OAG issue with Celestron Edge HD 8” - Forums

Topic starter

Oct 5, 2023 · Edited: Oct 5, 2023

So I finally figured out collimation and can shoot with Celestron EdgeHD 8".

But I'm having trouble getting the Celestron Off Axis Guider working. I have a Zwo ASI 174 MM mini guide camera that I know works.

I have the OAG pointing the correct direction. But my image for the Guide Cam looks like this, more or less no matter how much I push it in or pull it out.I'll try it tomorrow in the daylight but I figured I'd post this anyway though in case someone knows exactly what the issue is based on the screen shot, then I can fix it tonight. First clear night in a couple weeks.

Any ideas?
Thank you.

Joe Nidd

Oct 5, 2023

Given the long back-focus of that OTA and where the OAG sits in the image train, you may find you still need to add an extension nose piece to your guide camera to hit focus. I believe you need 55mm back-focus from the pick-off prism to the guide camera sensor with that OTA/OAG combo. You may well still be able to achieve focus with the helical focuser all the way out and the camera mounted as far out as possible but it may be difficult with a short nose on the 174MM.
You may also need to adjust the depth of the prism to ensure it is correctly in the light path. A little deeper will take it further from the guide sensor which should help you - keeping in mind any potential interference with your main sensor.
It would be a good idea to try in daylight if you don't manage to resolve the problem at night as it can be hard to ensure the OAG is correctly positioned if you aren't picking up stars.
Some good diagrams here: 93648-Off-Axis-Guider_Manual.pdf

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shenmesaodongxia

Oct 5, 2023

remove the extension of 174mm mini, let the cmos near the oag

Jan Erik Vallestad

Oct 5, 2023

I have the exact same setup myself but I haven't had any clear nights to test it myself yet. I have however tested the camera with an OAG on my Esprit 100ED and found that I had to remove the black extension tube that is attached to the camera in order to get it close enough to the prism, as mentioned above here.

Michael W. Dean

Topic starter

Oct 5, 2023 · Edited: Oct 5, 2023

I have the exact same setup myself but I haven't had any clear nights to test it myself yet. I have however tested the camera with an OAG on my Esprit 100ED and found that I had to remove the black extension tube that is attached to the camera in order to get it close enough to the prism, as mentioned above here.

Thank you. I don't have it in front of me but I think there are two tubes on it that unscrew. One is about 2 inches, one shorter. Do you mean remove both of them?

I think one has the UV / IR filter in it.

Jan Erik Vallestad

Oct 5, 2023

Michael W. Dean:
I have the exact same setup myself but I haven't had any clear nights to test it myself yet. I have however tested the camera with an OAG on my Esprit 100ED and found that I had to remove the black extension tube that is attached to the camera in order to get it close enough to the prism, as mentioned above here.

Thank you. I don't have it in front of me but I think there are two tubes on it that unscrew. One is about 2 inches, one shorter. Do you mean remove both of them?

I think one has the UV / IR filter in it.

Sorry, didn't even cross my mind. I only meant the larger one!

John

Jul 26, 2024

hi, i had he same problem, all was set up correctly, my guide camera is inserted into a zwo focuser on the OAG. In the end I found it earier to focus the edge onto a distant building wall during daylight and then focus the guide camera on the OAG, Once the guide camera was in focus this was locked in so there is no movement.
I found that the next image session I did, when my main camera was in focus the guide camera was in focus and showing guide stars.

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Jeramie

Jul 26, 2024

What's your back focus for the guide cam set at? It should be the same or close to the main cam.

dswtan

Jul 26, 2024

A helpful trick I've found is to search the internet for images with the combination of equipment that you have. People have posted their successful (and not!) configurations that work. From that, it can either get you in the ballpark or at least confirm you're along the right lines.

This is my EdgeHD 8 imaging train with the 0.7x reducer, Moonlite focuser, Celestron OAG, 174MM mini, EFW, and 294MM:
https://www.cloudynights.com/gallery/image/210795-20230527-035537303-ios/

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John

Jul 26, 2024

Back focus for main and guide are the same, guide is taken from the centre of the prism on the oag to the sensor on the guide camera. Zwo focuser helps for any slight error.

Kurt Bierbaum

Jul 26, 2024

I had a similar issue with my rig and a ZWO OAG plus a ZWO helical for the guide cam. I never got that into focus. The solution was that the distance from the main cam to the OAG prism must be identical to from the prism to the guide cam (as Jeramie said above). The ZWO helical had a distance that never allowed that the guide cam came into focus. So I used a different guide cam focuser (no helical) and getting the guide cam into focus was a matter of a few minutes. I suggest you use a caliper to measure the distance from the main cam to the OAG prism and then compare the distance from the OAG prism to your guide cam (check the manuals to find out where to measure the distances at the cams).

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