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Halo Artifacts From Alnitak

ZWO ASI6200MM Pro Equipment Filters H-alpha Artifacts
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IDDAN avatar
IDDAN avatar
I am using an AP 155 EDT f9 refractor with an AP flattener, Astrodon E series 2" unmounted Ha 3nm filter in a ZWO 7 position wheel and an ASI6200MM Pro camera on a photon cage and got these huge Alnitak halos. This is 10 frames integrated with Pixinsight WBPP. Each frame is 1200 sec exposure. I'm not sure if there is anything that can be done to prevent with this setup except frame a pic to avoid bright stars like Alnitak? Any ideas on the likely source of this stray light phenomena and how to eliminate or mitigate otherwise are welcome. I do not see these halos except with my Ha filter and with a very bright star. Moon was up and about 25% illuminated. Thanks!


Dunk avatar
Dunk avatar
I see a halo around Alnitak but I also see halos (or rather circles - they are not even around stars) all over the image.. I think you have something else wrong..?

 

Could this be your filter wheel?
GalacticRAVE avatar
GalacticRAVE avatar
this  looks like the pattern produced by the diffractive/reflective interaction of the lenslet array with the protective window of the camera. Mark Shelly on cloudy nights hat done a very thorough analysis of this including modeling it optically. So I think this is a well established issue when imaging in the neighborhood of bright stars.

The sad news is: very little you can do about it. Fixing it in post processing is in principle possible though tedious. you could create a pseudo flat from your master light (basically removing all structure but the lenslet reflections using MMT or the like, creative fill in photoshop can also be a very helpful tool). Alternatively you could use an image from a
different scope/camera combination and work with the multi-scale gradient technique developed by the pixinsight team to create a pseudo flat. I was able to rescue one of my images that way. 

Matthias
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Dunk avatar
Dunk avatar
this  looks like the pattern produced by the diffractive/reflective interaction of the lenslet array with the protective window of the camera. Mark Shelly on cloudy nights hat done a very thorough analysis of this including modeling it optically. So I think this is a well established issue when imaging in the neighborhood of bright stars.

The sad news is: very little you can do about it. Fixing it in post processing is in principle possible though tedious. you could create a pseudo flat from your master light (basically removing all structure but the lenslet reflections using MMT or the like, creative fill in photoshop can also be a very helpful tool). Alternatively you could use an image from a
different scope/camera combination and work with the multi-scale gradient technique developed by the pixinsight team to create a pseudo flat. I was able to rescue one of my images that way. 

Matthias

Would taking more/shorter exposures help?
GalacticRAVE avatar
GalacticRAVE avatar
Dunk:

not really, unfortunately - it is like any faint signal (wanted (the nebula) or unwanted (the reflection)): what basically counts is the total integration time, the time of the individual sub is basically irrelevant (within reasonable limits, ie you swamp the readnoise by sky, no oversaturated frames ...)
MaksPower avatar
MaksPower avatar
I’ve experienced similar before - grid pattern of circles is the result of light reflected from the microlens array on the sensor, which is then reflected back to the sensor by a flat surface - either a filter, or possibly the cover glass of the sensor itself.

in my case it was reflected off the filter - moving the filter  further away from the camera solved it well enough.

The ASI533 sensor can produce diffraction - this appears as 4 or 8 spikes around a very bright star.
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