ZWO OAG-L help or experiences needed - Forums

Topic starter

Dec 11, 2021 · Edited: Dec 11, 2021

Hello to all,

after a long thinking period I finally ordered a RC8 telescope and a CCD47 0.67x reducer. To guide the longer focal length, I also ordered a ZWO OAG-L which is the larger one of brands' guiders. I put everything together and ended op with this order:

1. ZWO ASI294MM Pro (6mm distance from front to sensor)
2. ZWO 7x2" filter wheel (20mm)
3. ZWO OAG-L screwed directly on the filter wheel (17.5mm)
4. ZWO M48 adapter plate for M48 (5mm)
5. different adapter pieces to get the backfocus distance right
6. CCD47 reducer (85mm backfocus distance)

So far, s good. Yesterday, I was able to try the new equipment. Everything works quite well until I had to focus the guide camera within the OAG. After some time, I realized, that I had to move the guide cam (tried both the Altair GPCam3 178MM and the ZWO ASI120MM Mini) in to reach the focus point. But there was no room left for that. I was already in as far as I could go. So there are two possibilities, aren't they? I could move the mirror slightly out to shift the focus point to the camera sensor. But I am already quite near on the edge of the M48s aperture. The other way could be to insert an app. 5mm adapter between the cam and the filter wheel. But the OAG and the filter wheel is not ment to be used this way (referring to the manual).

Has someone of you experienced those kind of problems and can suggest some solutions here? Am I missing something?

Thank you for your help!

CS
Christian

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Steven

Dec 11, 2021 · Edited: Dec 11, 2021

Just to confirm this, you mention the 120MM.. but, just to confirm that it is the "Mini" version of the 120mm?
And not the standard 120MM with an eyepiece adapter?

the Altair seems to have a 17.55mm distance to the sensor, and the ZWO mini guide cameras 8.5mm
So that could be the reason why the Altair doesn't work.

Christian Großmann

Topic starter

Dec 11, 2021 · Edited: Dec 11, 2021

Just to confirm this, you mention the "Mini" version of the 120mm?
And not the standard 120MM with an eyepiece adapter?

the Altair seems to have a 17.55mm distance to the sensor, and the ZWO mini guide cameras 8.5mm
So that could be the reason why the Altair doesn't work.

Hi Steven,

Thanks for the hint. Of course I have the 120MM MIni Version.

The ASI120MM showed an out of focus star at the minimum position. Thats why I realized I need to get more into the tube...

Christian

SemiPro

IOTD Staff

Dec 11, 2021

Is it possible to move the OAG further up the assembly? Maybe I have this backwards but if its closer to the telescope then the camera can be further up the OAG focuser. Either way changing the position of the OAG in the imaging train is your best bet. Just make sure it doesn't sit behind the filters.

I am not sure if this is an option, but on my RC8 I had to ditch the helix focuser on the smaller OAG because I had the same issue you did. Instead I used the simple brass compression ring it came with to reach focus. I don't know if your OAG-L came with that though.

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Christian Großmann

Topic starter

Dec 11, 2021

Is it possible to move the OAG further up the assembly? Maybe I have this backwards but if its closer to the telescope then the camera can be further up the OAG focuser. Either way changing the position of the OAG in the imaging train is your best bet. Just make sure it doesn't sit behind the filters.

I am not sure if this is an option, but on my RC8 I had to ditch the helix focuser on the smaller OAG because I had the same issue you did. Instead I used the simple brass compression ring it came with to reach focus. I don't know if your OAG-L came with that though.

Hi @SemiPro

thanks for the help. This seems to be the least attractive method for me, because I had to add a spacer between the camera and the Filter Wheel. This means, that the distance between the filters and the sensor increases, which may result in larger (but also weaker) halos!?! I use a set of Astronomik filters and the LRGB Set is creating some of them.

However, I guess this would be the only option I have. It means that I have to buy some other adapters and spacers. But that's how life goes in astro photography, isn't it?

Brian Boyle

Dec 11, 2021 · Edited: Dec 11, 2021

Hi Christian,
I have the same set-up with my RC8, except for a full-frame sensor on the back, the TS-field flattener (not reducer) and the 174MM mini as my guide camera.

I have almost the same experience as you. My guide camera only focuses at the very closest point in on the OAG-L.

I have my pick-off mirror quite a long way out to avoid obscuring the full frame sensor. It is very close to the edge of the M48 plate. The image structure out here is not great, and it would be be even worse at this distance you don't have a flattener, but a reducer.

It is possible to guide off the images (which are in focus, but are all comet shaped), since the guider only cares about getting a reliable centroid, but the rms is rather poor 0.8arcsec rms. I suspect it will be worse in your case.

If I use my 60mm/f4 guidescope instead, i get better than 0.4arcsec rms. So I have given up using the OAG-L on my RC. This is probably not what you wanted to hear. Sorry.

It is a little bizarre that an OAG designed for full frame sensors should have an M48 tilt plate packaged with it - since you have to push the pick-up mirror so far out of the way to avoid vignetting the sensor that it is partially blocked by the M48 tilt plate.

However, the closest you can physically place the guide camera means that the pick up mirror must be this far up to focus on the guide camera. While this makes sense (just) for a full frame sensor, I can’t understand why they didnt give a bit more slack to allow you to place the pick off mirror further into the beam where the image quality is better for use with smaller sensors.

ZWO should also have packaged this with an M54 or M68 tilt plate. But connecting to the telescope side would have been a challenge for a 2inch focuser!

I think you are also hampered by the non-flatness of the RC field - made worse by the reducer.

You are coming across the basic incompatibility between the non-flatness of an RC field at the >10mm off axis and the design of an OAG made for full frame sensors. I sympathise with your problem, one i have struggled with for a few months.

Brian

PS i just remembered that my 174MM has a black cover glass ring that can be unscrewed. The120MM mini has the same. Have you tried taking that off? It would get you 2mm or so closer.

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kuechlew

Dec 11, 2021

Admittedly I'm at a loss understanding this problem. I would expect that both cameras have to keep a backfocus distance of 85mm to your reducer. Given the fact that there is a 20mm thick filter wheel between the OAG and the main camera, you should need an 18 mm distance ring in front of your guide camera (20 mm for the filter wheel minus 2 mm for the additional back focus of the mini camera compared to the main camera) to have the proper distance to the OAG. There shouldn't be a problem with getting not close enough .

I can say for sure that placing a 5mm distance ring between the filter wheel and the OAG won't hurt, even if the manual advices to connect the OAG directly. This way you can skip 5mm in front of the OAG which may or may not resolve your problem.

Best regards
Wolfgang

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Christian Großmann

Topic starter

Dec 11, 2021

Brian Boyle:
Hi Christian,
I have the same set-up with my RC8, except for a full-frame sensor on the back, the TS-field flattener (not reducer) and the 174MM mini as my guide camera.

I have almost the same experience as you. My guide camera only focuses at the very closest point in on the OAG-L.

I have my pick-off mirror quite a long way out to avoid obscuring the full frame sensor. It is very close to the edge of the M48 plate. The image structure out here is not great, and it would be be even worse at this distance you don't have a flattener, but a reducer.

It is possible to guide off the images (which are in focus, but are all comet shaped), since the guider only cares about getting a reliable centroid, but the rms is rather poor 0.8arcsec rms. I suspect it will be worse in your case.

If I use my 60mm/f4 guidescope instead, i get better than 0.4arcsec rms. So I have given up using the OAG-L on my RC. This is probably not what you wanted to hear. Sorry.

It is a little bizarre that an OAG designed for full frame sensors should have an M48 tilt plate packaged with it - since you have to push the pick-up mirror so far out of the way to avoid vignetting the sensor that it is partially blocked by the M48 tilt plate.

However, the closest you can physically place the guide camera means that the pick up mirror must be this far up to focus on the guide camera. While this makes sense (just) for a full frame sensor, I can’t understand why they didnt give a bit more slack to allow you to place the pick off mirror further into the beam where the image quality is better for use with smaller sensors.

ZWO should also have packaged this with an M54 or M68 tilt plate. But connecting to the telescope side would have been a challenge for a 2inch focuser!

I think you are also hampered by the non-flatness of the RC field - made worse by the reducer.

You are coming across the basic incompatibility between the non-flatness of an RC field at the >10mm off axis and the design of an OAG made for full frame sensors. I sympathise with your problem, one i have struggled with for a few months.

Brian

PS i just remembered that my 174MM has a black cover glass ring that can be unscrewed. The120MM mini has the same. Have you tried taking that off? It would get you 2mm or so closer.

Hi @Brian Boyle ,

that's sad to hear. Especially for the fact, that the OAG is useless that way on all my setups. I didn't expect this problem. But I already guessed that it would be a hard-to-solve one.

Meanwhile, I switched back to my 60mm guide scope and the GPCam on it. This may work on the reduced FOV. But if it comes to the unreduced focal length, it may be problematic. Have you tried it? What are your experiences? I am a bit dissapointed at the moment.

I bought the scope with the galaxy season in mind and hoped, I could use it with the full reach. I try to be optimistic and may find a solution without busting the bank again.

@kuechlew
Placing a 5mm ring between the main camera and the filter wheel is basically possible. But I have to buy new adapters for that and I am sad, that ZWO sells the OAG-L with the M48 adapter plate that is nearly impossible to use. I bought it mainly, because I wanted to prevent exactly the problem I have now. Referring to the manual it seemed to work and the parts (at least in theory) fit very well together. The idea of this setup is great. Brian may have had the same idea as me a while ago.

CS

Christian

Brian Boyle

Dec 11, 2021

Hi Christian,

I found that a 240mm focal length guide scope system (with 120MM mini) worked well (0.3-0.4 arcsec rms) with my unreduced focal length of 1600mm.

But I do resample at x2 in the postprocessing as my typical seeing is not great, about 3-4 arcsec.

In hindsight, the OAG really only works for a large corrected f.o.v. I guess the ZWO would say that makes sense, since it is specifically for large sensors. Selling with a M48F tilt plate connection to the scope, however, reduces it usefulness right out of the box. It works OK with my corrected Esprit 100 field, but with a faster beam or larger scope then you really need to buy the larger tilt plate connection, for use in a larger focusser.

This hobby is bery much like the tale of the old lady who swallowed a fly….

Brian

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Howard Richard

Dec 19, 2021

I had to remove the eyepiece adaptor form the ZWO 120mm mini to achieve focus on the guide camera.

The other alternative is to move the prism toward the center of the telescope.

Adel Kildeev

Dec 19, 2021 · Edited: Dec 19, 2021

I have the similar issue with Meade 8’ ACF and Orion thin OAG. 1.Meade suggests 55 mm backfocus (???) for the attached camera. How it is physically possible if the OAG plus EFW and the ASI 1600 already have around 50 mm? What kind of T-adaptor has the thickness of 5 mm only? With extensive research I did find the correct back focus for this ACF -110 mm. One problem is solved. I would advise you to make the similar research- claimed backfocus in the manual may be not correct or ommited. 2.Orion OAG “suppose” to work fine with Orion autoguider camera, yes or no? No, it does not. The only possible way to reach decent focus -to play with expenders between EFW, the camera and between OAG and the camera, keeping in mind that you need to have correct backfocus (see par.1). From my experience, moving the OAG mirror up and down does not help so much. The only way is tidious experiments, unfortunately. 3. I like guider scope, I use it with a great success on my 80 mm APO. But ACF does not have upper plank for the scope, so I attached miniguider scope with 2xBarlow lense on additional optional viewfinder scope base, works poorely, generally I need at least 1/4 focal of the main scope guider, i.e 500 mm, I have only 325 mm.
Sorry for bothering you by my problems but the situation is the similar. Some OAG does properly work with certain cameras, some -does not work at all 🙀

Adel Kildeev

Dec 19, 2021 · Edited: Dec 19, 2021

Admittedly I'm at a loss understanding this problem. I would expect that both cameras have to keep a backfocus distance of 85mm to your reducer. Given the fact that there is a 20mm thick filter wheel between the OAG and the main camera, you should need an 18 mm distance ring in front of your guide camera (20 mm for the filter wheel minus 2 mm for the additional back focus of the mini camera compared to the main camera) to have the proper distance to the OAG. There shouldn't be a problem with getting not close enough .

I can say for sure that placing a 5mm distance ring between the filter wheel and the OAG won't hurt, even if the manual advices to connect the OAG directly. This way you can skip 5mm in front of the OAG which may or may not resolve your problem.

Best regards
Wolfgang

Wolfgang, At the first look your position is well based. Add 20 mm minus difference in camera backfocus ( for ASI 120 MM -2 mm) and everything is fine. BUT ( the devil is in small detailes), ZWO recommends the way to reach 55 mm back focus, it is well known https://astronomy-imaging-camera.com/tutorials/best-back-focus-length-solutions-55mm.html
In the case of adding 11+2 mm between FW and OAG as per recommendation what would be additional spacer before guider camera? If we try to skip 11+2 and attach the OAG directly to the FW- it is almost impossible, I tried this option. Again 55 mm is a trick, I did find that the correct back focus for my APO and flattener is 66 mm and for ACF -110 mm. Mission is impossible…😎 I would be glad if you prove otherwise, with 15 mm backfocus for Orion guidercamera I did try all possible combinations and still fail.

Steve de Lisle

Dec 19, 2021

Just my quick response. Maybe others have said this but one easy solution is to unscrew the end off the camera. This should give you a couple more millimeters.

Miguel Noppe

Dec 28, 2021

I had the same combo 294MM + EFW 7x2" + OAG L and i've put 6-7mm (spacer+adaptor) between EFW and 294MM.
This worked for me.

I've since sold the 294MM and switched up my gear, now i have another problem, which is to fit the OAG L to a 8x1,25" EFW.
A friend made u custom aluminum adaptor for me, going to pick it up today.

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surfinash

Jun 6, 2024

This is an old thread - but FWIW, I'll add my $0.02. The guide-cam (290 vs 120) is not an issue here.

With ASI294MM, 7x36 / 7x2 EFW, and OAG-L, you need to add a 6.5mm spacer between the 294MM Pro and the filter wheel (or between the FW and the OAG-L). That is the only way the guide cam will receive the focus. The 0.5mm makes the difference.

Here is the backfocus math:

Sensor > FW > OAG-L > Tilt Plate > Telescope

6.5mm + 20 mm + 17.5 mm + 5 mm = 49 mm (6 mm short of 55 mm back focus)

Sensor > Spacer > FW > OAG-L > Tilt Plate > Telescope

6.5mm + 6.5 mm + 20 mm + 17.5 mm + 5 mm = 55.5 mm (0.5 mm+ for the back focus, but it works for your guide-cam to achieve focus, and is within the tolerance for the camera/backfocus).

I am using this image train today (I use ASI290mm Mini) and it has worked without any problems.

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