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Stars stretched in one corner

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#1 Stephanh

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Posted 17 August 2019 - 10:24 AM

I am trying a new ES 127mm FCD scope and used an AT field flattener with it for first light.

The stars are stretch along one side (left) and even more in one corner (top left in the image below).  Would a different FF help with this or is it an alignment issue in the imaging train?

 

thanks

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#2 ngc7319_20

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Posted 17 August 2019 - 11:04 AM

Alignment issue.  Something is de-centered or tilted.


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#3 Gipht

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Posted 17 August 2019 - 11:53 AM

What is interesting about your picture is that the stars look good  for maybe 90% of the picture,  then get terrible very quickly.  A tilt issue would be more gradual and uniform.  If it were me, I would try moving the spacing in 1 mm and see what the result looked like.  You seem very close to getting the best out of the FF, but maybe a small change would be an improvement.  Even with the result shown, you have a large width of very good stars.



#4 Stephanh

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Posted 17 August 2019 - 12:59 PM

What is interesting about your picture is that the stars look good  for maybe 90% of the picture,  then get terrible very quickly.  A tilt issue would be more gradual and uniform.  If it were me, I would try moving the spacing in 1 mm and see what the result looked like.  You seem very close to getting the best out of the FF, but maybe a small change would be an improvement.  Even with the result shown, you have a large width of very good stars.

Thank you for the reply.  I agree, tilting would be more gradual.

Are you suggesting moving the distance between the FF and the camera by 1 mm increments by adding shims?



#5 ngc7319_20

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Posted 17 August 2019 - 01:15 PM

Probably the optical axis of the main lens is not coincident with the optical axis of the flattener.  Might be a tilt / mis-collimation in the main lens.

 

I suppose it is also possible that the lenses in the flattener are slightly mis-aligned or tilted.  Probably you could do some test like rotating the flattener 180 degrees, and see what happens to the images.  

 

Spacing change might help, but this is not a simple spacing problem.  Spacing error alone would make all the corners elongated together.

 

Are you using all threaded connections?  Or are there slip-in 2" connections with the focuser?  Wonder if the flattener + camera is tilted in the focuser...



#6 Stephanh

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Posted 17 August 2019 - 01:54 PM

Probably the optical axis of the main lens is not coincident with the optical axis of the flattener.  Might be a tilt / mis-collimation in the main lens.

 

I suppose it is also possible that the lenses in the flattener are slightly mis-aligned or tilted.  Probably you could do some test like rotating the flattener 180 degrees, and see what happens to the images.  

 

Spacing change might help, but this is not a simple spacing problem.  Spacing error alone would make all the corners elongated together.

 

Are you using all threaded connections?  Or are there slip-in 2" connections with the focuser?  Wonder if the flattener + camera is tilted in the focuser...

There is a 2 inch slip connection to the focuser.  When I saw the corner elongation,  I tried re-adjusting the FF inside the focuser thinking it was a bad connection but it changed nothing.   I have used this FF with 2 other telescopes without a problem.  This is a new scope and I am hoping it is not an issue with the scope.

This FF (2" ES flattener) was reported to work well with this ES 127mm scope by another user.  

 

But as suggested above, wouldn't a tilt give me a gradual elongation across the entire image, going from nothing to real bad.     Right now  it is good for most of the field and then goes bad all of a sudden. 



#7 ngc7319_20

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Posted 17 August 2019 - 02:14 PM

There is a 2 inch slip connection to the focuser.  When I saw the corner elongation,  I tried re-adjusting the FF inside the focuser thinking it was a bad connection but it changed nothing.   I have used this FF with 2 other telescopes without a problem.  This is a new scope and I am hoping it is not an issue with the scope.

This FF (2" ES flattener) was reported to work well with this ES 127mm scope by another user.  

 

But as suggested above, wouldn't a tilt give me a gradual elongation across the entire image, going from nothing to real bad.     Right now  it is good for most of the field and then goes bad all of a sudden. 

A tilt on the sensor would give a gradual error.  But a tilt on the main scope lens or flattener would give a sudden big error.  These flatteners seem to work well only on the specified image or sensor size, and then there is often a huge error just past that size.  In your image, I am guessing that the well-corrected region is slightly offset towards the bottom right.  Possibly the telescope main lens optical axis is slightly offset towards that bottom right corner. 

 

You can do some tests by rotating stuff -- rotate the flattener + camera 180 degrees in the scope and see what happens.  If the bad corner moves to the opposite corner, then it is an issue with the main scope.  If it stays in that corner, then it is an issue with the flattener + sensor assembly.  You might need to get a cheshire collimation tool and test the main scope.  Possibly there are tip / tilt collimation screws on the scope lens.  Or sometimes the focuser is mount on the scope with a slight tilt.  Or the flattener is not seating quite perfectly in the 2" focuser opening.



#8 Stephanh

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Posted 17 August 2019 - 02:27 PM

""These flatteners seem to work well only on the specified image or sensor size, and then there is often a huge error just past that size. ""

I used that camera/FF combination many times without a problem. ES sells that flattener as being an all around FF for refractors.
I am starting to think the issue is with the telescope.

#9 ngc7319_20

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Posted 17 August 2019 - 02:32 PM

""These flatteners seem to work well only on the specified image or sensor size, and then there is often a huge error just past that size. ""

I used that camera/FF combination many times without a problem. ES sells that flattener as being an all around FF for refractors.
I am starting to think the issue is with the telescope.

Yes, if it has worked well with other refractors of similar F-ratio, then I might suspect this new scope needs a small adjustment.  Its not very far off -- most of the frame is fine -- it is just the ~5% of the frame width near the top left.




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