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Why is this OAG image not to edges?

astrophotography equipment imaging Meade SCT refractor
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#1 adamchicago

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Posted 27 September 2020 - 04:50 PM

Does anyone know why this OAG image is in the middle of the screen and not out to the edges?  I've added a ZWO OAG with ZWO 174mini camera to both of my telescopes (refractor & SCT) and in both setups I get this same situation in the PHD2 tracking software...I didn't have this problem when using the same ZWO 174mini on a separate tracking scope.

 

The order of equipment on both setups is: (following focuser, reducer etc)

1. ZWO OAG (with ZWO 174mini tracking camera)

2. ZWO 2" 7 place filter wheel

3. ZWO 1600mm main camera

 

 

Attached Thumbnails

  • OAG image.jpg

Edited by adamchicago, 27 September 2020 - 04:51 PM.


#2 Ken Sturrock

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Posted 27 September 2020 - 04:51 PM

Because the prism on the ZWO OAG is too small to fully illuminate the large sensor on the ASI-174.


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#3 adamchicago

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Posted 27 September 2020 - 04:58 PM

Thank you.  But that is a bummer, so there's basically no solution outside of buying another OAG with a larger prism, I assume?

 

 

Because the prism on the ZWO OAG is too small to fully illuminate the large sensor on the ASI-174.

 



#4 Ken Sturrock

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Posted 27 September 2020 - 05:36 PM

Yeah. I'm not sure why ZWO sizes it like that. Like most things, probably cost.

 

I'm not aware of another solution.

 

The ZWO 8mm OAG will perfectly illuminate the 5.6x3.2mm chip in the ASI-290 or the 4.8x3.6mm chip in the ASI-120. The 11.3x7.1mm chip in the ASI-174 needs a larger prism like the 12.5x12.5mm one found in the Celestron OAG (or the similar sized 0.5x0.5 inch prism in the QSI WSG cameras).


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#5 ryanha

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Posted 27 September 2020 - 07:40 PM

FWIW, This is why I bought the Celestron OAG (larger prism) vs the ZWO one (plus easy camera/guider rotation on the Celestron). The OAG and the asi174 are on their way to my house this week so hopefully Ill know within a week if it works as hoped.

 

 

--Ryan


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#6 KTAZ

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Posted 27 September 2020 - 08:40 PM

So far I’m very happy with the Celestron OAG. It is super adjustable and easy to use. 

 

Plus, it has a BAP (Big A$$ Prism) grin.gif.

 

Ryan, be sure that you pick up a short SCT adapter to replace the Celestron OAG SCT adapter straight away. It is a space hog and will give you grief on attaining proposer back-focus if you use a .63 reducer.



#7 KNak

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Posted 27 September 2020 - 09:12 PM

Another option that has a 12mm prism and is almost half the price is the Hercules one (S8239) on AliExpress. I don't have it but there's another CN user who had good things to say about it (I'd try but already have an OAG with my ASI174).  Depends how adventurous you are, and how long you are willing to wait I suppose.



#8 adamchicago

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Posted 27 September 2020 - 10:12 PM

Ok great, I'm going to give the Celestron a try, thank you.



#9 jdupton

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Posted 27 September 2020 - 10:36 PM

Adam,

 

   The Celestron OAG is great. I use one with my ASI294MC camera. The adapters that come with it allow you attach almost anything. The dovetail adapter design allows it to be rotated as needed if you want.

 

   You should be aware though, that it is large. That is, it takes up a lot of back-focus room. For a color camera, this is not much an issue. If you use it with your ASI1600MM camera and filter wheel, you may run into an issue if you also use a focal reducer on your scope. You need to carefully add up the length of the adapters you will need and make sure they all total less than the back-focus requirement of your focal reducer.

 

   Here are the optical path lengths for the OAG body and the adapters it comes with. You will need one of the adapters between the scope and the OAG body, another between the OAG and the filter wheel. Then you have to add in the path through the filter wheel, any adapters or extension couplings between the filter wheel and the camera plus the depth from the camera's threads to the sensor. If all that adds up to more than the recommended back-focus distance for the focal reducer, then you will have problems.

 

Adapter optical path lengths:

      OAG Body: 29mm
      SCT Adaper:25.3mm
      Male M42 and M48 camera adapters: 12.5mm
      Female M42 and M48 telescope adapters: 4.5mm
      Short T-thread Spacer Ring: 6mm
      Medium T-thread Spacer Ring: 11.55mm
      Long T-thread Spacer Ring: 24.25mm
      Medium M48 Spacer Ring: 11.55mm

 

      ZWO Filter Wheel: 20mm

      ZWO ASI1600MM Camera: 17.5mm

 

   If you do not use a focal reducer, you are probably good to go. If you use a focal reducer or field flattener, add up all adapter lengths to make sure everything will fit. At the least, you will probably need the OAG Body and two adapters. That will be about 29 + 12.5 + 12.5 = 54mm in addition to your FW and camera. That may put you at roughly 91mm plus any other assorted adapters that could be needed because of not matching the correct thread pitch and gender.

 

   It can be done for monochrome cameras but is much more difficult than with color cameras that don't need the filter wheel.

 

 

John


Edited by jdupton, 27 September 2020 - 10:37 PM.


#10 imtl

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Posted 28 September 2020 - 12:54 AM

The sensor of the 174 is relatively very big for that sort of a camera. So you are seeing vignetting because the prism so not large enough to cover it. Why does it matter to you? Are you having trouble finding guide stars? If you're fine with that then it should not matter to anything. Especially since you mentioned you're using it with a refractor. With the SCT you might have some issues when not using the FR. But otherwise you'll have plenty of stars to guide on. And PHD2 knows what to do even with faint star. I would not stress about it too much.



#11 WadeH237

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Posted 28 September 2020 - 06:40 AM

If you are able to find guide stars, why not just configure the guiding software to use a subframe?

 

I have an Ultrastar guide camera, which also has a larger sensor than my OAG can illuminate and that's what I do.


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#12 ryanha

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Posted 28 September 2020 - 10:10 AM

The sensor of the 174 is relatively very big for that sort of a camera. So you are seeing vignetting because the prism so not large enough to cover it. Why does it matter to you? Are you having trouble finding guide stars? If you're fine with that then it should not matter to anything. Especially since you mentioned you're using it with a refractor. 

Oh yeah, that is the right question to ask!

 

No need to chase an issue unless it is an issue :)

 

--Ryan



#13 adamchicago

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Posted 28 September 2020 - 02:19 PM

Thanks for the help and that's a great point, I don't know what I don't know at this point, have only been doing this a few months.  Yes I was able to guide off of the current image using PHD2 with my SCT however when switching to a refractor using a similar OAG and setup I was having awful tracking issues and starting to get a flashing red screen in PHD2 which usually means PHD2 is losing the tracking star.  While I could see the star on the PHD2 screen, with all of the tracking issues I was having in general, I decided to follow the other's recommendation to get a larger prism OAG to see if this resolves it. THEN I saw your's and the previous message about not necessarily needing to fill the screen as long as you see/track stars.  So at this point the Celestron OAG is due to arrive tomorrow and I'll at least give that a shot on the refractor.

Oh yeah, that is the right question to ask!

 

No need to chase an issue unless it is an issue smile.gif

 

--Ryan

 



#14 adamchicago

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Posted 28 September 2020 - 02:25 PM

Thank you very much for the detailed response, very helpful.  I already have about 60mm of multiple tube extensions on the SCT so definitely have room pull some millimeters out to accommodate the larger Celestron OAG.  However based on the newer messages stating I don't necessarily need to fill the screen for tracking purposes I am going to first try the Celestron on my other scope which is a refractor, which had really bad tracking with the new OAG...it has a 0.82 reducer/flattener with no extension tubes so this may push that back focus out of range, I'll have to see.

 

Definitely a learning experience here!

 

Adam,

 

   The Celestron OAG is great. I use one with my ASI294MC camera. The adapters that come with it allow you attach almost anything. The dovetail adapter design allows it to be rotated as needed if you want.

 

   You should be aware though, that it is large. That is, it takes up a lot of back-focus room. For a color camera, this is not much an issue. If you use it with your ASI1600MM camera and filter wheel, you may run into an issue if you also use a focal reducer on your scope. You need to carefully add up the length of the adapters you will need and make sure they all total less than the back-focus requirement of your focal reducer.

 

   Here are the optical path lengths for the OAG body and the adapters it comes with. You will need one of the adapters between the scope and the OAG body, another between the OAG and the filter wheel. Then you have to add in the path through the filter wheel, any adapters or extension couplings between the filter wheel and the camera plus the depth from the camera's threads to the sensor. If all that adds up to more than the recommended back-focus distance for the focal reducer, then you will have problems.

 

Adapter optical path lengths:

      OAG Body: 29mm
      SCT Adaper:25.3mm
      Male M42 and M48 camera adapters: 12.5mm
      Female M42 and M48 telescope adapters: 4.5mm
      Short T-thread Spacer Ring: 6mm
      Medium T-thread Spacer Ring: 11.55mm
      Long T-thread Spacer Ring: 24.25mm
      Medium M48 Spacer Ring: 11.55mm

 

      ZWO Filter Wheel: 20mm

      ZWO ASI1600MM Camera: 17.5mm

 

   If you do not use a focal reducer, you are probably good to go. If you use a focal reducer or field flattener, add up all adapter lengths to make sure everything will fit. At the least, you will probably need the OAG Body and two adapters. That will be about 29 + 12.5 + 12.5 = 54mm in addition to your FW and camera. That may put you at roughly 91mm plus any other assorted adapters that could be needed because of not matching the correct thread pitch and gender.

 

   It can be done for monochrome cameras but is much more difficult than with color cameras that don't need the filter wheel.

 

 

John

 


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#15 Ken Sturrock

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Posted 28 September 2020 - 04:27 PM

The question was about what caused the light fall off.

 

There are a number of folks who successfully guide with a 174 on a ZWO OAG. It only matters from the standpoint that the camera has such a wide FOV and its nice to use it to it's full extent, especially on a narrower-FOV instrument like the SCT.



#16 fielderda

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Posted 07 October 2020 - 09:35 PM

I just installed my Celestron OAG last night with my new ASI174MM Mini and had good success with guiding. The decision was made with lots of the CN threads that a large prism is required and a good size guide camera sensor.

 

EDGE HD8 - FR - OAG-spacer-ZWO 8-position EFW - ASI1660MM  ~105.5mm BF

 

With high Gain and 2.5 sec exposures, PHD2 was able to pick out a several stars to guide on.


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