High ISO Noise Reduction
The draw back to using very high ISO settings is increased noise. Some if not all cameras do have an in camera Hi ISO noise reduction setting. Generally this should be used. There is one drawback to it however in that it takes about as long to remove the noise as the exposure. So if you are taking a 30 sec exposure, you can expect to wait another 30 sec while the camera is doing the noise reduction. Generally this is no problem.
Most every post processing program has a noise reduction feature. The drawback is they reduce the overall sharpness of the image. This technique does not.
With your camera on a good tripod, take 15 to 20 images of the sky. You can take them as fast as your camera is able. These can be at a high ISO 1000 to 6400 as you desire.
If there is a foreground you would like in the final image, optimize camera settings and take its image. This often is a lower ISO. It can be a simple image or complicated light painting. You do not need to do this step if the foreground in the star images is good, but I generally do shoot a separate image for the foreground and then a series for the stars. I do it in that order because if for some reason you get interrupted in the star shots, you will already have the foreground image (I once had the lens fog up during the star shots and had to move the camera to dry it. I had not shot the foreground image and was forced to use a much poorer foreground from the star images).
Reducing Noise in Night Sky
In Light Room:
If you wish you can develop the sky images to optimize them and then the foreground image before exporting to Photoshop. Or you can do all of the processing in Photoshop.
Export star images to Photoshop as layers. You can also export the foreground layer, but be careful not to use it while aligning the stars.
For the sky:
It is necessary to align all of the images as the stars will have moved from shot to shot. This is done, as normal, by selecting all of the star layers, Click on “Edit”, “Align Layers”, use the normal default of “Auto”.
If there is a lot of foreground in the images, this may not work and align the foreground rather than the stars. The solution is simple. Create a layer mask on one of the images and with a large soft brush paint black to cover the foreground. Accuracy is not required. Just get all of the foreground. Then copy the mask to each of the other sky layers (Copy with ctrl-click, select next layer and click add mask. Repeat for each layer).
Now you can align the stars with the auto align procedure described above.
I like to test the alignment by deselecting all of the images, go to the first (bottom of the stack) enable and blow up to 1X, then enable the top image. There should be no movement of the stars.
If you had to create a mask to align the stars, now delete all of the masks.
With all of the star layers selected, create a smart object (“Layer”, “Smart Objects”, “Convert to Smart Object”).
Set the stack mode to “Mean” (“Layer”, Smart Objects”, “Stack Mode”, “Mean”). I have seen some us “Medium” rather than “Mean” but I would go with “Mean”.
Boom, the noise in the sky is gone and the stars are clear!
It should be noted that this technique can be used to remove noise from an photo if you shoot a number of images!
Now all that is left is to blend the Stars with the Foreground.
Put the foreground layer on top and add a layer mask. With the layer mask selected, paint the sky black with a large soft brush.
Now you can adjust the sky layers and foreground layers independently from each other if you did not do this in Light Room.