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Tutorial – Creating contrast in nebula images

Since the weather isn’t cooperating enough for me to capture more images of NGC 7000, let’s use the H-alpha channel image to show a few ways to create contrast in otherwise “flat” looking nebula images.

Here is the image basically right out of calibration, with only a single quick curve applied to stretch it a little.  This is six hours of total integration time (20 minute subexposures).

It’s okay, but nothing stands out.  There nebulosity has no definition.  It’s more like fog than a distant cloud.  Before anything else, the first step is to shrink the stars, because the unsharp mask filter I’ll use in subsequent steps has will exaggerate the stars.  You could move the stars to separate layer, but this is a short example, and that would require another tutorial entirely.  Here, all we do is run the minimum filter (Filter > Other > Minimum) with a radius of 1.  The original image size is about 3100 x 2400, so even the small stars can withstand the Minimum filter at this setting.  Smaller images may need to be resampled to avoid the destruction of smaller stars.

 

A look at the histogram reveals that the image isn’t taking up the full dynamic range, so we’ll use Curves to bring the white point down.  We’ll also subtly shape the bottom end of the curve so we bring up the darkest parts of the nebula at the same time.  This is why I always use Curves instead of Levels, even for simple adjustments.

Now, let’s create contrast between the smaller structures in the image.  The next image shows the results after unsharp mask (Filter > Sharpen > Unsharp mask) with a radius of about 10, a threshold of 3, and a strength of about 150%.  This was done to a layer containing a duplicate of the original image, and the layer was then set to the Soft Light blending mode.  Every image will require different settings, but the beauty of the filter is that you can preview the results as you make adjustments to highlight the details you want.  Because this filter reduced some of the darkest areas around the periphery more than I’d like, I used a layer mask to mask those areas off from the effects of the filter.

 

Things are looking much better.  The edges are better defined now, but I’d still like to create more contrast within the core of the nebula.  So I’ll duplicate the image again to a new layer and apply the unsharp mask again, this time with a radius of 90 pixels.  This creates contrast across larger structures.

 

Finally, it’s time for one more application of curves.  I’m still not happy with the contrast across the body of the nebula.  A subtly shaped curve will allow me to choose the areas I want to differentiate.  By holding down on control while clicking the image, I can create anchor points, shown below as A, B, and C.

 

 

The image is in pretty good shape now, at least for a monochrome image:

Now, let’s hope the skies clear so I can capture some S-II and O-III images to combine with this one!

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