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ES .7x Focal Reducer/Flattener Really Bad Imaging Pattern/Vignetting

CMOS refractor astrophotography
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#1 astroBomber

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Posted 25 February 2019 - 11:28 PM

I am seeing an unusual pattern on all my images taken with a ES 127 ED Apo

 

My equipment is as follows:

  • ES 127 ED Apo
  • Moonlite Focuser
  • ES .7x reducer/flattener
  • ZWO filter wheel (8 filters)
  • ZWO ASI 1600mm Pro

h3LKjQb.jpg?1

 

I have exactly 55mm of distance from the reducer to the camera sensor. I am able to achieve focus just fine, but all my images have a giant ring around them as visible in images and flats. Here are examples:

 

Flat frame taken using an R filter.

 

HcSxnqM.jpg

 

90 second Light frame of Sombrero Galaxy using R filter.

 

wNeXDmh.jpg

 

At first I thought it was because I am using 1.25" filters instead of 31mm, but if I remove the filter wheel from the equation, the problem is still there. I have noticed as I decrease the distance from my camera sensor to the focal reducer, the ring begins to increase in diameter essentially pushing it further out of the frame and reducing the problem. If I increase the distance, the ring diameter shrinks and the ring moves closer towards the center of the image. I have yet to try to achieve focus (haven't had a clear night in a while) with the camera closer to the reducer but I am expecting to have issues here since the product's back focus is 55mm.

 

I can use flats to get most of the ring out of the way, but this pattern seems unusual to me and I have seen un-processed photos of very similar imaging platforms (same telescope, camera and reducer) in same light polluted skies and do not see this pattern. I am out of ideas on how to fix this issue or if it is even fixable. Maybe there is something wrong with the reducer/flattener or maybe I should just move the camera closer than 55mm and hope I can achieve focus still. I am curious if anyone else has run into anything like this. Any help is greatly appreciated.



#2 sg6

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Posted 26 February 2019 - 03:52 AM

Not in any way serious about imaging, it is something I occasionally play with for the heck of it. My first thought was there is or appears to be a lot of stuff between scope rear end and sensor. Should something be removed? I have no idea what but it just looks "busy".

 

I thnk ES supply extension tubes for visual use - is there one of those in the path that needs removing?

My scope arrived with 2 extension tubes, not sure if 2 was a case of one too many but 1 for visual appear fine, none if DSLR added, not sure about the other therefore, spare perhaps?

 

I know that flattener/reducers have a back distance but that is also dependant on the thing sitting at the correct position along the axis of the scope. It is a case of "If the FR is at the correct position then the distance to sensor should be 55mm", also expect something saying soemthing like +/-10mm lol.gif

 

Crazy question: There are not any nice shiny surfaces inside the optical path is there? Sort of thiing that happens.



#3 happylimpet

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Posted 26 February 2019 - 05:29 AM

But it all calibrates out OK?



#4 tonyt

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Posted 26 February 2019 - 07:41 AM

Perhaps try a 10mm wide cardboard baffle at the front of the reducer to see if reflection is causing the problem? 



#5 astroBomber

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Posted 26 February 2019 - 09:51 AM

But it all calibrates out OK?

Yea everything calibrates ok. Auto-focus doesn't have any issue. If I take really good flats I can cancel most of the vignetting out and produce a good image. The problem is that taking perfect flats to cancel out that ring is very time consuming. Even the stars inside the ring seem to be focused and round. I might give Surveryor1's suggestion a try and see if there is some kind of reflection causing the ring.



#6 astroBomber

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Posted 04 March 2019 - 10:24 PM

I figured out the issue. The focal reducer was giving off a really bad reflection on the edge that connects to the focus tube. After using flocking material from scope stuff, I was able to pretty much remove the ring. I ended up adding the flocking material to the entire focus tube as well to even further eliminate ambient light/reflections.

 

Here is a picture of flocking material being applied to half of the reducer:

 

EDnw5H3.jpg

 

Here is the result looking through the focal reducer from where the camera would sit:

 

zHfWO7y.jpg

 

You can see the ring that was causing the reflection ring very clearly and see that it has been cut in half.

 

Here is my flat after flocking everything:

 

Sv3u37D.jpg



#7 Jim Waters

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Posted 04 March 2019 - 10:29 PM

Its good that the reflections are gone but something is still wrong.  A Flat Frame should look something like this.

 

https://www.google.c...551842905341777

 

EDIT - How are you taking your Flats?


Edited by Jim Waters, 04 March 2019 - 11:32 PM.


#8 astroBomber

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Posted 06 March 2019 - 03:18 PM

Its good that the reflections are gone but something is still wrong.  A Flat Frame should look something like this.

 

https://www.google.c...551842905341777

 

EDIT - How are you taking your Flats?

I wrap the telescope in a white shirt and using an LED board to create a uniform light and then I use sequence generator pro's flat frame calibration wizard to get the exposure times for each filter. 

 

Here is the LED board I use:

https://www.amazon.c...e?ie=UTF8&psc=1

 

Maybe it is the case that my tshirt is too thick or not clean enough?



#9 riot1013

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Posted 07 March 2019 - 09:49 AM

I am seeing an unusual pattern on all my images taken with a ES 127 ED Apo

 

My equipment is as follows:

  • ES 127 ED Apo
  • Moonlite Focuser
  • ES .7x reducer/flattener
  • ZWO filter wheel (8 filters)
  • ZWO ASI 1600mm Pro

h3LKjQb.jpg?1

 

I have exactly 55mm of distance from the reducer to the camera sensor. I am able to achieve focus just fine, but all my images have a giant ring around them as visible in images and flats. Here are examples:

 

Flat frame taken using an R filter.

 

HcSxnqM.jpg

 

90 second Light frame of Sombrero Galaxy using R filter.

 

wNeXDmh.jpg

 

At first I thought it was because I am using 1.25" filters instead of 31mm, but if I remove the filter wheel from the equation, the problem is still there. I have noticed as I decrease the distance from my camera sensor to the focal reducer, the ring begins to increase in diameter essentially pushing it further out of the frame and reducing the problem. If I increase the distance, the ring diameter shrinks and the ring moves closer towards the center of the image. I have yet to try to achieve focus (haven't had a clear night in a while) with the camera closer to the reducer but I am expecting to have issues here since the product's back focus is 55mm.

 

I can use flats to get most of the ring out of the way, but this pattern seems unusual to me and I have seen un-processed photos of very similar imaging platforms (same telescope, camera and reducer) in same light polluted skies and do not see this pattern. I am out of ideas on how to fix this issue or if it is even fixable. Maybe there is something wrong with the reducer/flattener or maybe I should just move the camera closer than 55mm and hope I can achieve focus still. I am curious if anyone else has run into anything like this. Any help is greatly appreciated.

I had this issue too, I took the M42 adapter off of the FR and sprayed the inside with chalkboard paint.  The inside of that adapter is shiny anodizing and will reflect light back into the lens causing issues like you are seeing.  Additionally I would spray all of the spacers with chalk board paint on the inside.  The ZWO spacers are still a little shiny on the inside as well. 



#10 gustavo_sanchez

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Posted 14 March 2019 - 02:55 PM

I have that reducer, and I haven't seen that issue. On the other hand, there's seems to be a tilt issue with the reducer because I'm getting slightly elongated starts on just one side of the image. Don't know where to fix it because everything is carefully screwed to the focuser (no nosepieces or clamps used, the camera is screwed to the extensions, the extensions to the reducer, and the reducer to the focuser). The focuser seems square to the tube.




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