Hello,
I recently have gotten an Edge HD 8, but I am yet to get the accessories. I have settled on an Asi174mm Mini guide scope and a ZWO Oag-L, but the problem is, with my asi533mm pro (17.5mm with plate) and filter wheel (20mm), I cannot get the imaging train to reach the correct back focusing (133.35mm). The oag L needs an adaptor (5mm, m48) that cannot connect to the edge hd 8 t adapter either (78.35, m42). Would someone be able to help me figure this out?
Thanks in advance,
Henry
M48 to M42 adaptor with zero optical depth.
andrea tasselli · May 2, 2026, 08:16 PM
M48 to M42 adaptor with zero optical depth.
M42 tilt plate. Screws to the front of your ZWO OAG L .
You might already have one on your camera that should be moved to the front of the OAG.
https://agenaastro.com/zwo-m42-sensor-tilt-adapter-plate-m42-plate.html
Hi, Henry.
I have a C8 and OAG-L and filter wheel as well, so probably much the same setup as you.
If I understand correctly, your challenge is that you can’t connect your OAG to the C8. Once you have sorted that you can put any amount of spacers to get the desired backfocus.
I’ll proceed on the assumption that this connection issue is your main problem.
You may just need a SCT female to M48 male adaptor which converts the 2"-24 tpi male thread on the rear cell of the C8 to a standard 48mm thread, which your OAG-L uses with the front M48 frontplate.
Like one of these: 📷 image.png
This one has an optical thickness of 8mm. Then you can include any number of spacers and shims to get the back focus you want.
Once you go from SCT to M48, it’s just M48 all the way.
Here is an example image train (in mm):
SCT-M48 = 8
OAG-L = 22.5 (M48 front adaptor (5mm) which comes with the OAG, plus OAG body (17.5mm))
Filter Wheel = 20
Camera = 17.5 (as per your info)
Thats a total of 65mm. You make up the rest with M48 extenders and shims (about another 68mm or so) from the SCT-M48 adaptor to the OAG.
I hope I’m answering your actual question, Henry. Let me know if I am missing the point.
Also, I have added a rotator in mine (a manual one - very handy for framing) which is 12mm. The good thing about the ZWO OAG-L (compared to the Celestron one) is that you have plenty of space to add such things.
All the best.
Paul