tension screw.
