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Celestron OAG Guide Camera ASI174MM too Dark

ZWO ASI174MM ZWO ASI174MM Mini Planetary Camera Guiding
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RM Haley avatar
RM Haley avatar
So I have the Celestron OAG and the ASI174MM-Mini guide camera. The stars barely show up, even at 2 second exposures. Will the guiding be sufficient enough if I bump it up to 3 or even 5 seconds? I would think that the shorter exposure times would be much better for faster corrections. Do I need a better guide camera that is more sensitive? Here's what HighPointScientific has to say about the 174:

"Looking for a highly sensitive monochrome guide camera or planetary camera? Then look no further than this ASI174MM Mini!"

Not sure how this camera is "sensitive". I am using the Celestron 0.7 reducer on the 8" EdgeHD, bringing the f-stop to f7. Not sure what else I can change other than the exposure time, but I am worried that the longer exposures would make the whole OAG useless, since I can use a 200mm guide scope (not recommended on this scope) at 0.5 seconds with no issues.
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Lasse Skov avatar
Lasse Skov avatar
Hi.

I have the Celestron OAG and ASI174MM-Mini with my EdgeHD 9.25 and I have no problems getting stars. Is the prism maybe too far back? And/or have you tried without the reducer?
Dark Matters Astrophotography avatar
Dark Matters Astrophotography avatar
In PHD2 you can turn up the gain. The gain value shown in PHD2 is a percentage of the maximum gain on the camera. Try setting this to 60 or 70 and see if that makes a difference.
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RM Haley avatar
RM Haley avatar
Lasse Skov:
Hi.

I have the Celestron OAG and ASI174MM-Mini with my EdgeHD 9.25 and I have no problems getting stars. Is the prism maybe too far back? And/or have you tried without the reducer?

I think removing the reducer would make it worse at f10. I am also not sure if adjusting the prism would really make a difference, worth looking into though. Thanks.
RM Haley avatar
RM Haley avatar
Dark Matters Astrophotography:
In PHD2 you can turn up the gain. The gain value shown in PHD2 is a percentage of the maximum gain on the camera. Try setting this to 60 or 70 and see if that makes a difference.

I am using the ASIAIR, which uses PHD2, but I didn't see (or look for) a setting in PHD2 for gain. The camera is at max gain, but I'll look for a setting, thanks.

EDIT: There is no "gain" setting in PHD2 on the ASIAIR. The camera is at max gain though, max allowable. I did read this in another forum though:
"Go to 4, 5 even 6 secs, unless you have a rickety mount."

I guess I can try 5 seconds, I was just worried this would reduce the effectiveness of the auto-guiding.
Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
The reducer actually reduces the imaging circle, so even though it gets "faster" it delivers less light in the outer parts of the FOV. So I would still try without the reducer attached, it's not all about F-stop. Moving the prism is actually crucial to whether or not it will gather light, and bring it to the guide sensor.

I have to remove my reducer (permanently) for other reasons though I was able to successfully use my OAG with it, BUT not unless the prism protruded into the imaging cam's FOV. I'm guessing that a C8HD, APS-C and the 0.7x reducer might be pushing it beyond what it's capable of.
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RM Haley avatar
RM Haley avatar
The reducer actually reduces the imaging circle, so even though it gets "faster" it delivers less light in the outer parts of the FOV. So I would still try without the reducer attached, it's not all about F-stop. Moving the prism is actually crucial to whether or not it will gather light, and bring it to the guide sensor.

I have to remove my reducer (permanently) for other reasons though I was able to successfully use my OAG with it, BUT not unless the prism protruded into the imaging cam's FOV. I'm guessing that a C8HD, APS-C and the 0.7x reducer might be pushing it beyond what it's capable of.

Right, but on many forums, the Celestron OAG and ASI174MM are a perfect match. I don't have any vignetting, or blurry stars, it's just a faintness issue. I am going to try 5 seconds, since I haven't gone over 2 seconds, but mainly because I am worried that a higher exposure time would reduce the effectiveness of the guiding.
Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
RM Haley:
Right, but on many forums, the Celestron OAG and ASI174MM are a perfect match. I don't have any vignetting, or blurry stars, it's just a faintness issue. I am going to try 5 seconds, since I haven't gone over 2 seconds, but mainly because I am worried that a higher exposure time would reduce the effectiveness of the guiding.


Yes it is, but clearly you have an issue that needs sorting which is most likely related to what has been responded to you in this thread. You should not need those exposure lengths to see stars, especially with gain at max. If you are using HD mounts you would ideally need less than 1s exposures for guiding, EQ mounts could do with more than that.

If you have a camera with a small sensor then you would not see any vignetting at all. Those turn up with APS-C. Vignetting on your imaging cam also has nothing to do with whatever the guide camera sees or not. 

1. The reducer reduces the imaging circle, less light and more warped stars towards the corners. Setting the OAG prism too far up will result in: low light, faint stars, oblong stars and or issues with getting proper guiding for all the above reasons.

2. Removing the reducer will give the opposite effect, despite being F/10 which doesn't really matter as much because aperture is more important, much more important than F-ratio.

So I would re-check the position of your OAG prism. Make sure that you see it clearly getting light through the reducer but still not obscuring your imaging sensor. It should be very easy to do with small sensors up to 4/3.
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Richard Carande avatar
Richard Carande avatar
I use an OAG with the 174 on my Edge-11, with the 0.7 reducer installed.   My main camera is a full-frame ASI2400mc-p.  In order to keep the prism out of the camera's FOV, it is really into the vignetted drop-off area.  If I'm imaging near the galactic plane, finding guide stars is not an issue usually (max gain and using 1 sec exposures), but if I'm in a low-density star field area shooting, for instance, an isolated galaxy, I sometimes have to rotate the OAG (changing the target framing) to find a good guide star.  And sometimes, I do need to bump up the exposure time to make that work.

Curious if you (OP) have used the 200mm guide scope you mentioned with any success on your rig?
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RM Haley avatar
RM Haley avatar
Richard Carande:
I use an OAG with the 174 on my Edge-11, with the 0.7 reducer installed.   My main camera is a full-frame ASI2400mc-p.  In order to keep the prism out of the camera's FOV, it is really into the vignetted drop-off area.  If I'm imaging near the galactic plane, finding guide stars is not an issue usually (max gain and using 1 sec exposures), but if I'm in a low-density star field area shooting, for instance, an isolated galaxy, I sometimes have to rotate the OAG (changing the target framing) to find a good guide star.  And sometimes, I do need to bump up the exposure time to make that work.

Curious if you (OP) have used the 200mm guide scope you mentioned with any success on your rig?

Yeah, I was originally using a 200mm guide scope with the ASI120MM-S, and that worked great. What I have found though using the OAG is that the images are a bit more crisp. I originally did it for the stars, initially, but I have seen a dramatic improvement in the targets as well. I have looked into some forums, and it seems like I can bump it up from 2 second exposures to up to 5 second exposures without any real issue, as long as you have a good mount, and I am using the EQ6-R Pro.
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RM Haley avatar
RM Haley avatar


Yes it is, but clearly you have an issue that needs sorting which is most likely related to what has been responded to you in this thread. You should not need those exposure lengths to see stars, especially with gain at max. If you are using HD mounts you would ideally need less than 1s exposures for guiding, EQ mounts could do with more than that.

If you have a camera with a small sensor then you would not see any vignetting at all. Those turn up with APS-C. Vignetting on your imaging cam also has nothing to do with whatever the guide camera sees or not. 

1. The reducer reduces the imaging circle, less light and more warped stars towards the corners. Setting the OAG prism too far up will result in: low light, faint stars, oblong stars and or issues with getting proper guiding for all the above reasons.

2. Removing the reducer will give the opposite effect, despite being F/10 which doesn't really matter as much because aperture is more important, much more important than F-ratio.

So I would re-check the position of your OAG prism. Make sure that you see it clearly getting light through the reducer but still not obscuring your imaging sensor. It should be very easy to do with small sensors up to 4/3.

I am using the ASI533 and ASI294 on this EdgeHD 8", and I can't use the ASI2600 with the reducer. The prism on the Celestron OAG is massive, and I don't think I can move it any closer to the sensor as it barely fits as it is with the 294. I think without the reducer things might be fine, and I could probably do 1 second guiding exposures, but with the reducer I might be stuck just increasing the exposure up to 5 seconds. Is what it is I guess, and as long as it works, I don't care.
Dark Matters Astrophotography avatar
Dark Matters Astrophotography avatar
RM Haley:
Dark Matters Astrophotography:
In PHD2 you can turn up the gain. The gain value shown in PHD2 is a percentage of the maximum gain on the camera. Try setting this to 60 or 70 and see if that makes a difference.

I am using the ASIAIR, which uses PHD2, but I didn't see (or look for) a setting in PHD2 for gain. The camera is at max gain, but I'll look for a setting, thanks.

EDIT: There is no "gain" setting in PHD2 on the ASIAIR. The camera is at max gain though, max allowable. I did read this in another forum though:
"Go to 4, 5 even 6 secs, unless you have a rickety mount."

I guess I can try 5 seconds, I was just worried this would reduce the effectiveness of the auto-guiding.



Before you do that check the position of the prism and also make sure that enough light is getting in for the prism to be effective. If you can use adapters with the largest possible opening this should allow more light in for the prism which will in turn allow for more signal from the guide stars.
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RM Haley avatar
RM Haley avatar
So I don't know what happened, I didn't really make any changes but all of a sudden the stars were fine at 1 second with max gain.