Crayford focusors

[Craig.a.c (Craig)]

Could some one tell me why the crayford focusors have 2 screws under them? If I have the one closest to the OTA loose then the focusor will move in and out, if I have both of them loose then the focusor slips back down and if the one further away from the OTA is loose then the focusor dose not move at all? I don't understand it.


Cheers - Craig

[avandonk]

Here is a good how to

http://www.backyardvoyager.com/gsofocuser1.html

Bert

[Liz]

Yes Craig, I had lots trouble with mine too, not sure which one I was supposed to be turning to focus, then the whole thing goes loose, and wont do a thing. Finally have it under control and is great. Hang in there

[Omaroo (Chris Malikoff)]

Craig - the one closest to you adjusts the draw tension - i.e. the clamping force between the actual tube and the focus rollers. Adjust this one such that a large load (like your camera or a heavy eyepiece) doesn't slip when the unit is vertical, but the turning force you need to apply to the wheels isn't either excessive or sloppy. The focuser shoud be able to be moved to any point along its travel and when let go, the whole lot should stay put where you leave it. If the focuser slips then tighten it slowly until it doesn't.

The one furthest away from you disengages the tube from the rollers altogether - allowing to to "lock" the focus at the point you have it. Tighten this one and the wheels spin freely without moving the focuser tube.

[pjphilli (Peter)]

Hi Craig
My Meade 80mm refractor Crayford focuser works the way Chris has described. I was in the habit of locking the focuser when I had achieved exact focus with my DSI II camera. Unfortunately, I found that the lock had a nasty habit of moving the focus just slightly but enough to spoil focus when the lock was clamped down. On close examination I found that the locking action moved the focuser barrel slightly in or out in a random fashion. I now only set the draw tension screw enough to stop the weight of the camera moving the barrel. Does anyone know how I can cure the
locking problem? Cheers Peter

[Omaroo (Chris Malikoff)]

Probably the first thing to do, Peter, is pull it apart and check that the roller tracks on the flat of the focuser tube are free from any built-up dirt or grease. They should be dry and scrupulously (well, that may be going too far ) clean. The roller bearing' outer surfaces themselves need a good clean as well. I find that the rollers can "rise" onto a dirty bump and affect the focus lock right at that point - by moving when you lock.

When you move the focuser along entire path under nice even tension, is it liquid-mooth or a little jittery?

[Craig.a.c (Craig)]

Thanks heaps guys. I had no idea what the go was.

Cheers - Craig.

[pjphilli (Peter)]

Thanks Chris - It looks like "out with the spanners and into it".
My Crayford otherwise moves in and out very smoothly but I will check for
problems as you suggest. Cheers Peter

[leinad (Dan)]

On the topic of Crayford focusers.. the skywatcher dob focuser has two threaded holes, but only a screw in one of them closer to the eyepiece.
Anyone know why this is so, and is the second threaded hole of any use?

[Omaroo (Chris Malikoff)]

Originally Posted by leinad

On the topic of Crayford focusers.. the skywatcher dob focuser has two threaded holes, but only a screw in one of them closer to the eyepiece.
Anyone know why this is so, and is the second threaded hole of any use?

I'd say the knurled screw has probably fallen out..... I'd have thought that it should have two.... but the photo here has only one. They say "tension control knob plus focus lock". Where the heck is the lock?

[TrevorW]

Hi Craig

Is the focuser a single or dual speed focuser as the method for adjusting and tensioning does differ

[AlexN]

Daniel, the one that is fitted is the focus tension control, the empty one is for focus locking, I would have thought that there would be a screw in there standard, however ther wasn't... I've since added one.

[leinad (Dan)]

Interesting thanks.