Noise reduction
Personally, I prefer to have a certain level of sky noise visible in the background of the final image. This helps the eye to evaluate the actual depth of the image and to tell which objects are real and which are of spurious or noisy nature. If the sky noise level appears too high, it can be reduced by selecting the sky background with the Magic wand in Photoshop. The tolerance of the Magic wand should be set such that it does not select any visible objects. For me, this setting is usually 2 or 3.
Then do Filter -> Noise -> Despeckle to flatten the noise. If you feel the flattening is too much, then you can Edit -> Fade Despeckle and reduce the amplitude of the filtering. Note that this fading step only works as long as the selection is still visible.
The effect is subtle and might not appear strong enough, but this is the crucial point with all this photoshop science: to process images gently so that they don't appear overprocessed. This holds in particular for unsharp masks (see below).
Sky desaturation
Sometimes I lower the colour saturation of the sky background, as on some monitors pixels of certain colour appear more pronounced than others. To this end I select the sky background with the magic wand, using the same settings as above, and in Image -> Adjustment -> Hue/Saturation I set the saturation slider to a value of -30.
Unsharp masking is a way to make an astronomical image appear more crispy than it actually is. It has to be applied very carefully, otherwise more harm than good is done. In principle, an unsharp mask is obtained by smoothing the image on a certain scale (in photoshop: radius) and then subtract the smoothed image from the unsmoothed image. This difference image is then added back to the original, which leads to an edge enhancement effect.
Photoshop offers two more parameters in this process: The amount, which (probably) is a factor by which the difference image is scaled before it is added back to the image, and a threshold, which is a lower cut-off that allows unsharp masking only of brighter image areas (hence the noise does not get amplified). Hence, unsharp masking is a primitive realisation of wavelet processing, working with one length scale only.
Choosing the unsharp mask's parameters has to be done with great care in order to avoid overprocessing. The radius should be chosen such that it is the same or a bit larger than the image seeing (the FWHM) in pixels. It should not be larger than twice the image seeing (I usually pick about 1.5 times the seeing value, and an amount of 70% or max 80%). The larger the radius is chosen, the smaller the amount has to become (maybe 50%) in order to avoid unwanted effects.
The threshold is given in terms of background brightness. It should be clearly above the background level, so that the noise in the image does not get enhanced. In my images the background level is adjusted to a value of 20 in RGB, and I choose 30 for threshold of the unsharp mask.
Imho, the moment one can tell from the image that an unsharp mask has been applied, it is already too much. See the example below for an illustration. More enlightening details about unsharp masking are given e.g. by Keith Wiley.
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The image before unsharp masking |
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Unsharp mask: Amount: 80% Radius: 4.0 pixel Threshold: 30 According to my own taste, this is a good level of unsharp masking:
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Unsharp mask: Amount: 120% Radius: 8.0 pixel Threshold: 30 This image is imho already overprocessed by unsharp masking. Stars start to broaden up, which can be seen by the narrow double star in the image centre, and the small galaxy to the upper right has a fattened core. Anything beyond this would be clearly overprocessed, in particular if the stars in front of the large galaxy would feature dark rings around them. |
Less is more in Photoshop. What is not in the data cannot be made visible in particular with Photoshop science (nor with any other technique). When applying a smoothing or sharpening technique, or if you have to juggle the colour sliders because your colour calibration did not work out, don't overdo it. After a few hours, have a break and don't look at the image for a while.
If you think you are done, don't look at the result for two days, then come back and have a second look. It is a bit like wearing green sunglasses. After a while one doesn't notice them anymore, as the brain learnt to "renormalise" the vision. Staring at a computer screen for many hours is not so different from wearing green sun glasses, your eyes and brain need a break.