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Night Sky Photography Noise Reduction

Night Sky Photography Noise Reduction

Thursday November 17, 2016

I recently came across a very handy tutorial on night sky photography noise reduction which I thought I would share. It appeared on PetaPixel a few months ago and is well worth a look. It involves working in the image channels which can be quite daunting but there is a YouTube video on the site as well as written instructions from the photographer they are crediting the technique with – Dave Morrow. If you can find a quiet couple of hours, and you have been searching for a good technique to apply to your night sky photos make sure you spend some time on this :)

The first thing to do (if you haven’t done so already!) is download the free Nik Software Suite . After you have finished editing your chosen image to your own taste, run the Dfine 2 program in Nik and then get yourself a cup of tea and watch the video. Then watch it again. It will take a while if working in channels is new to you. The best news is that once you are a little familiar with how it works you can save the process as an action, so that the noise reduction becomes a one-click process (well two, if you count clicking on Dfine 2 to get the ball rolling!)

Basically what this technique is enabling you to do is reduce the noise in the darkest areas (where noise is more an issue) than in the bright zones. As you would know, noise reduction is typically a far from perfect process – the more vigorously you try and reduce noise the more detail you lose. That is why this technique is so useful – it only reduces noise where it is most pronounced. You can select what parts of the image will have noise reduction applied too, by selecting the “darks”, the “dark darks” or even the “ultra darks”! – watch the video, it will all become clear – pardon the pun ;)

Aurora Australis on the Overland Track, captured in November 2015

Sample Shot – Aurora Australis on the Overland Track, captured in November 2015. Settings: Pentax K3; ISO 2500, 20 second exposure

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Image 1: Adjustments in ACR and no NR at all.

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Image 2: Running the Nik Dfine 2 program cleared up the noise but as expected at the expense of some detail in the sharper edges eg starbursts and ridgeline

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Image 3: Working on the channels as per the tutorial and applying NR to the “dark” channel. Some noise returns, as does a little detail/sharpness.

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Image 4: This time NR has been applied to the “super darks” channel. More noise appears as NR is not applied to the bright areas of the image ie the aurora. More detail retained too.

Dave’s tutorial is about Night Sky photography and doesn’t mention the aurora, so a little experimentation is required. This technique is designed to reduce noise in the darkest areas of an image but the aurora of course is bright. I found NR in the dark (or dark dark) channels to be a good compromise for aurora photos. NR in the super dark channels acts just as described ie the dark areas are cleaned right up but the bright areas (ie the aurora) remain a little noisy.

As I mentioned, it is a much more complicated technique to get your head around but well worth the time invested in figuring it all out.

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