Reprocessing Historical Images, Looking for REALLY big challenges? Options

[GregM]

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[edstrick]

Mariner Mars 1971 was to be a two spacecraft mission. One spacecraft was to be placed in a high-inclination orbit optimized for mapping, the other in an orbit optimized for "variable features" and atmosphere studies, with viewing geometry repeating closely evey few orbits. Mariner 8 went in the drink due to launch failure, and the Mariner 9 mission was redisigned to do both tasks, with the emphasis on mapping.

The imaging system was an upgrade of the Mariner 69 system, but with a new, all digital, tape recorder, capable of storing some 33'ish 800x600 or so pixel images. As before, the narrow angle camera was monochrome, with I think a yellow "haze filter". The wide angle camera had a filter wheel with a quite nice set of (I think 8) filters: Violet/Blue/Green/Red, 3 orange polarizing filters, and maybe Clear.

When Mariner 9 got to Mars in Nov 71, the greatest global dust storm *EVER* observed on the planet, which had started in Aug or Sep 71 was still essentially obscuring the entire planet. The dust was starting to settle, but... the only things visible with any contrast were 4 dark spots (the tharsis volcanos, sticking out of the dust) and the small summer south polar cap.

Mariner started it's pre-programmed mapping sequences, but it was immediately obvious that they were 3/4 useless, and a couple of makeshift interim observation sequences were developed and put in operation. A better dust storm and south polar observation sequence followed about a month after arrival, and included test mapping pics to check on atmosphere clearing. Things progressively improved and mapping was started in <I think> early January 72.

*UNFORTUNATELY*.... by then, due to a hardware failure, the filter wheel on the wide angle camera had stuck on one of the orange polarizing filter positions. The data was acceptible for mapping purposes, but any further color imaging was lost. Some color images of dark albedo markings at high southern lattitudes were generated as well as color pictures of the south polar cap and surrounding hazy atmosphere and dimly visible surface, but that's all I've seen.

Mariner 9's camera had one severe design limitation that made quantitative decalibration of the images very difficult <The Mariner 69 cameras had the same problem>. Each vidicon image exposure was erased with repeat scans of an electron beam after electron beam readout from the camera, but the erasure was incomplete. Ghosts of previous images with some 5 to 10% of the brighness of the previous image, then fading slowly, persisted in subsequent images. In mapping strips of images, that wasn't terrible, but when the ghost images contained the bright limb of the planet, they were awful. The strength of each ghost depended on the pixel location, and the brightness of the previous image and the brightness of the current image. The resulting images were significantly degraded cosmetically and scientifically by the residual images.

Mariner Venus/Mercury 1973 (Mariner 10) solved the problem with perfect brute force engineering. Each of the twin cameras contained a ring of several small "wheat bulb" light bulbs! After each exposure was read out, the light bulbs INSIDE the camera were turned on, totally and uniformally saturating the vidicon surface with light. The cameras then erased the vidicon repeatedly, before taking a new exposure. Residual image was reduced to essentially undetectible levels.

The Viking Orbiters and Voyagers did the same thing. The "Light Flood" could be turned off when desired, as it left a background level of "glow" noise in the images, and when Voyagers, in particular, were doing long exposure low-light-level imaging, the light flood was "off".