55mm or 60mm Back Focus???

Figzphotos

ASI 2600MC Pro 17.5mm - M54 Filter Drawer 20mm - M68 OAG 17.5mm (with M54 sensor tilt plate adapter) = 60mm ???

Figzphotos

Tech@ZWO

Figzphotos The OAG version on this connection way is M68 OAG, not OAG-L, they are two different products, the former has been discontinued and has a smaller prism than OAG-L, it also does not need an extra tilt plate, because it has threads on the telescope side.

Figzphotos

Tech@ZWO

Can anyone tell me the best way to attach this same imaging train to a telescope without a tilt plate on the scope side. Adding a tilt plate to the telescope side will add 5mm more to the back focus.

Figzphotos

You are incorrect, The OAG version on this connection is the M68 OAG-L. Does the tilt plate add 5mm to the imaging train shown in the picture?

erossi

Figzphotos looks like the OAG you're using is indeed the newer OAG-L with the plate on the telescope side. This variant requires you to remove the tilt plate on the camera side to compensate for that extra 5 mm. This would not work for you as there wouldn't be any threads left on the camera side to screw the filter drawer onto. What ZWO is suggesting you is using the older non-L variant of the OAG, that one had no tilt plate and used M68 threads on the telescope side. Sadly the non-L variant was discontinued, you may find some leftover stock through some dealers.

The following diagram is for the older non-L OAG and requires an M54 tilt plate on the camera side as well as the M54 filter drawer (works for APS-C too provided you do attach an M54 tilt plate)

The only other option I see would be using the basic OAG as per the following diagram:

Figzphotos

erossi the non L is not an option. Is the current plate that I have adding 5mm to my train? Are there tilt plates that are thinner?

erossi

There's this alternative here, this would yield you the exact backfocus distance, though swapping filters is a pain as they do reside recessed within that M54 adapter that is screwed onto the whole system (yet again works on both full frame and APS-C)

erossi

This last solution works because the M54 adapter is 15 mm instead of the 20 mm used by the filter drawer, therefore it compensates for the extra 5 mm from the OAG plate

Figzphotos

I can not remove the filter draw because I use it for my LPro filter for light pollution.

w7ay

Figzphotos I can not remove the filter draw because I use it for my LPro filter for light pollution.

The M54 spacer that is 15mm thick can also take a M48 filter:

https://astronomy-imaging-camera.com/product/zwo-m54-adapter-replacing-efw/

Remember that your Optolong filter glass is 2mm thick, so for backfocus purposes, you also need to add 0.66mm to the backfocus specs of the OTA.

I.e., if your OTA manufacturer tells you the backfocus from the metal back needs to be 55.00 mm, adding a glass of thickness L will make the optical path have 55.00 + L/3 backfocus. See Katie Schwertz' Master's thesis that I have linked multiple times to this forum.

All of ZWO's parts are aimed at 55mm mechanical backfocus, which is wrong for an OTA with 55mm backfocus, once there is any glass filter between the OTA and the camera.

Chen

Figzphotos

Ok, Now Im really lost. How can I build the imaging train for both telescopes?

C-11 HD Edge - Focal reducer - TAdapter for Edge - OAG L (17.5mm)- Filter Drawer w-LPro (20mm)- ASI 2600 MC Pro (17.5) = 55mm (Imaging train only)

Askar 80 PHQ -Focal reducer - OAG L (17.5mm)- Filter Drawer w-LPro (20mm)- ASI 2600 MC Pro (17.5) = 55mm (Imaging train only)

w7ay

Figzphotos Askar 80 PHQ -Focal reducer - OAG L (17.5mm)- Filter Drawer w-LPro (20mm)- ASI 2600 MC Pro (17.5) = 55mm (Imaging train only)

The Askar focal reducer has a backfocus of 55mm. But you also have an Optolong L-Pro in the light path. The L-Pro is 2mm thick, so it adds 2.0/3 mm to the light path.

The mechanical distance needs to be 55.66 mm, not 55.0 mm. You need to add about 0.66mm of shims. Something like this:

https://agenaastro.com/blue-fireball-9-pc-fine-tuning-spacer-ring-set-for-m54-threads-0-1-to-1-0-mm-s-set9.html

That's assuming ZWO's camera flange to sensor distance is really 17.5mm.

Problem is that ZWO's camera specs at one time say 17.5mm +/- 0.5mm , so you really need to tune the backfocus distance by actually looking at the stars at the edge of the ASI2600's frame. See "How to Tell if Your Equipment's Back Focus is Off" in this web page:

https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus

Chen

Figzphotos

I found someone that is going to machine my 5mm tilt plate to 1mm. Will a total of 56mm be close?

Figzphotos

ok, I did some research and being off less than 1mm back focus is reasonable. thank you.

Tech@ZWO

Figzphotos What is the version of your OAG? OAG-L or M68 OAG?
If it is former, the 5mm tilt plate is needed, you can use this adapter to replace the filter drawer to achieve 55mm back focus.
https://www.zwoastro.com/product/zwo-70mm-diameter-m54-adapter-for-oag-l-and-asi-dso-cameras/