Crayford focusors

[Craig.a.c]

When looking through the specs of some telescopes I allways come across the Crayford focusor. Are they designed to make it easier (smoother) to focus your telescope without getting heaps of wobble?

Sorry for all of my questions lately, I have been interested in astronomy for many years and have only just gotten in to it.

[tbentley]

A "Crayford" or "Crayford style" (I presume there must be some form of trademark on the name) focuser uses friction to raise or lower the focus tube(?). I understand that this allows finer control of the focus and less backlash also. The only other type that I know of is rack and pinion (teeth and gears) and is not very precise. I've just changed the one on my dob to a crayford.
Hope I've got this right and have helped some.
Travis

[Karlsson]

'Crayford' is not a registered trademark, nor was the design ever patented - scroll down to the bottom of this page: http://www.observatory.org/featurmr.htm

Yes, they operate smoother than bog-standard R&P's, but R&P can be manufactured to be smooth and almost backlash-free too... but I suspect that would be more expensive now than mass-producing serviceable crayfords. Some Japanese and German high-end scopes of a few years ago came with good R&P focusers.

There is a third type, which is less common on telescopes: the helical focuser, like the focus ring on a camera lens. I have seen them recently on some ultra-light designs, but they have always had their fans...

[GrahamL]

Don't forget the helical crayford I like mine a lot :)
http://www.kineoptics.com/HC-2.html

A little more on its history
http://en.wikipedia.org/wiki/Crayford_focuser
pretty cool that he decided not to patent the idea

[OneOfOne]

If you mainly want to get rid of the "wobbles" when focusing, I would suggest getting an electric focus motor and attach it to the scope. You will still get wobbles when focusing with a crayford as your hand will still bump the scope around. I have the Orion one, but there are several alternatives. They cost from around $70 and up. Trust me....you won't regret it.

[erick]

Ditto here. Early on I managed to get a second hand Orion Accufocus. It was designed for an R&P, but I found a way to fit it to my GSO Crayford. I now have a GSO 10:1 Crayford on my scope, but have still fitted the Accufocus. Had to make the connection more rigid, since the torque required to turn the main focus axis of a 10:1 is more than a regular GSO Crayford.

Though the 10:1 is sufficiently fine focus control, my clumsy hands always wobble the scope to much - so I prefer no-touch electric focussing.

Here is the relevant thread:-

http://www.iceinspace.com.au/forum/showthread.php?t=10782