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QHY OAG with Zwo ASI 120mm won't focus

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#1 photoenthu

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Posted 28 February 2021 - 02:18 AM

I have a QHY OAG (Medium). Tried to use it with a Zwo ASI 120mm guide camera on a Celestron SCT 9.25" and 6.3 reducer. Guide cam won't focus in any position. Main camera focus worked fine.

 

Anyone else here using this combination successfully?



#2 michael8554

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Posted 28 February 2021 - 12:28 PM

Where is the OAG situated in the image train ?

 

The imaging camera sensor has to be 105mm back from the Focal Reducer, so I expect you have the OAG between the FR and the 294MC ?

 

How far is the 294MC sensor from the centre of the prism when focused ?

 

Use a ruler to help visualise that same distance from the centre of the prism to where the guidecam sensor needs to be located, to be in focus when the 294MC is in focus.

 

Once you have got that roughly right, finetune the 120MM focus on stars, lock everything down and don't change the 120MM focus ever, only focus the 294MC.

 

When that's in focus the 120MM  will be in focus too.



#3 Peter in Reno

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Posted 28 February 2021 - 12:32 PM

Is your 120mm the old style or new "Mini" 1.25" barrel style? The "Mini" would be inserted inside OAG's drawtube so the sensor is closer to OAG's prism and better chance to get focused.

 

Peter 



#4 joeytroy

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Posted 28 February 2021 - 12:57 PM

So the guide camera unscrews to get focus. You will also need to move the camera as well to get some back focus. It doesn't need to be perfect, to check focus go to the guide settings main view here is a video I did on it. (sorry for the dust my mount was sitting around for a week with no use due to weather when I took the pictures)

 

http://www.youtube.c...aTeX661o&t=1m5s

 

Screen-Shot-2021-02-17-at-1-43-04-PM.png

 

Screen-Shot-2021-02-17-at-1-44-58-PM.png

 

Also make sure to update the firmware as well for the 120

https://astronomy-im...oftware-drivers


Edited by joeytroy, 28 February 2021 - 12:58 PM.


#5 bokemon

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Posted 01 March 2021 - 08:28 AM

I have a QHY OAG (Medium). Tried to use it with a Zwo ASI 120mm guide camera on a Celestron SCT 9.25" and 6.3 reducer. Guide cam won't focus in any position. Main camera focus worked fine.

 

Anyone else here using this combination successfully?

You probably have the OAG too close to the camera



#6 michael8554

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Posted 01 March 2021 - 09:19 AM

So the guide camera unscrews to get focus. You will also need to move the camera as well to get some back focus. It doesn't need to be perfect, to check focus go to the guide settings main view here is a video I did on it. (sorry for the dust my mount was sitting around for a week with no use due to weather when I took the pictures)

 

http://www.youtube.c...aTeX661o&t=1m5s

 

Screen-Shot-2021-02-17-at-1-43-04-PM.png

 

Screen-Shot-2021-02-17-at-1-44-58-PM.png

 

Also make sure to update the firmware as well for the 120

https://astronomy-im...oftware-drivers

 

 

 

OP is using an OAG, not a Guide Scope



#7 photoenthu

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Posted 01 March 2021 - 11:24 AM

Here is the imaging train (to achieve the 105 mm backfocus from the reducer to main cam):

 

From the reducer side:

1) 50mm Celestron T thread Adapter

2) 10mm extension ring

3) 16mm QHY OAG M

4) 22mm ZWO EFW Mini (20mm + 2mm adapter ring)

5) 6.5mm ZWO 294mc Pro

 

Total: 50+10+16+22+6.5 = 104.5

 

So the prism is roughly 35mm from the main camera imaging sensor. 

 

I am using the latest ZWO 120mm-S Guide cam (https://astronomy-im...duct/asi120mm-s) with the 1.25" nosepiece attached. The nosepiece allows me to move it in and out of the OAG - tried all sorts of distance from the prism along with the helical focuser in the OAG - still no luck. I tried during the daytime on a street light post - can see a slightly out of focus light post - was hoping will work in the night on stars. 

 

I did update the firmware of the camera.

 

I see the problem now - due to the design of the guide cam 120mm-S - it won't go in like a regular 120mm. As a result, the minimum distance to the prism I am able to achieve is about 60mm. That's a difference of (60 - 35) =  25mm from the distance between main cam sensor to prism. I can change the location of the 20mm ring to be behind the EFW - that will increase the distance to the main sensor from prism to 55mm. But there will still be a 5mm gap and no give for the helical focuser. Interesting problem to solve! 

 

Also, some more research tells me I have to rotate the guide camera and OAG around as there may not be any guide stars on one side of the object (I was on Orion nebula). How do we do that? The Celestron brand OAG comes with a mechanism to allow rotating the guide cam. But no such mechanism on the QHY OAG.

 

Also an YouTube video suggests the 120MM-s sensor is too small for a OAG use case - and prescribes to buy a larger sensor guide cam. Not sure if true.


Edited by photoenthu, 01 March 2021 - 01:39 PM.


#8 bokemon

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Posted 01 March 2021 - 06:58 PM

U need to get a 120mm to fit into the OAG, but u still need a spacer btw the OAG and FW

1)celestron adapter

2) spacer

3) oag

4)spacer

5)FW

6)camera



#9 Peter in Reno

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Posted 01 March 2021 - 07:16 PM

Let's assume the minimum distance between the prism and guide camera's sensor is longer than distance between prism and main camera's sensor, then you need to increase distance between OAG and main camera and reduce spacing between OAG and FW to preserve required back focus.

 

Peter 



#10 photoenthu

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Posted 01 March 2021 - 08:55 PM

U need to get a 120mm to fit into the OAG, but u still need a spacer btw the OAG and FW

1)celestron adapter

2) spacer

3) oag

4)spacer

5)FW

6)camera

Thinking about the same - but will 120MM work OK for a 1480mm and 2350mm scope (when not using the reducer)? I am getting conflicting info on this, some seem to believe 120MM has too small a sensor for this longer focal lengths.



#11 photoenthu

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Posted 01 March 2021 - 08:57 PM

Let's assume the minimum distance between the prism and guide camera's sensor is longer than distance between prism and main camera's sensor, then you need to increase distance between OAG and main camera and reduce spacing between OAG and FW to preserve required back focus.

 

Peter 

Thanks - but currently I have zero space between the OAG and FW - directly attached. No way to reduce further. See imaging train info above. 



#12 Peter in Reno

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Posted 01 March 2021 - 09:04 PM

Thinking about the same - but will 120MM work OK for a 1480mm and 2350mm scope (when not using the reducer)? I am getting conflicting info on this, some seem to believe 120MM has too small a sensor for this longer focal lengths.

QHY OAG prism is 8mm x 8mm (11.3mm diagonal) and 120mm sensor is 6mm diagonal so it may be better to get larger sensor close to 11.3mm diagonal. This would be a better but pricey guide camera (13mm diagonal) : 

 

https://astronomy-im...174mm-mini-mono

 

This will allow sensor to be much closer to the prism.

 

Peter 


Edited by Peter in Reno, 01 March 2021 - 09:05 PM.


#13 Peter in Reno

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Posted 01 March 2021 - 09:08 PM

Thanks - but currently I have zero space between the OAG and FW - directly attached. No way to reduce further. See imaging train info above. 

Can you remove 10mm extension ring and insert 10mm spacer at main camera? 

 

Peter 



#14 Peter in Reno

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Posted 01 March 2021 - 09:13 PM

If your C925 is not EdgeHD, then back focus is either not critical or not well defined. Try incrementally add spacer at camera until guide camera is focused and see if you get good images.

 

Peter 


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#15 bokemon

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Posted 02 March 2021 - 12:01 AM

Thinking about the same - but will 120MM work OK for a 1480mm and 2350mm scope (when not using the reducer)? I am getting conflicting info on this, some seem to believe 120MM has too small a sensor for this longer focal lengths.

Focal length doesn't matter - it's an OAG, it comes after the scope.

 

Thanks - but currently I have zero space between the OAG and FW - directly attached. No way to reduce further. See imaging train info above. 

Peter said something wrong.

The distance from the reducer (or scope) to camera is always fixed.

The filter wheel is always right in front of the camera (to reduce vignetting)

You need to scoot the OAG forwards in the imaging train.

INCREASE the distance between OAG and FW, DECREASE distance between OAG and reducer / scope
 



#16 photoenthu

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Posted 02 March 2021 - 09:46 AM

If your C925 is not EdgeHD, then back focus is either not critical or not well defined. Try incrementally add spacer at camera until guide camera is focused and see if you get good images.

 

Peter 

That's good to know - I was stuck as I thought 105mm is mandatory. Let me try this. Thanks.



#17 photoenthu

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Posted 02 March 2021 - 09:47 AM

Focal length doesn't matter - it's an OAG, it comes after the scope.

 

Peter said something wrong.

The distance from the reducer (or scope) to camera is always fixed.

The filter wheel is always right in front of the camera (to reduce vignetting)

You need to scoot the OAG forwards in the imaging train.

INCREASE the distance between OAG and FW, DECREASE distance between OAG and reducer / scope
 

That makes sense - thanks



#18 Peter in Reno

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Posted 02 March 2021 - 09:51 AM

Focal length doesn't matter - it's an OAG, it comes after the scope.

 

Peter said something wrong.

The distance from the reducer (or scope) to camera is always fixed.

The filter wheel is always right in front of the camera (to reduce vignetting)

You need to scoot the OAG forwards in the imaging train.

INCREASE the distance between OAG and FW, DECREASE distance between OAG and reducer / scope
 

Your method is no different than my method. It will work either way. My method also preserves back focus distance of 105mm, just different way of extending the distance between OAG's prism and main camera's sensor to help parfocaling with guide camera.

 

Peter


Edited by Peter in Reno, 02 March 2021 - 09:51 AM.


#19 Peter in Reno

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Posted 02 March 2021 - 11:18 AM

That's good to know - I was stuck as I thought 105mm is mandatory. Let me try this. Thanks.

Be careful not to extend back focus too far with focal reducer. The longer the back focus the greater the vignetting. It's perfectly natural the imaging circle shrinks using focal reducer. Also the longer the back focus, the shorter the focal ratio at the cost of increasing vignetting. 

 

Peter 


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#20 photoenthu

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Posted 03 March 2021 - 03:53 PM

Be careful not to extend back focus too far with focal reducer. The longer the back focus the greater the vignetting. It's perfectly natural the imaging circle shrinks using focal reducer. Also the longer the back focus, the shorter the focal ratio at the cost of increasing vignetting. 

 

Peter 

Quick question here. Even with a perfect 105mm backfocus, I am getting some vignette when using the reducer. Is this expected? Vignette goes away when reducer is removed.  



#21 Peter in Reno

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Posted 03 March 2021 - 04:08 PM

Quick question here. Even with a perfect 105mm backfocus, I am getting some vignette when using the reducer. Is this expected? Vignette goes away when reducer is removed.  

Yes, it's expected.

 

Peter 


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