Derek - That's a fantastic result, and no, your post was spot-on. Many of us would like to know the specific steps you took in PS to do that - some of us because we use both PI and PS and your technique could be used to squeeze the last little bit out a given image, and others because they use PS exclusively after stacking in something like DSS. I'm thinking you're using a layer that's been filled with a neutral gray that you then adjust the intensity of to match the average background of the astrophoto, and then one of the blending modes to accomplish filling in the "holes" or subtracting out the "hills".
Thanks. I've had to do a lot of experimenting, imaging from a white zone, and your guess is correct. I created a flat background a few years ago, basically by taking one of my best color images that had a very sparce starfield and a small PN that were easy to remove, then with a combination of gaussian blur and dust and scratches filter, similar to JPMs tone mapping techniques came up with this background, which I adjust accordingly with color balance to taste and levels to achieve my desired background with most subsequent images.
With images in general I do the following:
1) If it is monochrome image I desaturated the flat background. If it is a color image, I adjust the color balance to roughly match that of the target image (or leave it if I prefer the color of the flat background).
2) My flat background has a "k value" of 80%. You find this in the window /info tab /CMYK levels. In the reference image, you find a uniform area of background devoid of stars and nebulosity, and run the cursor across this area watching the k value change as you move the curser.
3) In an image with splotchy but otherwise uniform target background, I'll determine the lowest k value (say 70%) and adjust the levels in the flat backround to perhaps 1-2% lower (brighter, say 69%) than that of the target background.
4) Copy the adjusted flat background and paste it over the target background.
5) Change the blending in layers to "lighten"
6) Adjust the opacity % to taste and flatten image.
With your image, because it was more a salt and pepper issue it was more guesswork and trial and error. I checked the k value of individual pixels in a magnified segment. The stars had a k value from 0 in the center to 5-10% at the edges and the darkest pixels (pepper) had a k value of 87%. So getting a general sense of the target background level and with successive trial and error, I adjusted the k value of the flat background to 35%, then 25%, then finally 15% which is what you see in the three examples in the previous post, again layering with a lighten blend. At 15% the image overall was ridiculously bright, so I adjusted the resulting background levels to a k of 50%. Normally, this method has very little effect on star quality and nebulosity, and even on fine nebulosity if done carefully. But in your example, because of the extreme nature of the background neutralization required (wide salt and pepper variation), the brighter stars took on a harsh edge. So a little "less crunchy more fuzzy" application with Carboni's Action Set would fix those up nicely. Anyway, that's about it.
Edited by schmeah, 30 April 2017 - 08:33 PM.