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Dual-Narrowband filter artifacts

imaging lens making
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#1 GroundControlTo

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Posted 20 September 2020 - 09:52 PM

For my first foray into narrowband imaging, I selected the ZWO Duo-Band Filter that is for use with color cameras and has transmission for the H-alpha and O-III lines. This seemed like a good choice since I live in a relatively light polluted area (Bortle class 5 skies), and I'm currently using a Canon 70D DSLR for the camera. Overall I'm happy with the filter except for the artifacts in the brightest stars (see attached). Any suggestions for how to eliminate these artifacts, either physically or using deconvolution? I've tried building up an artificial PSF but so far they have not had the spikey parts that I want to remove.

 

--Tom

 

LagoonNebula-20200911-BNSCNRPCC-stretch-mlt-hdr-crop.jpg



#2 SilverLitz

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Posted 21 September 2020 - 07:26 AM

I would not expect the spike are being caused by the filter, but by something in the scope.  This looks more like diffraction spikes possibly cause by pinching.  A filter artifact will be halos, like a big bubble around the bright stars, cause by reflections.



#3 GroundControlTo

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Posted 21 September 2020 - 05:29 PM

I would not expect the spike are being caused by the filter, but by something in the scope.  This looks more like diffraction spikes possibly cause by pinching.  A filter artifact will be halos, like a big bubble around the bright stars, cause by reflections.

I thought it was diffraction spikes as well, but it goes away completely when I remove the filter (which is screwed into one end of the T-ring mount adapter. And for super-bright stars (like Vega for example), you do see a halo as well, but it is not apparent for fainter stars (and not such a bother as a result). 

 

-Tom



#4 SilverLitz

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Posted 22 September 2020 - 03:46 PM

I thought it was diffraction spikes as well, but it goes away completely when I remove the filter (which is screwed into one end of the T-ring mount adapter. And for super-bright stars (like Vega for example), you do see a halo as well, but it is not apparent for fainter stars (and not such a bother as a result). 

 

-Tom

Are you using the same exposure times w/ and w/o the filter?  (Probably not, as these extreme filters cut out a lot of light.)

 

I wonder if it shows up w/ the filter due to exposure time differences or stretching difference, e.g. would be there w/o the filter, but shorter exposures or less stretching hid them.  Just guessing.



#5 GroundControlTo

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Posted 25 September 2020 - 11:32 PM

I think I solved it (at least the spikes are gone). The issue was the filter was pretty far away from the sensor (4 - 5"), and it was contributing to some reflections off the tube between the filter and the sensor. Since I'm working toward a monochromatic cooled camera and want to have a filter wheel for that setup, I got the filter wheel first (a ZWO WFW Mini 5 position) and it holds the filter much closer to the sensor. Looking at the images tonight, it is much cleaner. There is still a bit of a halo with extremely bright stars (like Vega), but it is not noticeable otherwise -- at least so far. 

 

--Tom


Edited by GroundControlTo, 25 September 2020 - 11:33 PM.



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