Histogram Question

[tdgm]

Hello Everyone

Just a quick question in regards to the Histogram. Most recently it seems the red green and blue lines do not match up in the Histogram . This is a OSC image. It is just STF un-linked with BN/CC done to it. Refer to the attached image for reference . My question is should the histogram colors match up or is this histogram normal. On gallery images the lines eem to overlap each other. Obviously this is an image of a nebula.


Steve Herrman

[Rob Friefeld]

I've found that doing a Linear Fit before RGB combination gives well aligned peaks. Whether it looks better or not is your decision!

[jkmorse]

Steve,

The image doesn't look off but a simple test is to look at the background in various places.  It should come out relatively uniform across the three color channels.  Key is to make sure you do the testing where there is no nebulosity.  Linear Fit, as mentioned by Rob is an easy way to fix if the background is out of alignment but needs to be tested since in can throw off an image that naturally has a staggered histogram which often happens with nebulosity.

Best,

Jim

[tdgm]

Hello Jim


Thanks for the response. I looked at the readout numbers for the image and Red 0.0019 Green 0.0024 and Blue 0.0030. When I do a Linear fit numbers do become closer together. Thanks for you input.

Does the numbers out of balance effect non linear stretching?


Steve

[jkmorse]

Steve,

They can have a significant effect since you are stretching an unbalanced background that can create a fair amount of havoc when what you are trying to get is a smooth, grey background in your final image.  There are lots of tricks to help with that process, but recently I have been taking a different tack and waiting until after I have stretched the channels before combining them (several methods you can use but I like the result I get with the SHO-AIP script).   

As far off as your numbers are in the linear stage, you may want to try both methods to see which produces the best results.  Note you can also use Linear Fit after stretching.  The trick, however is making sure that linear fit or any other method used to fix the background doesn't mess up the color balance of your nebulosity.  That you manage with masks, protecting the area you DO NOT want to change and that's a good solution in both the linear and non-linear phases of your processing.

Best,

Jim