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DBE Normalization

Gordon Aspin
in PixInsight
Hi Adam

I'm currently processing an OSC image of the IC405 area. I've used SXT to remove the stars and am using DBE subtraction to remove some gradients in the starless image. I found that after applying DBE I lost a lot of contrast as the mean level in the image shot up from around 0.03 in each channel to 0.35. This surprised me, I'd never noticed this happening before and I'd expected that the mean should be lower after subtracting the gradient, not higher.

But then I noticed the normalise checkbox (I've always wondered when I should use this). This was unchecked, so I checked it and with this setting the mean levels after gradient removal were similar to those before doing DBE and the resulting image looked much better. Can you explain when I should and shouldn't use the normalise option in DBE?

Thanks
Gordon

Comments

  • BlockHeadBlockHead
    There is no problem with using the Normalize option. It will apply the correction around the calculated median of the image (the background). It is necessary to use this when adding starless and star-only images in their linear form. I explain and demonstrate in a number of videos.

    By *not* re-normalizing you can take advantage of the extra contrast of further subtracting from background like areas. This raw subtraction (or addition as you found) can help show the other features in the field. 
    I use this to my advantage on non-linear images of nebulosity or dust. It is a little secret sauce thing unique to my method. I show this in the most recent workflow example (NGC 1491)- but it is better demonstrated in my NGC 1333 (dusty) workflow video.

    -The Blockhead
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