Post processing filtered images

[malc-c]

Guys,

I use a CLS clip filter in my Canon 400D to remove light pollution. Whilst this removes the orange glow really well, it gives all my images a green/blue tinge. For example here is a stack of 16 x 7min exposures taken last night. I would love to get the lovely jewels and colour that other capture in the same galaxy.

I've tried shifting the red peak in DSS after stacking, but that just makes the background change. - any pointers would be welcome

[JustinLT]

The light pollution and other filters tend to outbalance the colors, this is normal. I use semi-apo filter (as I'm learning astrophotgraphy with an achromat), so this filter cuts violter, light pollution and IR. In total it gives a total mess to the color balance. It is always quite difficult to fix the colors after stacking, but it is manageable in PS using levels for different color channels.

First thing, what you should consider, is the white balance of the camera you are shooting. Some guys recommend to use daylight, some simply automatic. The goods news are that as the files are RAW, the white balance can be altered during the processing. The 2nd good thing, that you do not need to alter each RAW file, as DSS can do it for you.

Load one of your light images in DSS, select it, so it would be visible on the DSS window. If it is too dark, use the upper right tool to make the image lighter (just slide the middle triangle to the left side).

Ok, now what is important, is to tell DSS how it should interpret the image. For example your camera can show a total different color balance that your computer, as each machine uses different algorhytms for color interpretation and how it is shown in the final image. In DSS you can change some of the values. Open RAW/FITS DDP settings... and you should see two choices: RAW and FITS. For the moment forget the FITS, as you are using raw files. The first thing you shold notice are the white balance options. It is difficult to suggest an option for your setup, but I use both options ticket (meaning to use camera white balance, which is usually automatic and also let DSS to automatically calibrate the WB for you). You could try removing one or both and see what results it gives to your image.

Now the most interesting part is with the Color adjustments. I usually leave the brightness to default (1.0000). Now, depending on your setup, you may consider changing the values of red and blue scales.

Your filter takes away the red glow from your pictures, so usually it removes some amount of the red light. You may consider trying to increase the amount of red color and reduce the amount of blue color. Just play with the digits and you should see how it affects your image.

The one more important thing to note is the absence of the green channel. It is done so because usually the bayer CFA (color filter array) is made of RGBG for each pixer. This means, that green gain is 2x higher than red and blue, so, some guys, who shoot with DSLR's under black skies and do not use any filter tend to change the red and blue scales to 1.5 (as each pixes gets only 0.5 amount of green and red separately). There is one more but... the red color passes more easily through the atmosphere, so it would be the strongest one that reaches your lens if it would come from a perfectly equal RGB target, but the the bayer CFA would filter it and pass the most amount of green light, so the weakest would be the blue one that reaches the photodiode of the pixel. In your case the LPS filter clearly takes out some larger amounts of red and green, leaving the image blueish. As mentioned, try to play with the digits, also, other DSS options also have some impact (like chanel calibration, black point etc.). I suggest not to use DSS for color processing after stacking. Try to work on some stretching techniques in PS (if it is available for you) or you can even dive deeper to a more astro based software like Pixinsight, Iris (quite difficult to handle). With good techniques (levels, curves etc) it is possible to even the colors even in those images.

I hope I did not cause any more confusions

P.S. here is a 5 min very basic rework of your image, using levels, curves, color balance, selective color filter and some masking. The center and some background of the galaxy could use more yellow color, but this would require a longer work.

[JustinLT]

Btw, from the image you will clearly see the gradient, which needs to be removed as well, didn't bother to do that. Later would come some sharpening and noise removing techniques. After several hours of work this could be quite a nice image. Almost 2h of data is already quite good.

[malc-c]

Justin, thank for the in depth advice. I'll give that a go and see what comes out

[malc-c]

LOL - I made some adjustments as suggested and kicked off the stacking process. Right at the end after producing the autosave file it popped up an out of memory error. The PC has six cores and 16GB of ram, but apparently the way the application is written, being 32bit, can only address the max 32bit memory of 3.45GB or there about. - Time to look for another stacking program

[Cloudwatcher]

Like the widefield......hope you don't mind a bit of colour tweaking.

[malc-c]

Like the widefield......hope you don't mind a bit of colour tweaking.

How the hell did you do that !! - it's just what I was hoping for

[malc-c]

Well with a little (lot actually ) help from a friend the attached image was the result.

[AndyUK]

That's MUCH better - I hope you made a note of what he did... !