Fig. 1 The photo has a purplish cast overall, and is speckled with dots (see detail).
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Fig. 1 The photo has a purplish cast overall, and is speckled with dots (see detail). |
At first I thought this was random noise of some kind; perhaps cosmic rays, perhaps thermal noise, or something that causes a pixel to become "on" randomly. But as I looked at other long-exposure photos I took that night, I found that the noise was the same in every picture. So I began to suspect the camera.
(At this point, I didn't think anything of the purple color. I thought
it was some weird effect of the moonlight or of the long exposure
time, or something.)
 Fig. 2 A photo taken with a 10-minute exposure with the lens cap on also shows a purple haze, and the same pattern of dots (see detail). |
 Fig. 3 Subtracting Fig. 2 from Fig. 1 cleans the image up considerably. |
I edited the original photo (Fig. 1) with the Gimp, and added the "noise" photo (Fig. 2) as a second layer, using "Difference" mode in the "Layers, Channels & Paths" dialog box ("Subtract" also seems to work, in this case).
The result is in Fig. 3. As you can see in the detail view, the speckles are almost gone, as is the purple haze.
You may notice "haloes" around the remaining speckles. I'm not sure, but these might be compression artifacts: the original photo is a JPEG file, whereas the "noise" image is a full-size TIFF image. Since JPEG uses lossy compression, the haloes may have been introduced when the image was compressed.
The colors still seem a bit wrong, but it's hard to tell how, exactly.