I have asked the people responsible for AstroImageJ to implement something similar to ASTAP's "Extract raw Green pixels and combine". I think it would be a useful feature to have in there. They asked the exactly how it should work:
"Can you confirm the functionality you are looking for? We can consider implementing in AIJ. Are you wanting to extract the two green pixels in each RGGB Bayer group, average them together, and then produce an image from all of the averaged green pixel pairs which will result in being 1/2 x 1/2 the size of the original image? If you are looking for something different, please explain. "
Does ASTAP average the two gren channels or is ti doing something else. I just want to make sure they are doing the same thing.
Thanks,
Bill
Last edit: Bill Tschumy 2022-02-23
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You described it correctly. From each 2x2 pixels Bayer pattern the two green sensitive pixels are averaged. So the final image will be 1/2 x1/2 size of the orginal image.
The practical problems is that you have to know the demosaic pattern. Otherwise you start to combine red or blue sensitive pixels instead of green sensitive pixels.
Han
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The AstroImageJ people have implemented a Red, Green, Blue channel extraction based upon the above understanding. I tested it and got different results with ASTAP and AIJ. I posted in the AIJ forums about the descrepancy and Karen (one of the developers??) relplied.
She seems to think that ASTAP is interpreting the bayer matrix differently than AIJ. The FITS file (created by ASIAir) indicates the Canon 700D camera is generating RGGB. That has always been my understanding. However, if in AIJ, you specify the file is really GBRG, then the green extraction between the two matches. So one of you is in error. I'm not sure who.
I have asked the people responsible for AstroImageJ to implement something similar to ASTAP's "Extract raw Green pixels and combine". I think it would be a useful feature to have in there. They asked the exactly how it should work:
"Can you confirm the functionality you are looking for? We can consider implementing in AIJ. Are you wanting to extract the two green pixels in each RGGB Bayer group, average them together, and then produce an image from all of the averaged green pixel pairs which will result in being 1/2 x 1/2 the size of the original image? If you are looking for something different, please explain. "
Does ASTAP average the two gren channels or is ti doing something else. I just want to make sure they are doing the same thing.
Thanks,
Bill
Last edit: Bill Tschumy 2022-02-23
You described it correctly. From each 2x2 pixels Bayer pattern the two green sensitive pixels are averaged. So the final image will be 1/2 x1/2 size of the orginal image.
The practical problems is that you have to know the demosaic pattern. Otherwise you start to combine red or blue sensitive pixels instead of green sensitive pixels.
Han
Thanks
Han, I'm hoping you can chime in on this.
The AstroImageJ people have implemented a Red, Green, Blue channel extraction based upon the above understanding. I tested it and got different results with ASTAP and AIJ. I posted in the AIJ forums about the descrepancy and Karen (one of the developers??) relplied.
She seems to think that ASTAP is interpreting the bayer matrix differently than AIJ. The FITS file (created by ASIAir) indicates the Canon 700D camera is generating RGGB. That has always been my understanding. However, if in AIJ, you specify the file is really GBRG, then the green extraction between the two matches. So one of you is in error. I'm not sure who.
Here is the link to her findings:
http://astroimagej.170.s1.nabble.com/Extracting-green-channel-from-FITS-td1750.html#a1800
I suppose an ambiguity could arise from the assumed ordering in the superpixel. Does RGGB mean
RG
GB
or
RG
BG
In the second case you order the pixels in a clockwise fashion.
Hello Bill,
I can't acces the forum of AstroImageJ yet. I have send a request to the admin. Can you send the following two images?
The first is a marked up of the image from AstrimageJ. The left bottom pixel is position 1,1 as indicated.
The second image is a special test file i made some time ago. RG means RGGB
Your test image is RGGB. That looks correct looking to the star colours.
https://www.otherwise.com/Light_RAur_30.0s_Bin1_20220207-200746_0001.fit
Cheers, Han
Last edit: han.k 2022-03-02
This is what you see if you specify RGGB pattern. Only the yellow marked is correct red, green, blue
I just got access to the forum. so posting the images is no longer required.
Cheers, Han