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Here is a 60-second exposure of lava entering the ocean on the
Big Island of Hawai'i from Kilauea volcano.
Look at the zoomed in image at 100% below. No, that's not flying
lava...it's hot pixels!
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Create a new layer, call it "Dark Frame". You can fill it with
anything.
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Open the dark frame image in a separate window.
Here you can really see the hot pixels in the zoomed in
view. Because this exposure was taken (with the lens cap on)
immediately after the main exposure, and using the same
exposure settings, the ambient temerature of the CCD sensor
should be about the same and the hot pixels (dark current
noise) should be more-or-less the same as in the previous image.
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Select All (Ctrl+A) and Copy (Ctrl+C) in the dark frame image.
Go to the original image with the new layer and Paste (Ctrl+V)
it into the "Dark Frame" layer.
In the Layers dialog, set the Blend mode to "Difference" and
adjust the layer Opacity to suit (100% for this example). At
this point you should see black specks instead of hot pixels;
these occur where the hot pixels from each image cancel each
other out.
(Optional) Duplicate (Ctrl+D) the resulting image at this point
to have a backup and branch point for fine-tuning the final result.
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Flatten (Layers/Flatten) the duplicate image.
Run Filters/Enhance/Despeckle. In this example I used both the
Adaptive and Recursive options with a 3 pixel
radius. This should remove the black specks.
Notice the silky tones now! Hardly a trace of those pesky hot
pixels.
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(Optional) Add some noise back to the image, if desired.
Unfortunately, the Despeckle filter does degrade the
image in subtle and sometimes not-so-subtle ways. In this case
some posterization occurred in the orange-red smoke and steam
clouds.
In this example I used Filters/Noise/Noisify with a value of 0.06
on all channels to add some texture back into the clouds.
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(Optional) If you add noise, you may also want to blur slightly
to smooth it.
Here I blurred very slightly to smooth the added noise.
(Optional) Smart sharpen the
image to counteract the softening due to the Despeckle and
subsequent blur, if desired. With this image there weren't any
real defined edges to sharpen, so I skipped this step, but for
any image with edge detail I would do it.
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Compare. Top right, original zoomed. Bottom right, processed
as described here.
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