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PixInsight => General => Topic started by: Lightpath on 2018 October 22 20:34:53

Title: asi1600mm Bright Star Reflections
Post by: Lightpath on 2018 October 22 20:34:53
Hey all, I'm curious if anyone has any idea how to process out these reflections from Alnitak...  I'm using an ASI1600mm pro cool and an 80mm refractor with a 300s exposure.

https://i.imgur.com/mycuMHO.png

Maybe a synthetic flat?

Thanks very much for any thoughts about how to correct this, I have a fair bit of data with this problem.
Title: Re: asi1600mm Bright Star Reflections
Post by: Theresamarie1 on 2018 November 30 11:14:00
Hi Lightpath,
I know this is old and there are no responses, but I've been seeing what appear to be the same kind of spikes on my ASI1600 images.   While they're fainter (maybe less exposure and fainter stars) they are there.   On my images, it almost looks like spider spikes, while I'm using a refractor!   I can see that yours are much more extensive and beyond spider-spikes.   Really odd.   So it's either the camera or some odd artifact of having an OAG in the field. Are you using an OAG?  Do you have any further insight from other sources?

Terri


Hey all, I'm curious if anyone has any idea how to process out these reflections from Alnitak...  I'm using an ASI1600mm pro cool and an 80mm refractor with a 300s exposure.

https://i.imgur.com/mycuMHO.png

Maybe a synthetic flat?

Thanks very much for any thoughts about how to correct this, I have a fair bit of data with this problem.
Title: Re: asi1600mm Bright Star Reflections
Post by: RickS on 2018 November 30 13:57:04
The artefact appears to be a combination of diffraction from the microlens array on the sensor and reflection from the inside surface of the coverglass.  Other cameras using the same Panasonic sensor have shown similar patterns on bright stars.

I've not had to deal with this yet but my first approach would be to build a mask that isolates the worst of the artefact and then reduce the brightness with CurvesTransformation and maybe blur it a little with Convolution or one of the wavelet processes.  For the mask try RangeSelection to grab middle brightnesses (you don't want the background or the core of the big star) and then multiply that mask by the original image.  If the mask includes a lot of stuff you don'r want, draw a preview around the section you do want and then use the SubstituteWithPreview script to copy the preview into a black image (create with PixelMath "0".)  It will probably take some messing around and experimentation to get the mask right.

Cheers,
Rick.
Title: Re: asi1600mm Bright Star Reflections
Post by: sharkmelley on 2018 November 30 16:58:28
There was some analysis done on these diffraction patterns here:
https://www.cloudynights.com/topic/635937-microlens-diffraction-effect-explained/?p=8967693

It doesn't help with removing them though!

Mark
Title: Re: asi1600mm Bright Star Reflections
Post by: Geoff on 2018 November 30 18:59:46
For the mask try RangeSelection to grab middle brightnesses (you don't want the background or the core of the big star) and then multiply that mask by the original image. 

Cheers,
Rick.
Hi Rick
Could you elaborate on what the multiplication achieves?
Geoff
Title: Re: asi1600mm Bright Star Reflections
Post by: RickS on 2018 November 30 23:04:26
Could you elaborate on what the multiplication achieves?

G'day Geoff,

I have just discovered that the "Screening" option for RangeSelection does the same thing, so that's an easier way to achieve it :)  Maybe it wasn't an option when I started using RangeSelection or perhaps I just missed it?  Anyway, the point is for the final mask to match the brightness levels of the data rather than using a simple binary mask.

Cheers,
Rick.