Digital Lunar Imaging and Enhancement

The map in the Larousse Encyclopaedia of Astronomy (1967) drawn by the French astronomer L. Rudaux is to my knowledge the only map of the moon that attempts to show colour on the lunar surface; colour so subtle that very few observers have attempted to describe it.

 
General Map of the Moon drawn by L. Rudaux from the Larrouse Encyclopeida of Astronomy 1967

I believe that in the apparent absence of any other colour reference to the visual appearance of the moon, this should be used as a standard when processing digital images.

It is very easy to over-process digital images and distort the end result. On Photoshop 5.5 I have used autolevels to adjust brightness and contrast then adjusted the colour saturation until surface colour could be easily distinguished. Alternatively, manually adjusting brightness and contrast was employed to produce a slightly dark, contrasty, image before altering the colour saturation to reveal Rudaux's lunar colours, without going OTT or, by increasing graininess, damaging the scientific content of the image.

Members of the MAS lunar imaging group have already provided several dozen digital images for me to process. With the exception of Andrew Fearnside, who processed his own pictures, I have tried to extract sufficient colour information from prime, unprocessed, digital images donated by Jerry Grover, Barry Henshall and Anthony Jennings, together with my own, to demonstrate that lunar surface colouration is independent of camera brand (Nikon Coolpix, Canon DSLR or Philips ToUcam), or telescope…so far these have been either Meade, Celestron or Skywatcher catadioptric instruments.

Some patterns are already emerging:

1. Processed image quality is dependent on the pixel size of the camera sensor. Andrew's images, taken on a 3mp sensor, are comparatively grainy. This can be overcome by stacking images before processing (see Alves) but in practice it has been found that poor seeing can affect image quality so badly that images don't register properly…there is a trade off.

2. Slightly under-exposed images process better than over-exposed pix. Under-exposure can be adjusted, over-exposure cannot.

3. Low resolution images stand less colour saturation enhancement before deteriorating in quality. This can be partially overcome by increasing the saturation in small steps, saving, and then repeating the process…see Alves.

4. Atmospheric extinction can give a strong orange bias to digital images that is difficult, if not impossible, to balance out before colour saturation enhancement. So far, the best images for processing have been taken with the moon high in the sky.

5. I used a Nikon Coolpix 5000 to take ca30 second MOV files, converted to AVI and then stacked on Registax. The results were OK as lunar images but would not process to increase colour saturation without horrendous colour distortions…mainly to the blue and green end of the spectrum and then in coarse blocks of colour rather than as a smooth continuum. In comparison, AVI files with the same resolution, taken on the ToUcam and stacked on Registax, produced images that gave colour fidelity comparable with digital stills. Jerry Grover's pix and those of Anthony Jennings amply support this.

There is more to learn.

Kevin J Kilburn, FRAS. 26 Jan 2006
Manchester Astronomical Society and the Society for the History of Astronomy


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