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Frustrating OAG Backfocus spacing problem QHYOAG-M/asi174mm

Astrophotography Beginner Equipment Imaging Optics
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#1 davidmalanick

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Posted 19 April 2021 - 06:53 PM

Attached is a photo of my new OAG, QHY OAG-M, setup. I tried it out Saturday night and could not get the guide camera,  asi174mm, looking in good focus when viewing it in the Star Profile cross hairs with PHD2.

 

Researching for a camera, learning from previous experiences, I went the "Buy once, cry once" route and got an asi174mm.

I was totally amazed and happy I was getting a minimum of 7-8 guide stars for multi-star guiding, although I could maybe only see 1 to 3 stars in the main PHD2 screen.

 

I tried to focus on distant trees during the day and had the same problem that night.  Would have to lower the guide camera in the guider base to get better focus.

However,  it is at the maximum lowest level. Hence my problem.

 

The back focus of my flattener is 51.5mm +/- 5mm . With my previous setup I had 50.5mm back focus with just the camera adapter, shown separate in photo, connected directly to the flattener and a 1.5mm shim.

 

With trying to keep the same back focus, 50.5mm, and the OAG as far away from the camera as possible the distance from center of the prism to main camera sensor is 36mm.

The distance from the center of the prism up to the guide camera compartment, 36mm, puts it at the inside bottom of the guide camera compartment.

The guide camera stops 4mm from the bottom of the compartment and it still has another 8.5mm to reach the sensor for a total of 12.5mm extra focus length.

 

I've been reading everything I can on OAGs and know you keep the "sensor to prism to sensor" distance equal.  Just don't know what to try.

If I add 12.5mm worth of spacers between the OAG and main camera, changing the "flattener to sensor" back focus distance above 50.5mm will it mess up my image focusing?

Or will I just need to adjust the main focuser in or out more?

 

Any thoughts or comments much appreciated!!!

 

David Malanick

BACK SPACE PROBLEM

 

 

 



#2 Dynan

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Posted 19 April 2021 - 07:28 PM

Looks to me you might need to switch to the OAG-S. You would be 12mm closer to the backfocus required by the reducer. It looks like that  would be enough.

 

OAG-M.png  OAG-S.png

 

However, it would also decrease your diameter by 12mm. If that is impinging on your light cone to the 11.3 x 7.1mm sensor, this wouldn't work.



#3 Andynator

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Posted 19 April 2021 - 07:30 PM

Hi David. You want to erase the 36mm dimension from your mind. The dimension you are looking for is to match the 51.5mm for the imaging camera. So from your flattener, to the prism, and up the tube to the guide sensor, same 51.5mm length. Add an extension ring to the nose of the guide camera or use a helical focuser to dial it in.

I hope this helps,

#4 davidmalanick

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Posted 19 April 2021 - 07:40 PM

Looks to me you might need to switch to the OAG-S. You would be 12mm closer to the backfocus required by the reducer. It looks like that  would be enough.

 

attachicon.gifOAG-M.png attachicon.gifOAG-S.png

 

However, it would also decrease your diameter by 12mm. If that is impinging on your light cone to the 11.3 x 7.1mm sensor, this wouldn't work.

I got the OAG-M because it was the one QHY recommends for my QHY247C camera. I would be worried the small OAG might impede the image train.

Then I might end up having problems with finding guide stars by needing to move the prism higher up.


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#5 rgsalinger

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Posted 19 April 2021 - 07:40 PM

+1 for Andynator's explanation. The prism in an OAG is placed just above the long side of the imaging chip and parallel to it. No matter where you place there's no effect on the back focus point for the guide camera. It a prism, not a lens. I suspect that you are actually going to need to put a spacer between your main camera and the OAG rather than an extender.

 

Rgrds-Ross



#6 davidmalanick

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Posted 19 April 2021 - 07:50 PM

Hi David. You want to erase the 36mm dimension from your mind. The dimension you are looking for is to match the 51.5mm for the imaging camera. So from your flattener, to the prism, and up the tube to the guide sensor, same 51.5mm length. Add an extension ring to the nose of the guide camera or use a helical focuser to dial it in.

I hope this helps,

Flattener to prism is 14mm, minus the 50.5mm is still ruffly 36mm needed to the guide camera sensor. Still 12mm short.



#7 Dynan

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Posted 19 April 2021 - 08:01 PM

I got the OAG-M because it was the one QHY recommends for my QHY247C camera. I would be worried the small OAG might impede the image train.

Then I might end up having problems with finding guide stars by needing to move the prism higher up.

My mistake. I mistook the 174mm to be your imaging camera...



#8 davidmalanick

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Posted 19 April 2021 - 08:07 PM

+1 for Andynator's explanation. The prism in an OAG is placed just above the long side of the imaging chip and parallel to it. No matter where you place there's no effect on the back focus point for the guide camera. It a prism, not a lens. I suspect that you are actually going to need to put a spacer between your main camera and the OAG rather than an extender.

 

Rgrds-Ross

Yes, I have the prism above the sensor.  I'm just wondering what would happen if a spacer is added between the main sensor and the OAG. It would increase the 50.5mm but what would that do to the main image focus wise?



#9 rgsalinger

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Posted 19 April 2021 - 08:16 PM

@davidmalanick. You do not care what the distance is from the prism to the guide camera because the prism does not refract the light, it reflects it. His problem is happening because he needs to add a spacer between the OAG and the main camera. Then the guide camera and the main camera will become par focal. The OP doesn't need a new OAG, he just needs to get the two chips par focal at 50.5 mm.

 

The way to do this involves the back focus of the guide camera with some residual imposed by the metal between the guide camera and the OAG (normally just a couple of millimeters). That OAG has 10 to 13mm depending on how it's connected to the rest of the system. The ZWO mini has 8.5mm of back focus and the camera has 18mm of back focus. So, with 50.5 to work with there's no problem at all here that I can see.

 

He just needs to add a spacer between the OAG and the main camera that takes the main camera to the correct back focus for the flattener.  Then move out the guide camera from the bottom of the OAG using the helical focuser. You should have plenty of room. 

 

What's tricky is to get the main camera at exactly the right back focus from the flattener. Once that's done the rest is straightforward.

 

Rgrds-Ross



#10 Andynator

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Posted 19 April 2021 - 08:19 PM

Flattener to prism is 14mm, minus the 50.5mm is still ruffly 36mm needed to the guide camera sensor. Still 12mm short.


Right. And you need to add that 12mm to the guide camera only. Disassemble the retention ring on top of the OAG and replace it with a helical focuser, or use an extension threaded on the front of the guide camera.

#11 davidmalanick

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Posted 19 April 2021 - 08:32 PM

@davidmalanick. You do not care what the distance is from the prism to the guide camera because the prism does not refract the light, it reflects it. His problem is happening because he needs to add a spacer between the OAG and the main camera. Then the guide camera and the main camera will become par focal. The OP doesn't need a new OAG, he just needs to get the two chips par focal at 50.5 mm.

 

The way to do this involves the back focus of the guide camera with some residual imposed by the metal between the guide camera and the OAG (normally just a couple of millimeters). That OAG has 10 to 13mm depending on how it's connected to the rest of the system. The ZWO mini has 8.5mm of back focus and the camera has 18mm of back focus. So, with 50.5 to work with there's no problem at all here that I can see.

 

He just needs to add a spacer between the OAG and the main camera that takes the main camera to the correct back focus for the flattener.  Then move out the guide camera from the bottom of the OAG using the helical focuser. You should have plenty of room. 

 

What's tricky is to get the main camera at exactly the right back focus from the flattener. Once that's done the rest is straightforward.

 

Rgrds-Ross

I am the OP. 

I get what you mean by adding a spacer between the OAG and camera. Say I add a 12mm spacer.  That would get me the equal distance. but also wouldn't that increase the distance between the main camera sensor and the flattener by 12mm, thus the back focus distance would now be at 62.5 instead of 50.5?


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#12 davidmalanick

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Posted 19 April 2021 - 08:58 PM

Maybe this will help to understand my problem.

The asi174mm camera doesn't fit all the way down into the guide camera holder.

It's like I need to lower it the 12.5mm to get focus.

Drawing
 
I really want to know the effect of adding a spacer and increasing the 50.5mm distance.

Edited by davidmalanick, 19 April 2021 - 09:00 PM.


#13 Dynan

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Posted 19 April 2021 - 09:03 PM

The problem you're facing being that you can't get the Guide Camera CLOSE ENOUGH to the prism.

 

The other suggestions are ADDING length to the guide camera train. No?

 

There is a statement on the QHY page that if you're using THEIR GUIDE CAM, you can remove to shorten the distance to the prism.

 

From QHY page (https://www.qhyccd.c...catid=30&id=251:

"If the distance is too short and even the guider is in shortest position, guider can still not get focusing. You can remove the QHY5-II's front part to reduce the distance a little."

 

But that is THEIR guide cam. I'm not sure if this can be done with the 174MM.



#14 Dynan

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Posted 19 April 2021 - 09:05 PM

As far as remedies, I can only think of three.

 

A thinner OAG. You'd have to respace the Reducer to Camera 50.5mm. But that wouldn't make up the blocked inward travel of the guide camera, unless the OAG was micro thin.

 

A different Reducer with more backfocus.

 

Getting the QHY guide cam.

 

All involve returns or new purchases, so blahhh. Not a nice solution.



#15 davidmalanick

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Posted 19 April 2021 - 09:10 PM

The problem you're facing being that you can't get the Guide Camera CLOSE ENOUGH to the prism.

 

The other suggestions are ADDING length to the guide camera train. No?

 

There is a statement on the QHY page that if you're using THEIR GUIDE CAM, you can remove to shorten the distance to the prism.

 

From QHY page (https://www.qhyccd.c...catid=30&id=251:

"If the distance is too short and even the guider is in shortest position, guider can still not get focusing. You can remove the QHY5-II's front part to reduce the distance a little."

 

But that is THEIR guide cam. I'm not sure if this can be done with the 174MM.

Yes, I believe I need to get the camera closer to keep both sensors par-focal with prism.  The asi174mm only has a removable 4mm thick part in the front, unfortunately it contains the sensor protector.


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#16 Dynan

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Posted 19 April 2021 - 09:10 PM

Adding to the alternatives above, since we can't avoid new/returned gear, possibly get a different OAG that allows closer approach of the guide cam to the light train?

 

I've never used a reducer, but can it be moved to the back of the OAG? Or is guiding unreduced on a reduced image a no-no? Then you'd DEFINITELY have to add length to your guide cam path!



#17 davidmalanick

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Posted 19 April 2021 - 09:14 PM

Adding to the alternatives above, since we can't avoid new/returned gear, possibly get a different OAG that allows closer approach of the guide cam to the light train?

 

I've never used a reducer, but can it be moved to the back of the OAG? Or is guiding unreduced on a reduced image a no-no? Then you'd DEFINITELY have to add length to your guide cam path!

It's not a reducer, it's a field flattener.

total backspace 38mm

Here's what it looked like before the OAG.



#18 Dynan

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Posted 19 April 2021 - 09:19 PM

Doing a little math, the effective imaging area of your sensor is 23.55mm x 15.73mm...giving a diagonal of 28.32mm.

 

The OAG-S is 38mm inside diameter. Are you sure this would impinge on your light train? That is 5mm beyond each corner the DIAGONAL of your sensor. And the guide cam would fit.

 

Spacing the Reducer to the sensor after that change would be easy in comparison to your present problem.

 

EDIT: Field Flattener...not reducer.


Edited by Dynan, 19 April 2021 - 09:20 PM.


#19 rgsalinger

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Posted 20 April 2021 - 01:29 AM

Your original diagram shows 50.5mm of distance between the flattener and the main camera chip. However, the QHY OAG is 13 mm thick and appears to be directly threaded into the flattener. The main camera has 18 mm of back focus to the chip and is threaded into the OAG. That's 31mm total between the camera chip and the flattener. Yet, you keep saying that you have the main camera chip 50.5mm from the flattener. So, what I can't figure out is why you are saying that. There must be some other component that's adding in 19 mm of back focus but I don't see anything like that in your setup. 

 

If you are actually putting spacers of some kind in already, then get rid of them by getting the right adapters for all of the components. I just don't see any. 

 

If, in fact, the main camera sensor is 50.5mm from the flattener then you cannot add a spacer and use the flattener. That's out. You will need a thinner OAG or a guide camera that has less back focus or can be fully inserted. I just can't see what's causing you to lose all of the back focus. Sorry. 

 

Your little diagram shows spaces between the flattener and the OAG and the OAG and the camera. What's in those spaces is what I can't figure out.

 

Rgrds-Ross




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