Before reading this, I would like to share these thoughts with you first:

Thoughts about processing L+OSC data (one shot color)

Thoughts about monochromatic and one shot color CCDs (OSC).

This is a recommendation of how to preserve the colors in OSC, DSLR as well as in RGB -processing:

No all astrophotographers are privileged with an observation site that provides clear-sky-nights "back to back" which can be utilized to gather real deep color-data. Many of us suffer from the disadvantage of being cast into a place with the equipment, where weather grants a couple clear nights within a couple months... Tragical, but real. This could be considered as a burden, but also as a chance to baldly take some steps which might be considered unconventional, in order to squeeze out all information in a data-set whch is comparably thin. Particularly in processing the colors in OSC (and DSLR)-images it is quite easy to loose the faint colors (mostly blue) and more importantly the color-distinctions (as RGB's most important advantage to OSC is the significant better color-distinction).

The acquired RGB-data; OSC-data or DSLR-data has to be pre-processed as usual in the software you are familiar with, including some DDP. When you bring the stacked and preprocessed image (Master-RGB, Master-OSC,...) to Photoshop for post-processing, you will have to strech the image of course. However, no matter what you use, be it levels, curves, multilayer (multiply+screen) or highlight/shadow tool (or at best a combination of all) in many times you get an image which is streched, but has lost intensitiy of colors, hence looking quite pale. Many people tend to boost colors with "saturation" in photoshop AFTER they have streched the color-image. This will certainly yield to severe color-noise, and a significant loss of a serious amount of colors, consequently. When you create your L-RGB image (or L-OSC or Pseuo-L-OSC, etc...) you will get heavily washed out colors including chrominance-noise in the end.

The alternative here is easy: Always boost the colors immediately after each and every single step of your streching procedure (bending the curves, pushing levels, etc). That way you will be able to perserve the faintest color-data in your color-image, particularly in OSC- and DSLR-data. (Is it possible to get too much colors? Well, as we always take small steps in streching, going back and forth, we can monitor what is happening; and quite frankly if you really should get too much color: it is always easier to get rid of too much, than to increase once the colors are washed away,...)

Color-enhancement can best be done by using 2 nice techniques, which are just about to become more and more popular.

Methode 1: The GaBany technique (please see footnote):

Create an adjustment-layer of the data you are about to boost the colors in. Change blend-mode to "soft-light". Copy this layer again and change blend-mode to "luminance". You will immediately notice some nice color-enhancement without any kind of color-noise, but you are not done yet. In "history" right click the last line and make a new document. Flatten all three levels, mark and copy the entire data. Delete this new document as we no longer need it to be open in Photoshop. Go back to the original image (3 layers should still be open) and delete the 2 new adjustmentlayers (softlight and luminosity). This should bring you back to where you have startet to enhance the colors. Now paste the copied data onto the image and a new layer will pop up. Change blend-mode to "color". You can repet this again and again (but I would next strech the entire image again and repeat color-enhancement AFTER the next streching procedure). You might have to change the opacity if you get too much colors. And you can decide to use the boosted colors where you want the to show up. (see below)

Copy the background-layer.

Copy again and change the blend-mode of "copy" to "soft light" and blend-mode of "copy2" to "luminosity" (you can also rename the ne layers if you wish,...need to).

Make a new document - right click in the last history-line to do so.

A new doc pops up - flatten all 3 layers

Mark and copy this new document - then delete it (it hovers in the memorya-bank)

Go back to the original image and and delete the 2 adjustment-layers you created. (Notice: the image looks darker now, as I have already deleted the luminosity layer! - softlight darkens the image...)

Paste the color-data (your created as the new document) onto the original image and change blend-mode to "color".

Adjust the opacity if needed. (see further below for masking this layer if needed.)

The alternative methode is more simple:

Create the adjustment layer and change blend-mode to "color". Go "image"-"adjustments"-"match-color". Push the "color-controll-slider" to the right (+20 or 30%) and maybe change the opacity, if you think you get too much of the colors. Sometimes, and particularly in OSC-data the blue-channel is a bit weaker than r and g (check in channel-mixer!). You might wanna change the opacity and also use this color-data-layer as a mask, in case you have a huge discrepancy between well saturated colors (red, and redish color tones) and blue. In this case you can create a mask - go"layer"-"hide all" and bring the colors only where you want them be boosted with the "brush"-tool. Feather the mask (not the layer! by making sure you have highlighted the mask) with a gaussian blur (4-5 pixels) to get a nice color-transition. (Alternatively you can use a "reveal-all" layer-mask in case you wan the colors to be pushed in the entire image except for some specific areas). As we are only boosting data which is already in the image, we do not create things being not available here. (remember: when you balance the colros in the pre-processing software you usally use, you do something similar to the color-data wou want to balance - like R.G:B=1.3:1:1.5 (just some sumbers)).

Create adjustment-layer and change blend-mode to "color"

Go "Image"-"Adjustments"-"Match-color"

Push the color-controll slider to the right (+20-30%)

Create a layer-mask "hide-all"

Use the brush-tool to paint the colors where you need them. NOTICE: the foreground-color must be WHITE!

Alternatively you can also use a layer mask "reveal all"

For using the "reveal-all"-mask the brush-tool fore-ground must be BLACK!

Finally flatten the image and your're good to take the next step.

Please notice: The chronology should always be:

Strech the image softly - enhance colors gently - strech the new image softly - enhance colors gently ... until you feel you are good to go with the image. NEVER strech the image in one single process, you might discard lots of good data you have spent so much time in gathering! AND I also would recommend not to boost color with "saturation". I almost never use this (except for some +5%).

No matter how you do it - make sure you have fun processing your data.

Dietmar Hager, M.D., F.R.A.S.

(1) Methode 1 was handed to me by the world famous CCD-astrohptographer R Jay GaBany (but has also been developed in several places as I learned - nevertheless I am very grateful for Jay's hints and tips he gave me over the past years)