L+RGB Processing

This tutorial starts after you've stacked your luminance images and your RGB images to create two .tiff files for processing in Photoshop or GIMP.

However, where possible, do remember to add dark and flat frames to your stacking procedure. Incorporating dark frames (exposures of the same length as your image files, with the lens cap on you camera) will greatly reduce image noise by subtracting light from defective pixels; while flat frames (fast exposures of a uniformly white background) will reduce the pale hue at the edge of your images - known as vignetting - and any artefacts caused by dust on any of your lenses.

There are many Photoshop tricks and formulae that can be employed to draw the detail out of your images, but the truth is that every image will need to be treated in subtly different ways. There is no one 'cure all' procedure that will get every image to look the best it can.

So this tutorial will be a starting point - or an average process - that 'works well' on every astroimage while giving you enough information to be able to try things out for yourself and learn how to get the best out of your images. Your own eyes are your best judge.


So, the first thing you'll need to do is open the luminance and RGB files in Photoshop as layers. This is done by clicking 'File' in the left hand corner, selecting 'Browse in Bridge' and selecting the two files via this application (Ctrl+left click allows you to select them both individually). Once selected, click 'Tools' on the Bridge Desktop menu, select 'Photoshop' and click on 'Load Files into Photoshop Layers'.

Look at the 'Palettte' on the right hand side (where your layers now appear) and make sure the Luminance file is above the RGB file - if they're the other way around, click on the luminance file and drag it to just above the RGB. Then, with the luminance layer highlighted, click on the drop down box and choose 'Luminosity'. This will blend the images together but still allow you to modify them separately for your desired result.

If at any point during the following stages, you are not happy with the result of an action, just click 'Edit' in the Photoshop/GIMP menu and select 'Step Backward' (Windows shortcut:Ctrl+Alt+Z).

Ctrl+Z allows toggling between your current and last action, so you can appraise your latest action.


1. The first step is to open up the Curves window by selecting Image from the menu at the top of the screen, hover over Adjustments and select Curves. The shortcut in Windows is Ctrl+M. 

You'll see the histogram has a grey peak. The more data you've collected while imaging, the more you'll be able to widen this peak. The grey peak is the data is that you want to bring out.

By clicking on the diagonal line in the histogram and dragging the line up or down, there are many ways of bringing this data out. Some favour clicking in the halfway point and raising it up slightly. Some suggest clicking in the middle and then clicking the halfway points between the middle and edge so you can turn the diagonal line into a gentle 'S' shape. Others have elaborate 10 or more points in the diagonal to really customise it.

Gentle use of the Curves tool should be used on both the luminance and RGB layers to enable you to bring out the detail and the colour in your image. As you've already blended your layers together you'll be able to see how each tweak affects your overall image.


2. If you have a grey peak in your luminance and RGB histograms of sufficient width to manipulate (such as the images in steps 1 & 3) then you can ignore this step and move on to step 4. If you see a grey line instead of a peak it just means the peak is very narrow and needs to be stretched out a little first.

This can be done by simply selecting Image from the menu at the top of the screen, hover over Adjustments and select Curves. The shortcut in Windows is Ctrl+M. Then click in the middle of the histogram and gently draw the line to the upper left (like drawing a bow). Don't over do it as repeating this step is better than a single aggressive manipulation here.

You may find that the peak is very near to the left hand edge - also making it difficult to manipulate but, as you stretch the peak, it'll start moving to the right.


3. Since the peak in your histogram is the data you want to enhance, a simple and effective way to bring this out is to left-click on the diagonal line, right on the right-hand edge of the peak, and drag the line up slightly. 

Then left-click on the diagonal line at the left-hand edge of the peak, and drag the line down slightly. 

You'll notice that your image is, very subtly, beginning to show.

If you have hours worth of exposures on bright galaxies, you may only need to do this step once but often you'll need to repeat this step.

However, on this first step you should left-click just to the right of the grey peak and pull the line down very slightly - but never far enough that the black line crosses over the original diagonal line. This stops over-exposure.

As you stretch the RGB layer you might find that the peak appears as two or three peaks instead of one. If this occurs click here for a tutorial on aligning the RGB channels.


4. Duplicate step three but don't repeat the final part - only click the left and right edges of the peak to stretch the peak out once more.

On multiple iterations you'll really notice the data in the image coming through. 

At this point if you zoom into the image by clicking Ctrl and + (Ctrl and - zooms out), you'll be able to see if your image can take another stretch.

As multiple gentle tweaks are far more effective than one giant stretch, repeat the gentle stretch step with curves until you start introducing noise - this will become apparent as the image object will start looking grainy - ctrl+alt+z will take you back a step if you've stretched too far.

5.At this point you can look at your image more critically and ask yourself if you are happy that you've drawn enough detail out without going too far and introducing noise - if you feel you have made it too noisy, you can click Edit > Step Backwards on the luminance layer. If you've overcooked the colour you can click Edit > Step Backwards on the RGB layer. Or you can do extra iterations of steps 2,3 or 4 if you think you could do with bringing more out in either layer. 

When you're happy with the level of detail you've obtained (and remember: you can only play with the data you have - 20 minutes of exposures will not have as much detail to manipulate as an hour's worth), click 'Layer' on the top left toolbar and select 'Merge Visible'.


6. The final step is to get rid of the milkiness in the image without darkening the detail you've just worked hard to bring out. So we'll use the Levels tool here.

Click on Image from the menu at the top of the screen, hover over Adjustments and select Levels. The shortcut in Windows is Ctrl+L.

The black peak has the black level slider at its far left, the white level at its far right and the mid point in the middle.

What you'll want to do is move the black point slider slightly to the right to either meet, or just pass, the start of the peak and then move the mid-point slider to the right until the background is dark but the object remains bright. Your eyes will tell you when you've got the balance right.


There are a few more advanced techniques you could employ that would enhance your images further, but getting used to the basics and learning how to personalise your curves will pay dividends later.