ST120 with dual speed focuser asi224

[rwc23q]

Good evening all,

I am in the process of getting my equipment together for some DSO imaging.

Currently have ST120 with Crayford style dual speed focuser, EAF, zwo 224 mc (i think, colour camera) asi120mm guide camera with 120mm zwo guide scope.  eq5 goto pro controlled by stellarium, sharpcap pro, eqmod and ascom. I am using a startech starlink from the mount to the computer.

I think I have everything covered to start imaging, apart from experience and knowledge (but trying).

I am struggling to find focus using the 224 as the main camera. Is there a calculation I can use, or does anyone have an idea as to the length I need the draw tube to be at to get focus on objects? Will I need extenders?

The reason I ask is that I had a little play while setting it all up (on one of the rare cloudless nights in my area), and the drawtube was at its maximum and still did not seem to be in focus (was aiming at the moon).

I hope these questions make sense, but happy to try and clarify if that would help.

Rob 

[Clarkey]

I'm not sure about the ST120 but I use an ST80 for guiding and that needs about an extra 10cm on top of the focuser being nearly fully extended. Try adding some extra extensions and check it during the day. Hopefully you will not end up wasting imaging time trying to sort it out.

[JamesF]

I did have my ST120 set up as a guide scope a while back, but I think I've removed the camera now.  I may still be able to measure something useful.  I'll nip out to the observatory and have a look.

James

[JamesF]

Ok.  From the back edge of the main tube (not the focuser adapter) to the camera sensor is about 235mm in my case, or alternatively it's about 600mm from the join between the dew shield and the lens cell.  I added a 60mm (T2, I think) drawtube extension to reach focus after removing the clamp and 2" to 1.25" converter that is normally on the end of the drawtube (I think that's what's normally on the end of the drawtube -- I no longer have the stock focuser on my ST120, but I believe in that respect they're the same).  My preference is to have everything screwed together rather than clamped in place by a few screws with the potential to drop out or not be square to the optical axis.

James

[JamesF]

The colour correction on the ST120 is not brilliant by the way (as you'll almost certainly know if you've used it to view the Moon, for example).  I imagine you'll find that stars are a bit bloated as a result.  If that's what you've got to work with however, then that's what you've got to work with, so give it a try and see how you get on.

James

[rwc23q]

3 hours ago, JamesF said:

Ok.  From the back edge of the main tube (not the focuser adapter) to the camera sensor is about 235mm in my case, or alternatively it's about 600mm from the join between the dew shield and the lens cell.  I added a 60mm (T2, I think) drawtube extension to reach focus after removing the clamp and 2" to 1.25" converter that is normally on the end of the drawtube (I think that's what's normally on the end of the drawtube -- I no longer have the stock focuser on my ST120, but I believe in that respect they're the same).  My preference is to have everything screwed together rather than clamped in place by a few screws with the potential to drop out or not be square to the optical axis.

James

Thank you very much for taking the time to reply. I will look to add some extention tubing in the next few days, and see how that goes.

Rob

[rwc23q]

6 hours ago, Clarkey said:

I'm not sure about the ST120 but I use an ST80 for guiding and that needs about an extra 10cm on top of the focuser being nearly fully extended. Try adding some extra extensions and check it during the day. Hopefully you will not end up wasting imaging time trying to sort it out.

Thank you for the reply. I think you are right about the extentions. Just got to find the right ones now 😉