September 13, 2007

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Be Gone, Obnoxious Framelet Lines!

Lunar Orbiter image - original (right) and destriped (left) by Niels Noordhoek

Frequently, high resolution Moon images are presented in LPOD that contain "framelet lines". The images are stitched together from film strips, that were chemically processed (!), scanned and transmitted to Earth from Moon-orbiting spacecraft such as the Lunar Orbiters. Though it is probably scientifically most correct to present the raw data as-is, a potentially pretty image is spoilt by the missing data between the framelets and the bands of varying brightness at their edges. The film data were finally lost, because the orbiters were purposely crashed onto the Moon’s surface to prevent possible hazard to future Apollo missions. To perform plastic surgery on the images, a filter was written in Matlab, an easy to use numerical software package, which can be used for image display and processing. The code reads in the image, low-pass filters it in a horizontal direction, and high-pass filters it again in a vertical direction. The result is an image that shows only the framelet lines and intensity modulations. The original image is divided by this image, removing most of the artefacts. A side effect is that the contrast of the image is somewhat reduced. For those who are mathematically inclined but do not want to spend money on a commercial Matlab license, free alternatives are available to run the code: Octave, SciLab and FreeMat.

Niels J. Noordhoek

Note by CAW: This is the general technique used by the US Geological Survey to destripe all the Orbiter IV images to make a (so far unreleased) nearside mosaic, and by Charles Byrne in his Lunar Orbiter Photographic Atlas of the Nearside of the Moon and his forthcoming companion farside atlas. In the mid-1970s, Tim Mutch at Brown University tried to destripe Orbiter images using the first generation tapes. Unfortunately, no tape drive could be found anywhere in the country that could read the decade old tapes and the project was abandoned.

Technical Details:
A version of Niels’ article with more illustrations and Matlab code is here. If you don’t recognize the crater, click here.

Related Links:
Niels’ website

Yesterday's LPOD: Rimae Fantasticus

Tomorrow's LPOD: A New Era of Lunar Exploration!


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