Suggestions on how or what I can do to utilise a OAG to reach focus before I sell it on?

[Cumbrianwolf]

I have a GSO 10" 254 mm parabolic imaging Newtonian F4 at 1016 mm, supposedly optimised for back focus!
Now after a lot of thought and advice from others I decided to purchase an OAG for the reason of weight reduction and more accurate tracking.
But for the life of me I cannot reach focus on the imaging camera as it want to go deeper into the focuser off which I cannot do, and to add insult to injury the guide camera (SVBONY 305 PRO) cannot meet focus as I was hoping to use a helical focuser for that, but even with adapters removed the camera needs to be flat onto the actual top of the guiding prism and still barely reaches focus. This is not a thick OAG, but rather slim, so any suggestions as I am well and truly stuffed!

Edited February 2, 2022 by Cumbrianwolf

[len2376]

You haven't posted links to the scope or coma corrector but if it's this one TS-Optics-NEWTON-Coma-Corrector-1-0x-TSGPU with a 55mm working distance, then the DSLR will swallow a good bit of that with a T-Ring adapter? Maybe let us know what is actually in the imaging train?

[Cumbrianwolf]

15 minutes ago, len2376 said:

You haven't posted links to the scope or coma corrector but if it's this one TS-Optics-NEWTON-Coma-Corrector-1-0x-TSGPU with a 55mm working distance, then the DSLR will swallow a good bit of that with a T-Ring adapter? Maybe let us know what is actually in the imaging train?

Here goes, first up is the coma corrector: https://www.teleskop-express.de/shop/product_info.php/info/p10875_Skywatcher-2--2-Element-Newton-Coma-Corrector-for-Astrophotography.html Secondly is the actual OTA https://www.teleskop-express.de/shop/product_info.php/info/p10225_TS-PHOTON-10--F4-Advanced-Newtonian-Telescope-with-Metal-Tube.html

[Padraic M]

The back-focus on that CC is 55mm, and the back-focus on your DSLR is also 55mm. No coincidence, as those CCs and field flatteners are designed to be used with SLRs with 55mm BF. Dedicated astro cameras usually have much shorter back focus requirements - for instance, the ZWO ASI1600mm is 6.5mm, which means that you have lots of space between the camera body and the CC edge to fit filter wheels, OAGs, rotators etc. and you usually even need to add spacers to make up the difference.

 

Canon SLRs have a focal plane mark which shows how far back the sensor is recessed into the body. You can verify the SLR back focus distance my measuring from this mark to the front edge of the T-ring.

https://www.photokonnexion.com/definition-focal-plane-mark/

 

Unless I'm mistaken, you can't put anything between the T-ring and the CC as the DSLR body/T-ring swallows up the entire 55mm. Moving the primary mirror forward won't help as the critical distance is between CC and camera.

 

The back focus requirement is the same for imaging camera and guide camera. While your guide cam will usually have a much shorter back focus requirement than an SLR, the 'stalk' on the OAG is quite long and holds the camera away from the prism. 

 

I have a C8 SCT with a field flattener/focal reducer with a back focus requirement of ~110mm. Focal length is 1300mm so an OAG is recommended. However, I have to say tuning the OAG is a complete PITA so I'm very much inclined to fall back on a guide scope which is so much easier to manage, and mirror flop be damned. I would certainly advise to avoid using an OAG unless you're very comfortable with spacing and measuring.

[Cumbrianwolf]

9 hours ago, Padraic M said:

The back-focus on that CC is 55mm, and the back-focus on your DSLR is also 55mm. No coincidence, as those CCs and field flatteners are designed to be used with SLRs with 55mm BF. Dedicated astro cameras usually have much shorter back focus requirements - for instance, the ZWO ASI1600mm is 6.5mm, which means that you have lots of space between the camera body and the CC edge to fit filter wheels, OAGs, rotators etc. and you usually even need to add spacers to make up the difference.

 

Canon SLRs have a focal plane mark which shows how far back the sensor is recessed into the body. You can verify the SLR back focus distance my measuring from this mark to the front edge of the T-ring.

https://www.photokonnexion.com/definition-focal-plane-mark/

 

Unless I'm mistaken, you can't put anything between the T-ring and the CC as the DSLR body/T-ring swallows up the entire 55mm. Moving the primary mirror forward won't help as the critical distance is between CC and camera.

 

The back focus requirement is the same for imaging camera and guide camera. While your guide cam will usually have a much shorter back focus requirement than an SLR, the 'stalk' on the OAG is quite long and holds the camera away from the prism. 

 

I have a C8 SCT with a field flattener/focal reducer with a back focus requirement of ~110mm. Focal length is 1300mm so an OAG is recommended. However, I have to say tuning the OAG is a complete PITA so I'm very much inclined to fall back on a guide scope which is so much easier to manage, and mirror flop be damned. I would certainly advise to avoid using an OAG unless you're very comfortable with spacing and measuring.

Your words ring loud and clear, on this occasion I am going to fall back on my guide scope which albeit large and heavy did the job I needed with no oval stars and of course the corrector worked well with the camera and the images were coma free as it was designed (nothing can fit in-between the mount and the corrector as you thought). As they say, if it is not broken, why fix it? After all, the mount was at its suggested weight limit for imaging and not once has it grumbled with consistent tones from the motors whilst slewing and no signs of backlash or momentum wobble when coming to a stop.

 

I might keep the OAG for future instances but for now I am out of commission as the focuser failed on the guide camera, and I am awaiting a part, not that it matters as the skies have put paid to that. I agree on the OAG being a total pain to get each part to work as one as I am glad I tested in the house as otherwise I would be very annoyed spending a few hours of my night trying to work it out. The original set up was becoming streamlined at around half an hour before I could retreat to the house and remote operate.

Edited February 3, 2022 by Cumbrianwolf

[GazAstro]

On 2/3/2022 at 9:43 AM, Cumbrianwolf said:

I am glad I tested in the house

Just a thought, if you tried focusing the OAG / scope in the house what were you focusing on ? was it at infinity ?

[Cumbrianwolf]

11 minutes ago, GazAstro said:

Just a thought, if you tried focusing the OAG / scope in the house what were you focusing on ? was it at infinity ?

It was the mountains over fifty miles away on the horizon, it was the best I could do.

[GazAstro]

Just checking 🙂