Processing a Milky Way image using the Photoshop “Stack Median” filter

Milky Way from Whitesands Bay
Milky Way from Whitesands Bay

This image of the Milky way was taken a few weeks ago in Pembrokeshire from the car park at Whitesands Beach at about 10pm. On the far right you can see the South Bishop Light which stands on its own small island West of Ramsey Island; it is about 9 km away to the South West.You can see it in my Astro Gallery.

The final image is made from 14 identical 30 second exposures at ISO1600 taken in sequence using a Sony NEX7 camera with a Samyang 12mm lens at f2.0. I have in camera noise reduction turned on. The lens is manually focussed, and I have the camera set to manual for exposure to keep everything the same between successive images. Once imported into Lightroom, I adjusted the colour balance of one of the images and then synced that across to the other 13. The 14 images were then exported to Photoshop CC14 as layers in one image. Below is the method I used:

You need to create two separate sets of 14 layers – one for the foreground and background – in fact “the ground”! And one for the sky and stars. So the first step is to duplicate all 14 layers, and group the 14 copies into a smart group which you can name “ground”.

In each of the successive images the stars will have shifted, and for what we want to do with the “sky” set of 14 we need them all aligned. We can get Photoshop to do this automatially but we need to get PS to ignore the ground – we do that by roughly masking out the ground in each image so that it just has stars to work with.

So, you hide all the layers except one of the sky layers to work on; create a layer mask for the visible layer and then thoroughly mask out the ground and any fixed lights etc. It doesn’t matter if you mask out some of the sky, but make sure you mask out all of the ground. You can then copy that mask to each of the other sky layers.

Unhide the sky layers, select all 14 and then Auto Align the layers; then delete all 14 layer masks. Select the 14 sky layers and convert them to a smart object which you can name “sky”.

What you now have is two smart objects: one “sky” with the stars lined up; one land which was already lined up unless you accidentally kicked your tripod!

Here is the clever bit – it does require the extended version of PS, or CC14. We are going to use Layers > Smart Objects > Stack Mode > Median to perform some clever processing which will substantially reduce the noise, particularly colour noise. Where there is a pixel at a particular position in the image with the same value in each of the 14 images, that same value will be used in the final result – eg a star. Where there is a pixel whose value changes randomly in each of the 14 images then the median value will be used in the final result – eg noise and plain sky. As the noise is random and if there are sufficient images as a sample to work on, then the result of this process is to cut out a large amount of the noise.

So we do the Layers > Smart Objects > Stack Mode > Median on each of our two smart objects.

Then Finally, select the foreground, add a layer mask and paint out the blurry sky to reveal the noise free sky layer – here, more care is needed in the masking to get the boundary between ground and sky just the way you want it.

After that it is “just” a matter of processing to bring out the details in the Milky Way, etc. I do this back in Lightroom.

Each of the 14 files from the NEX7 is a 25Mb dng file – thats a lot of pixels for PS to handle when it has these 14 images loaded up as layers in one file. So the processing is not instantaneous particularly when converting to smart objects and running the median filter. I noticed that the processor was running at about 60% but that 97% of my 8Gb of RAM was being used, so perhaps it would be quicker with more RAM. There are also issues with saving TIFF files over 2Gb. In any event, I flattened the layers after the PS processing was complete as there was no need to save the layers as it could all be done again if necessary and my final processing was in LR which is non destructive anyway.

This is the first time I have used this method and I am very pleased with the noise reduction. What would I do different next time? I would use a higher ISO – 3200 – and a shorter exposure time to cut the star trails.

Detail of unprocessed image
Detail of unprocessed image
Detail of processed image using median filter
Detail of processed image

What’s it all about?

Here are my jottings about my photographic projects and activities. I have been working on a focus stacking macro photography rig. There are quite a few posts about that. In addition I write about other photographic activities as and when!

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