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LRGB Channel Pre-Processing

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#1 Andy T.

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Posted 25 May 2018 - 07:19 AM

Hello everyone!

 

 

I finally had one clear night I was able to get the telescope out and attempt to do some LRGB imaging with my new ZWO camera. I picked the Whirlpool Galaxy as a target and did (as a test) five 120s images on LRGB filters. The individual frames turned out pretty good and I figured I'd get at least a somewhat modest image in the end (for the short duration of exposure). This is where I hit an issue. Processing the LRGB files.

 

I used Pixinsight and the Batched Preprocessing script like I've done on other images in the past (from the DSLR). I had prefixed the individual files so that Pixinsight recognized the individual filter files and in the Light Files section of the script it had the individual channels separated correctly. This raised the big question though: do I WANT to do ALL the channels at once?

 

I ask because in the end, after the files were run through BPP, I opened the Master files at the end and used the LRGBCombination tool, with everyone set at 1 for weights. The end result? I got what (for practical purposes) was a B&W image. The individual channels (for RGB) were all perfectly aligned and seemed equally intense. So I had really no color to the image. Do I need to process each channel through BPP separately (with the same reference file)? Or did I just have crappy luck? Or even yet, something further I need to do to the individual channels before combining them in the Combination Tool?

 

 

Scouring some tutorials this morning (while at work) to see if I can find something, but nothing so far is leaping out yet, so thought I'd ask. Did some searches on the forum here too but nothing specific to an "all white image" or anything similar. Heh.

 

Thanks!



#2 Ishtim

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Posted 25 May 2018 - 08:12 AM

The few times I've used BPP, I calibrated images for each filter separately.  As for combination, I use the LRGB process to create an RGB image (unchecked the L), then after further processing on Lum and RGB images, I go back to the LRGB process, uncheck the RGB, check the L, select the L image and apply it to the RGB.

 

Lots more detail on the BPP and LRGB process in various places (harrysastroshed videos, lightvortex astronomy, trappedphotons, and of course Keller's book) 

 

My PI advice is to (have fun) take your time and take good notes cause you will revisit the same processes over again.



#3 Alex McConahay

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Posted 25 May 2018 - 09:12 AM

You can make an RGB and a separate L. Process them each separately and then combine them. Or you can combine into one LRGB and process that. Either process will work.

In either case you can have a problem if your L overwhelms your colors. A linear fit to equalize the strength of the four channels in the first place overcomes this.

Alex

#4 BenKolt

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Posted 25 May 2018 - 10:32 AM

Andy:

 

There's no one way to do LRGB, of course.  In my experience I've found it more straightforward and efficient to calibrate and combine each channel separately, then combine just the RGB channels without the L.  I then process L and RGB separately.  This allows me to concentrate better on establishing the best detail and noise reduction in L as well as best color combination and noise reduction in RGB, and by doing it separately I'm less prone to messing up L when working on color or vice-versa.  Depending upon the situation, I may do quite a bit of the processing separately before combining, or I may just do a little and then carry on with the last processing after combination.  Again, there's no one way to do it, but this is my usual practice.

 

Ben



#5 Andy T.

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Posted 25 May 2018 - 11:43 AM

Thanks for all the replies everyone! Poured over most of the tutorials out there on the processing and the comment above about the L channel washing out the others looks to have been the key. I can safely BPP all the files together it seems, but the actual LRGBCombination tool I had to do the RGB and L seperately. Doing that, I got a nice color out of the image to the point it actually looks fairly decent. Adding in the L channel afterwards (after processing it too) resulted in a very nice brightening of key parts to bring out more details. Given it was only about 10 minutes of each channel, turned out surprisingly well! Yet more to learn at this point. Heh :)




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