Processing Your Images in Photoshop

I won't go through how to stack or align your images here because the software you use - such as Deep Sky Stacker or AviStack - is set up to work well with the default settings and, if you want to play around with the settings, you'll have to understand the deeper principles of the software far better than I could do justice to here.

Suffice to say, where possible, do remember to add dark and flat frames to yout 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.

Once you've done this you'll have a grey .tiff file with very little visible detail - don't worry here comes the magic!

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.

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So this is how your image should look when you first open the stacked .tiff file into Photoshop. As I said, it's not pretty, but there's tons of detail in there just waiting to be brought out.

To get your head around this important process, I'm going to limit this tutorial to the basic functions you'll need. These are:

Curves - a grid showing where the light pixels are across the range of the image. If you click on the black diagonal line a dot appears which can be dragged up or down to draw detail out in that part of the image. 

Levels -  the black and white sliders at the bottom of the histogram let you adjust the the levels of the darkest and lightest pixels, while the grey slider in between lets you adjust the mid point. This allows you to keep your galaxy or nebula bright and the background sky dark after you've used the curves tool.

Shadows/Highlights - this isn't always necessary but can be very helpful to make your colours more vivid when you haven't been able to take hours of exposures. DSLRs don't collect as much colour as a CCD so a tweak in this area can make all the difference 

Brightness/Contrast - brightness is like levels but adjusts the light, dark and mid-range all in one go, so is rarely used. But contrast can be a useful tool for drawing out detail in dust lanes or the boundaries of nebulae.  

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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.

But it's worth knowing that moving the cursor over the image will turn the cursor into a pipette icon and left-clicking on the image will put a temporary dot in the Curves diagonal line so you know where to emphasise a particular part of the image - such as an area of nebulosity.

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2. If you have a grey peak in your histogram of sufficient width to manipulate (such as the images in steps 1 & 3) then you an ignore this step and move on to step 3. 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 drawing 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.

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3. Since the peak 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.

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4. Duplicate step 3 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 coming out in the image.

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.

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5. This next step is a matter of personal preference but if you haven't got a lot of exposures, or your DSLR isn't very colour sensitive, you might feel your colours are a little flat and need enhancing too.

If this is the case, click on Image from the menu at the top of the screen, hover over Adjustments and select Shadows/Highlights. You'll notice that the image immediately becomes brighter but don't worry, we'll sort that in Levels in the next step.

In the Shadows/Highlights box, go to the Adjustments and move the Color Correction slider up. Its default setting is +20 but +50 will nicely enhance the colour, while +70 will constitute an aggressive enhancement if necessary.

Again, if you think you've over-cooked it, you can always hit ctrl+alt+z to go back a step or two.

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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.