Focus issue.

[Jm1973]

Hi. I've been imaging for about 5 months or so now, and getting along ok unguided so far. I can get 3 minutes subs without too much hassle.

However, quite often I have a lot of what seems to be 'walking' noise. I have bought a GPCAM2 and adaptor and had planned to use it with my Skywatcher 9x50 finderscope to autoguide, partly so I can dither and partly to have the chance of longer exposures.

However, I have been unable to get it to focus. I can get a bright star in view, but it is very large and diffuse. I can bring it towards focus to a point by screwing the two front parts of the finderscope clockwise, but when they are tightened as far as they can go inwards, the star is still very much out of focus - by what seems to be quite a long way. 

I am still a bit clueless about focusing. Does this mean I need more backwards travel, ie. an extender or spacer of some kind, or do I need to look into cutting down the finderscope tube? Or something else?

If anyone has come across this before and has any helpful suggestions, I would be very grateful.

Edited February 27, 2021 by Jm1973
typos

[Jm1973]

Thinking about it, if I'm screwing in the front part of the finderscope then that should mean I am shortening the focal length. So I think that means I need more inward travel. Does this mean I'm looking at cutting down the finderscope tube?

 

 

[Budgie1]

I used a Celestron 9x50 finder for guiding and with that I had to remove the locking ring from the finder scope tube to be able to get enough travel on the front lens to focus. Just unscrew the front lens from the tube, then the locking ring and screw the lens back on.

I was using the Astro Essentials Sky-Watcher 9x50 Finder to C Adapter with ZWO ASI120MM Mini.

It means you can't lock the focus but I found it doesn't really move due to the thick grease on the threads.

[Jm1973]

48 minutes ago, Budgie1 said:

I used a Celestron 9x50 finder for guiding and with that I had to remove the locking ring from the finder scope tube to be able to get enough travel on the front lens to focus. Just unscrew the front lens from the tube, then the locking ring and screw the lens back on.

I was using the Astro Essentials Sky-Watcher 9x50 Finder to C Adapter with ZWO ASI120MM Mini.

It means you can't lock the focus but I found it doesn't really move due to the thick grease on the threads.

Hi Budgie, thanks for the detailed reply. That sounds like it could do the trick. I will try it tomorrow assuming the clear weather persists.

Many thanks.