Guide Scope to OAG

[Budgie1]

I'm thinking about moving from my current guiding set up (Celestron 9 x 50 finder with ZWO ASI 120MM Mini) to an Off Axis Guider with the ASI 120MM Mini. This is instead of getting a guide scope to piggy-back on to the main scope because, looking at the prices, I can get the ZWO OAG V2 for about the same price as a cheap 60mm guide scope, with the advantages of the OAG.

The main scope it will be attached too is the SW Evostar 100ED with 0.85 reducer/flattener and the camera is an astro modded Canon EOS 1300D. 

Is this set up likely to work or will there be issues with the spacing of the OAG attaching to the camera because of the built-in flash casing overhanging the mounting flange?

Any other things to consider with this setup that others have experienced would be gratefully received.

[wimvb]

If I understand your concern correctly, you can avoid the problem by just hanging the camera up side down. The oag prism will be on the bottom side of the camera.

[Budgie1]

Thanks Wim, that would work.

So there's no issues with back focus on a DSLR when adding a OAG into the image train? At the moment the EOS adaptor screws on to the back of the reducer/flattener, I just wondered if added the OAG between them would have any effect of focus.

Thanks.

[prusling]

Yes, unfortunately it does. The amount of back focus required is dictated by the reducer/flattener - many are designed for a 55mm back focus which matches a DSLR plus T adaptor.  With such there is no opportunity to include any additional length in the back focus and an OAG will require anything from 9mm upwards (mine are relatively thin at around 16mm).  Some reducers are designed for greater back focus and these may be able to accommodate a thin OAG with a DSLR.

[prusling]

I vaguely recall coming across a T-adaptor with integral OAG so it may be worth searching for that.

[prusling]

TS make them and they are on offer at present!  TSOAG11

[michael8554]

If your FR/FF has 55mm backfocus for use with a Canon DSLR, then assemble OAG - FF/FR - Canon

That's what I've done.

Also the DSLR won't foul the OAG.

And you'll have enough backfocus to add a helical non-rotating focuser into the guidecam path. 

So much easier than trying to get good focus by sliding up and down the periscope tube.

Be aware that the guidestar may be distorted, but PHD2 will cope.

Michael

[Budgie1]

2 hours ago, michael8554 said:

If your FR/FF has 55mm backfocus for use with a Canon DSLR, then assemble OAG - FF/FR - Canon

That's what I've done.

Also the DSLR won't foul the OAG.

And you'll have enough backfocus to add a helical non-rotating focuser into the guidecam path. 

So much easier than trying to get good focus by sliding up and down the periscope tube.

Be aware that the guidestar may be distorted, but PHD2 will cope.

Michael

It's a nice idea, Michael, but the Evostar 100ED has M56 thread on the focuser tube and the scope end of the FR/FF. So I'd be going M56, M56 to M48 adaptor, OAG, M48 to M56 adaptor, FR/FF, M48 to DSLR adaptor then camera.  Surly having that dip in the focal train to accommodate the OAG would cause a shadow on the DSLR sensor?

[michael8554]

11 hours ago, Budgie1 said:

Surly having that dip in the focal train to accommodate the OAG would cause a shadow on the DSLR sensor?

If you're going from M56 to OAG then isn't that going to happen to an extent wherever you place it ?

I use a Full Frame 6D because of it's well known advantages, and accept some cropping even after Flats are applied, my largest hole is M48.

Michael

[Budgie1]

16 minutes ago, michael8554 said:

If you're going from M56 to OAG then isn't that going to happen to an extent wherever you place it ?

Not necessarily because the RF/FF is M56 on the scope end and M48 on the camera side so it's designed to focus the light with those apertures. If you reduce the aperture on the scope side from M56 to M48 by putting the OAG in there and dangle the mirror into the light stream then you're cutting 8mm off the dia of the light entering the RF/FF before it's done its job. So, my thinking is, it must be reducing the amount of available light getting to the camera.

Agreed, if I had M48 from the focuser tube back to the camera then it wouldn't matter too much.

I stand to be corrected in my thinking by those more experienced.

[Starflyer]

This is what you're after;

https://www.teleskop-express.de/shop/product_info.php/info/p2722_TS-Optics-Off-Axis-Guider-for-Canon-EOS-cameras---replaces-the-T-ring.html

I'm not sure of the guide camera connection, drop them a message explaining what you want to do, they're very helpful.