How to know if OAG-L prism needs to be adjusted? - Forums

Topic starter

Feb 22, 2023 · Edited: Feb 22, 2023

Hi, I have recently set up my ZWO camera, guide camera, filter wheel and OAG-L as adviced on ZWO webpage to achieve the 55 mm backfocus needed.

I have read that the prism needs to be ajusted (up and down, see attached image for ilustration purpose Prism_ADJ.png) so it won't affect / block the main camera sensor and/or the guide camera sensor.

My main doubt here is: how do I know if the prism needs to be adjusted or not? How do I find if it is on the "perfect" position or if it needs to be pulled in or out, like the attached picture shows?

I ask this because I'll perform the adjustment during the day (since it's easier) and I'm trying to figure out if I will have anything that let me know, in both camera's live view, that the prism is not correctly positioned - for example, some kind of double reflection? Vignetting?

Also, after adjusting the prism during the day, is it necessary to double check anything else during the night, such as star shapes?

Thanks
Clear skies

Engaging

Rob Johnson

Feb 22, 2023

I can only say the way I did mine (not a ZWO OAG) was to look at your images, if there is a slight dark area at the top of the image then move the OAG prism very slightly out until the next image you take doesn’t show the shadow. May be possible to do in the daytime but I haven’t tried. Also I found rotation of the OAG camera slightly improved star shapes.

cs, Rob

Eugen Naiman

Feb 23, 2023

In addition to the previous answer.
Take the whole group (OAG, FW, camera) off the scope (make sure you have a clear filter in the window), then look down to your camera sensor, The prism needs to be aligned to the largest side of sensor. If that is OK, try to see if it overlaps the sensor when you are looking straight down. If it doesn't you will be OK. Final check will be when you take the first image.

Joon Ren

Feb 23, 2023

After adjusting the prism following the other replies, take some flats and check whether the OAG is casting any shadows. You can stretch the flats to further discern fainter shadows. You can also take flats from the guide camera attached to the OAG to check how much the guide camera is seeing (Make sure its in focus you can check by tweaking the focus until you see sharp edges in flats or daytime images). Darker regions will mean that the guide camera will see less or no stars in those areas. Again, stretching will bring out fainter shadows.

From there you can make a decision to balance out between the shadows and how much light the OAG needs to see sufficient guide stars. Flats/DBE can easily take care of faint shadows so don't stress too much about it. Conversely, guiding only requires a few stars to be seen and with an L version, there shouldn't be any problems.

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IS_17m

Topic starter

Feb 24, 2023

Thank you all for your replies and help!

Eugen I followed your advice and took some pictures. The prism does not overlap the edge of the sensor of the main camera. I believe that everything is fine

I will test it as adviced by the other users and take some day shots.

I have not touched the prism height since I bought it, so I believe that even if I leave it as it is right now, I will not have any problems. The two pictures below are the same, the first is a crop of the second. There's a L filter in front of the sensor, mounted on the filter wheel, althouth it doesn't look like.

I also tried to take the pictures as "real" as possible - in other words, at the most realistic and correct angle, so it doesn't give the wrong idea about the prism overlaping or not the sensor.

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jewzaam

Perfect post

Feb 27, 2023

My process is what others have mentioned, take flats. It's something you can do any time. I move the prism all the out, take a flat. This is the reference for no blockage. Move the prism all the way in, or at least a little bit further in than you think you should do. Take a flat. Compare. Do you see a shadow? I haven't done any crazy stretching, I just use the image as displayed in NINA. Move prism out, take a flat, review, repeat as needed. The reason to want the prism as far in as possible is to get better exposure of the image circle on the prism. But you functionally need is enough light across the field for the guide camera that you can detect stars. That being said, the prism being further out than optimal it's fine if you can resolve stars across that guide camera sensor. I hadn't tried taking flats for the guide camera, that's interesting. It would require focusing first after the prism adjustment. So far I haven't had any problems with coverage though.

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Stuart Taylor

Perfect post

Feb 27, 2023

My process is what others have mentioned, take flats. It's something you can do any time. I move the prism all the out, take a flat. This is the reference for no blockage. Move the prism all the way in, or at least a little bit further in than you think you should do. Take a flat. Compare. Do you see a shadow? I haven't done any crazy stretching, I just use the image as displayed in NINA. Move prism out, take a flat, review, repeat as needed. The reason to want the prism as far in as possible is to get better exposure of the image circle on the prism. But you functionally need is enough light across the field for the guide camera that you can detect stars. That being said, the prism being further out than optimal it's fine if you can resolve stars across that guide camera sensor. I hadn't tried taking flats for the guide camera, that's interesting. It would require focusing first after the prism adjustment. So far I haven't had any problems with coverage though.

I have the OAG-L too and I use it with an APS-C camera. I also look at the flats. It's very obvious if there is a slight dark shadow from the prism. Adjust until they just disappear.

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