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ZWO OAG tilt - bad sample? Pls, share your experience.

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#1 astroby2

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Posted 01 December 2021 - 06:04 PM

Hi everyone,

 

I've been living with a slight tilt causing non-round stars in the corners of my images for a while. Recently I confirmed that the tilt is caused by ZWO OAG (https://astronomy-im...product/zwo-oag) by essentially replacing the OAG with a spacer in the image train and seeing perfectly round stars in the corners.

 

Now I'm considering buying a replacement. Since I'm used to ZWO OAG, it mostly satisfies my needs, and is relatively inexpensive, should I just buy the same model again, with a hope that I've got a bad sample first time? I think a lot of people are using this model, could you, please, share your experience with ZWO OAG in terms of introduced tilt?

 

Or maybe you know some other OAG you trust and getting great results with? Please, share. I'm using OAG with newtonian scopes and coma correctors, that usually means 55mm back focus, so it must be relatively narrow as I also need to fit a filter wheel in the image train.

 

Thank you!

 



#2 ChrisWhite

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Posted 01 December 2021 - 06:10 PM

How long have you had it? I'd warranty it if it's within the warranty period. I would not buy another zwo to replace. That seems like rewarding someone for doing a bad job.
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#3 astroby2

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Posted 01 December 2021 - 06:22 PM

How long have you had it? I'd warranty it if it's within the warranty period. I would not buy another zwo to replace. That seems like rewarding someone for doing a bad job.

I think I've had it for more than 2 years, so probably the warranty period is gone - while noticeable, the issue was not super pronounced, so unfortunately I did not do anything to identify what's causing it until now.



#4 ChrisWhite

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Posted 01 December 2021 - 06:25 PM

Do you have a set of calipers? Can you measure the thickness?

#5 astroby2

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Posted 01 December 2021 - 06:28 PM

Do you have a set of calipers? Can you measure the thickness?

Aye, looks like it is 16.5mm thick.



#6 ChrisWhite

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Posted 01 December 2021 - 06:51 PM

Aye, looks like it is 16.5mm thick.

Well, if it's introducing tilt it is thicker at some point and thinner at another. That's what I mean.

I know someone who had a tilt plate that was 0.1mm ticker at one end than the other.

Edited by ChrisWhite, 01 December 2021 - 08:02 PM.

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#7 astroby2

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Posted 01 December 2021 - 07:05 PM

Well, if it's introducing tilt it is thicker at some point and thinner at another. That's what I mean.

I know someone who had a ult plate that was 0.1mm ticker at one end than the other.

I see, I think calipers I have would not be able to catch it - it a rather cheap plastic instrument and the measurements differ by +-0.2mm depending on how hard I press.



#8 andysea

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Posted 02 December 2021 - 12:22 AM

I typically use a micrometer for this kind of measurements. It’s more accurate and repeatable. https://www.amazon.c...TS1RYW04Q0BZ5TB

Edited by andysea, 02 December 2021 - 12:22 AM.

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#9 Mr Astro

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Posted 02 December 2021 - 01:20 AM

If my memory still working ! ..I had this issue before.

The problem was when you screws whatever you had into the ZWO OAG from the telescope side it doesn't go all the way in.. It stops by the OAG stalk..

In my case it was the ring of my reducer that touches the OAG stalk and never go all the way in ..

So I went to the lathe store and just cut 0.5 or 1 mm from that ring....bang ! it solved ! and it screws all the way and eliminated any tilt or gap caused by that stoppage .

I hope this will help you..

 

CS!

Ali 


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#10 rockstarbill

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Posted 02 December 2021 - 02:21 AM

I typically use a micrometer for this kind of measurements. It’s more accurate and repeatable. https://www.amazon.c...TS1RYW04Q0BZ5TB

Purchased, thanks for the tip!


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#11 astroby2

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Posted 02 December 2021 - 03:33 PM

I typically use a micrometer for this kind of measurements. It’s more accurate and repeatable. https://www.amazon.c...TS1RYW04Q0BZ5TB

Thank you, this looks like a much more precise and reliable way to measure these things.



#12 astroby2

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Posted 02 December 2021 - 03:40 PM

If my memory still working ! ..I had this issue before.

The problem was when you screws whatever you had into the ZWO OAG from the telescope side it doesn't go all the way in.. It stops by the OAG stalk..

In my case it was the ring of my reducer that touches the OAG stalk and never go all the way in ..

So I went to the lathe store and just cut 0.5 or 1 mm from that ring....bang ! it solved ! and it screws all the way and eliminated any tilt or gap caused by that stoppage .

I hope this will help you..

 

CS!

Ali 

Thank you for the tip, but unfortunately this is not my case. Not sure it is clear from the photo below, but the extension ring I use doesn't actually touch the stalk when all the way in.

 

oag_res.jpg




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