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Source of Bright Star Reflections?

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6 replies to this topic

#1 Gary Imm

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Posted 07 November 2016 - 10:11 AM

I have been pretty happy with my setup for deep sky objects:

Orion EON 130mm ED Triplet Refractor / Orion EON 3" field flattener / ZWO Mini EFW with Astrodon Filters / ASI1600MM-Cool Camera

 

But for very bright stars, I always see a distinct symmetric pattern around them.  Attached is an example, cropped of a Pleiades star from a 5 second blue filter sub.  Of course, the pattern is much more obvious in the integrated master.

 

What is your view on what is causing this?  I assume it is the CMOS window or the flattener.

 

Is there any way to prevent it (aside from not imaging bright stars)?

 

Thanks,

Gary 

Attached Thumbnails

  • pleiades_5s_bluesub_.jpg

Edited by Gary Imm, 07 November 2016 - 10:13 AM.


#2 nikao

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Posted 07 November 2016 - 10:19 AM

doesn't look like reflection, but more something like blooming? 
I'm not familiar with this camera, so not sure if it is a known 'problem' of it? 



#3 Gary Imm

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Posted 07 November 2016 - 10:26 AM

Just to add - my camera settings are typically 200 gain, 35 offset, and 40 USB.  For this camera, that gain is a bit higher than the unity setting of 139. 



#4 happylimpet

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Posted 07 November 2016 - 10:34 AM

Rotate the camera and see if the pattern rotates with it. I expect it will. I suspect reflections/diffraction from the sensor, a little like a diffraction grating. Is there anything else in the system with that symmetry?


Edited by happylimpet, 07 November 2016 - 10:34 AM.


#5 james7ca

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Posted 07 November 2016 - 10:34 AM

I think that's a combination of diffraction (perhaps from the lens spacers) and also artifacts from the micro lenses on the sensor's photo sites.



#6 Gary Imm

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Posted 07 November 2016 - 11:46 AM

A few more points:

 

- This pattern is usually only apparent in the final integrated master when a bright star saturates an area which is more than 10 pixels by pixels wide in the sub.   Smaller than that, and the pattern does not appear.

- When it occurs, the pattern seems to always be aligned with the CMOS dimensions, with the four main spikes being oriented exactly up/down/right/left on the image.



#7 happylimpet

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Posted 07 November 2016 - 12:40 PM

A few more points:

 

- This pattern is usually only apparent in the final integrated master when a bright star saturates an area which is more than 10 pixels by pixels wide in the sub.   Smaller than that, and the pattern does not appear.

- When it occurs, the pattern seems to always be aligned with the CMOS dimensions, with the four main spikes being oriented exactly up/down/right/left on the image.

So I would guess its a reflected diffraction effect from the CMOS, being reflected back by some optical element close to the chip. Maybe a filter. You could jig things around if possible but probably hard to eliminate.




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