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ASI120MM in an OAG at ~1000mm FL, bad idea?


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I have played around with framing in stellarium and for most targets there appears to be a guidestar with little or no fiddling required, but nothing beats real life experiences, so im looking for feedback if anyone has used the very small sensor 120MM in a similar or longer focal length setup and how it went.

Looks like many use the 174MM for off axis guiding, but since i already have a 120MM and am not thrilled about the cost of the 174, would be convenient to be able to use the existing one.

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I have used the ZWO ASI 120mm as guide camera with a skywatcher ST 80mm mounted on 8" SCT, 10" newt, also with WO slide guide on WO Redcat and the combinations.

Yet to be proven/tested is with an OAG; have this sitting on an 8" SCT, I expect that to be successful - once there is enough good weather to get focus :(

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1 hour ago, ONIKKINEN said:

Looks like many use the 174MM for off axis guiding, but since i already have a 120MM and am not thrilled about the cost of the 174, would be convenient to be able to use the existing one.

It actually makes more sense to use smaller sensor than larger for OAG.

This is because prism is too small to illuminate large sensor. I use ASI185mc - it is somewhat larger than ASI120 (8.6mm vs 6mm diagonal) - and it gets vignetted on my OAG:

image.png.9117dd01ae0183607af4b856c676254f.png

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If you plan to use the original asi120 and not the mini version, make sure that it will fit. On my scope, I had trouble trying to fit the asi120 between the focuser and the filterwheel. I bought the asi290 mini, and it just works. You can use it with the standard zwo oag. The 290 is more sensitive than the 120. So far, I've had no trouble finding guide stars in the fov. Multi star guiding doesn't always work, though.

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2 hours ago, wimvb said:

If you plan to use the original asi120 and not the mini version, make sure that it will fit. On my scope, I had trouble trying to fit the asi120 between the focuser and the filterwheel. I bought the asi290 mini, and it just works. You can use it with the standard zwo oag. The 290 is more sensitive than the 120. So far, I've had no trouble finding guide stars in the fov. Multi star guiding doesn't always work, though.

Mine is the mini version, so i think it will work backfocus and size wise to any design of OAG, especially since i dont have a filterwheel.

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2 hours ago, vlaiv said:

It actually makes more sense to use smaller sensor than larger for OAG.

This is because prism is too small to illuminate large sensor. I use ASI185mc - it is somewhat larger than ASI120 (8.6mm vs 6mm diagonal) - and it gets vignetted on my OAG:

image.png.9117dd01ae0183607af4b856c676254f.png

I see, is this with an 8x8 size prism OAG? I dont have one yet, but the 8x8 ones are cheaper than the bigger ones, so another way to save cash right there.

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I'm using a ZWO120MM mini with a OAG on my C8 at 2032mm focal length, and I don't remember having a situation where I didn't find at least one guide star within the search area.
On my 500mm focal length refractor, I don't think I ever seen a starless field. 

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8 hours ago, ONIKKINEN said:

I see, is this with an 8x8 size prism OAG? I dont have one yet, but the 8x8 ones are cheaper than the bigger ones, so another way to save cash right there.

Yes, it is TS 16mm OAG

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8 hours ago, MarsG76 said:

I'm using a ZWO120MM mini with a OAG on my C8 at 2032mm focal length, and I don't remember having a situation where I didn't find at least one guide star within the search area.
On my 500mm focal length refractor, I don't think I ever seen a starless field. 

If it works for 2000mm FL then sounds good for my plans.

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2 hours ago, david_taurus83 said:

Interested as will be doing the same in the future. I would bin 2x2 though.

 

1 hour ago, ONIKKINEN said:

If it works for 2000mm FL then sounds good for my plans.

Yes I BIN2x2 my 120MM mini.... As far as imaging goes, I dont see my self going back to guide scope again... once the OAG is setup as running... and thats not that hard to do one you do it a few times... takes care of flexture and you're guiding at the same focal length as imaging.
One important thing to note is make sure the OAG is parfocal with the camera, half the problems in not finding a guide star was that the OAG was not in exact focus, limiting to the brightest stars if not completely eliminating seeing any of them.

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2 hours ago, MarsG76 said:

 

Yes I BIN2x2 my 120MM mini.... As far as imaging goes, I dont see my self going back to guide scope again... once the OAG is setup as running... and thats not that hard to do one you do it a few times... takes care of flexture and you're guiding at the same focal length as imaging.
One important thing to note is make sure the OAG is parfocal with the camera, half the problems in not finding a guide star was that the OAG was not in exact focus, limiting to the brightest stars if not completely eliminating seeing any of them.

Im thinking of using a helical microfocuser between the camera and OAG for peace of mind. Id rather not deal with the headache of figuring out the exact shims required to keep it parfocal.

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22 minutes ago, ONIKKINEN said:

Im thinking of using a helical microfocuser between the camera and OAG for peace of mind. Id rather not deal with the headache of figuring out the exact shims required to keep it parfocal.

That sounds like a good plan, just be careful about how much space you create between the OAG and the camera as there is a limited amount of distance that you'll be able to pull out of the OAG before using extenders (which is not ideal).

 

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Indeed - do mind not only the spacing - but also how close you put the prism to main sensor.

Faster the system - closer it needs to be to minimize vignetting.

Simple calculation says that for example 8mm prism on a fast system of say F/5 - needs closer than 40mm if you want to avoid prism size stopping down guide assembly. Even at 40mm (in F/5 case) - there will be only one point 100% illuminated.

OAG really needs to be close to camera for best performance (and it works better with slower scopes in that regard).

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On 17/05/2022 at 19:22, vlaiv said:

Indeed - do mind not only the spacing - but also how close you put the prism to main sensor.

Faster the system - closer it needs to be to minimize vignetting.

Simple calculation says that for example 8mm prism on a fast system of say F/5 - needs closer than 40mm if you want to avoid prism size stopping down guide assembly. Even at 40mm (in F/5 case) - there will be only one point 100% illuminated.

OAG really needs to be close to camera for best performance (and it works better with slower scopes in that regard).

Math feels like rolling a square barrel - up a hill - for me so not sure i would find the calculation simple 😬, but did i get the idea that a bigger prism would be desirable even with the tiny sensor as it will be sitting in a rather fast scope at ~F/5? My RA worm has some rough spots that undergo a large periodic error within a 10s or so time period so ideally i wouldn't have to push guide exposures too high due to prism size stopping the aperture for the guider making for dimmer stars.

Askar makes an OAG with a 10x10 prism, bunch of adapters and a helical focuser all in the box so looks like this might be the way to go.

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