Guidescope Focus

[groberts]

I am about to embark on the dark art of guiding.  I have a William Optics 50 mm guidescope and a ZWO ASI 120MC camera and last night tried the two together in order to assess the focus of the two together.  

The WO only has a few mm travel for focusing using a helical screw and, needless to say, I could not get focus without manually pulling the camera back beyond this limit.  What would be the best way of establishing the camera with the guidescope in order to fix the focus?  I am presuming some sort of extension tube or spacers might do the trick - can somebody please point me towards something suitable.

Thanks.  

[Tinker1947]

I used extension tubes they make for a very stable camera.....the silver bit is the end of the focuser the then 3 tubes plus the adapter to clamp the CCD....

[Photosbykev]

It's worth holding the camera away from the scope but in line with the optical axis to see how much extension you actually need to position it within the focussing range of the adjustment. On my BST guidescope I had to remove a built-in 25mm extension piece on the scope because my ASI120MC was too far away from the correct focus.

Worth noting the farther away you move the camera the closer the scope will be focussing so if the camera is already focussing too close you need to shorten the distance between camera and scope.

[Geordie mc]

If you can get focus by pulling the camera back a bit in the focus tube then you can get rings which clamp in place on the nosepiece of the camera so that it inserts ti the right point each time. People often use them to make different eyepieces parfocal.

[groberts]

If you can get focus by pulling the camera back a bit in the focus tube then you can get rings which clamp in place on the nosepiece of the camera so that it inserts ti the right point each time. People often use them to make different eyepieces parfocal.

Thanks, sounds about right - where can I get them from / what are they called?  

Was also thinking of an extended nosepiece - again any suppliers?

[Tinker1947]

There are the c-mount adapter fits 1.25" CCD's

http://www.firstlightoptics.com/adapters/low-profile-125-nosepiece-c-threads.html

And T2 extention tubes

http://www.firstlightoptics.com/adapters/baader-t2-extension-tube.html

[ollypenrice]

You need to be reasonably in focus but not perfectly so. The point on which you guide is a calculated centroid and slightly soft focus helps that calculation - according to Craig Stark of PHD. All I can say is that he knows a sight more about this than I do...

Olly