Can't focus QHY268M & QHY174M with OAG

[memnoch]

Hi, I need help focusing my new QHY268M (main imaging camera) and my QHY174M guide camera on OAG-M.

The setup is composed by: QHY268M, QHYCFW2M-US filter wheel, QHY OAG-M with QHY174M, and some spacers, to achieve the backfocus of 55mm, as per the schematics on your website.

I cannot focus both cameras at the same time, the focus point of the guide camera is much closer (about 250mm) than the focal point of the imaging camera.

I clearly cannot move the guide camera closer to the scope, by adding spacers between the guide and the imaging cameras, because then I would go past the backfocus of 55mm that I need to achieve for the flattener/reducer.

Since my whole setup is QHY and it seems I have done everything according to your schematics, I would like your help in solving the issue.

Thanks

[memnoch]

is it possible to have an answer from the QHY technicians who created and published the optical schemes which are obviously wrong and deceptive?

https://www.qhyccd.com/uploadfile/2021/0113/20210113013635780.png

[llsong]

Hello

You can adjust the order of the spacers according to the pictures in the attachment to shorten the back focus of the guide camera. In addition, if you extend the OAG prism pole too long, it will also cause the guide star to fail to achieve focus, so you need to adjust the extension length of the prism pole.

[johnrogerjennings]

I had a similar problem. I think the issue is that the minimum back focus distance for the guide camera is really about 61mm to the center of the off axis guider. That's 47mm to the base of the guider (Manual diagram), plus 12mm back focus (QHY462C), plus 2mm extra for focusing.

Lets assume the best case that you remove the front screw on AR window (-5mm) from the 462C guider. That means you have 7mm+2mm+47mm=56mm, not an idea situation, but the chip is still covered in the focuser from dust.

That means the back focus to the main sensor from the center of the off axis guider must be exactly 56mm for the two cameras to be in focus at the same time.

To get the calculation to the front of the OAG, add 8mm to the front of the OAG. (OAG 5mm, M54 plate adapter 3mm)  The total back focus to the main camera from the front of the OAG adapter is now 64mm.

Unfortunately that means that the 55mm back space requirement from the front of the OAG adapter to the main camera sensor plane can not be reached if the 2 cameras are parfocal. That's the issue with the diagrams on the QHY adapters website. The different back space adapters at the camera for the 268M or 268C is a red herring for this issue.

I'm using an astro-physics TCC Quad reducer/flattener on my AP130GT. It has 102.9mm back focus so I have a lot to play with. I use the full 69mm for my reducer/flattener spacer calculations and it should work ok.

There may be other quider/focuser housings that could be attached to the top of the OAG that would reduce the 47mm from the center of the OAG to the base of the focuser, or other guide cameras with less back focus, but I'm not aware of that. In reality, the stalk of the OAG may be too long. I also think other manufactures may have issues too. The real issue is the back focus of the OAG guider and camera.

I hope there is something I missed here and I'm wrong.

John