I’ve just had my mono camera come back from repair and have been setting it up on my Edge HD8. In the course of doing this, I found and resolved a back spacing issue that was affecting my ability to achieve focus on my OAG camera. Previously, I had a set up that was able to get close to a focus on the OAG, enough to use it but I was never really happy with it. Obviously, I changed something in my optical train spacing between when I had previously used it and now. Thought I would share my findings in case others find it useful.
I use an Esatto 2” Low Profile inline focuser, a ZWO OAG-L (with a ASI174MM Mini), a ZWO 36mm x 7 filter wheel and a ZWO AIS1600MM Pro mounted in an ASG Photon Cage. The manual for the OAG calls for it to be connected in front of the OAG with the camera screwed into the filter wheel. I found that when I used this arrangement and set up the back focus to suit the ASI1600MM Pro, I could not achieve anywhere close to focus with the OAG camera, even with the prism fully extended and the camera fully inserted with no spacers and the OAG focuser fully wound in.
To solve the issue, I needed to reduce the spacer distance between the Esatto 2” LP Focuser and the OAG to get within the range of the OAG focuser and add backspace after the filter wheel using my ASG Photon cage to get the right back spacing for my imaging camera. In my old set up, I used the cage and I must have been right on the edge of the correct spacing for the OAG. I could get close enough to a focus to be usable, but my guiding always suffered a little.
I measured the spacing and found that I needed to remove 3mm from the spacing between the inline focuser and the OAG to get into the range where the OAG focuser barrel could be used, and I then needed to add that 3mm back into the imaging train between the filter wheel and the camera. I was able to achieve this using different spacers and the ASG Photon Cage.
My imaging train set up where I have the correct back spacing for the imaging camera and plenty of adjustment focusing range for the OAG camera is:
133.5 mm - target back space for Edge HD8
0mm thickness - Esatto 2" LP Adapter for SCT with 2" thread - Allows the Esatto focuser to mount onto the Edge HD8
32mm thick - Esatto 2” LP Focuser - as well as focusing, it allows me some back focus adjustments. The focuser has 400,000 steps (10mm) of adjustment and I aim to have it in the middle of the range. With the spacing described here, it is coming in at around 220,000 steps.
7.5mm - at around 220,000 steps in, the focuser adds this into the back focus distance.
2.5mm thick - Esatto 2" LP Adapter M60 - M48 with stop ring - allows M48 spacers to be connected
34.5mm of spacers.
22.5mm thick in the back spacing for the imaging camera - ZWO OAG-L - to set it up to be in the middle of its focusing range - prism fully extended, ASI174MM Mini fully inserted with no spacers
21mm thick - ZWO 36mm x 7 EFW - with the filters factored in
2mm thick - M54 to M42 adapter - to connect the ASG Photon Cage to the EFW
5mm - ASG Photon Cage - I had previously purchased this to fix tilt issues on my Newtonian scope. The cage has both tilt and backspace adjustment. For my setup, I set the tilt adjuster and back space to the distances below. I have a small bit of tilt but not enough to worry about at this time:
3mm - base of the ASG Photon Cage
1mm - Tilt spacing set (allows for future tilt adjustments)
1mm - back spacing adjust set.
6.5mm - ZWO1600MM Pro
Hope others find this useful.