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My own interest in Mars dates back about a dozen years, when I devised a simple technique that I hoped would stabilize the image of a planet while it was being photographed.] During the summer of 1956, when Mars made its last close approach to the earth, I took hundreds of color pictures of the planet [on 16-millimeter film, using my stabilizing technique in conjunction with the 60-inch reflecting telescope on Mount Wilson. As luck would have it, a large dust storm developed midway through the most favorable picture-taking period and partly frustrated not only my efforts but also those of astronomers at other observatories.]

A few years later, [when the National Aeronautics and Space Administration, working through the Jet Propulsion Laboratory of the California Institute of Technology, began to plan spacecraft that could carry out missions to the nearby planets, it was natural for some of us at Cal Tech to consider the possibility of taking close-up pictures of Mars. Accordingly Bruce C.] Murray [Robert P.] Sharp and I proposed [to NASA] that Mars be photographed by a tele­vision camera placed aboard a spacecraft.

[The proposal was accepted in 1962 and we were invited to develop our ideas in collaboration, with the technical staff of the Jet Propulsion Laboratory. The spacecraft then being designed for admission to Mars at the next favorable opportunity – 1965 – was known as Mariner B. This was to be a spacecraft weighing from 1,200 to 1,500 pounds launched by an Atlas—Centaur vehicle. When the liquid- hydrogen-fueled Centaur ran into delays,] (4) the mission was [recast] to make use of an Atlas—Agena launch vehicle, which could send [only about a third as much weight] to [the vicinity of] Mars. [The craft for this mission, designated Mariner G ultimately became the successful Mariner IV.

For the heavier Mariner  we had planned to use two television cameras, one to provide 20 close-ups and the other to provide 20 views in two colors (red and green) of the entire disk of the planet. The close-up pictures were to have had a resolution of one kilometer and the full-disk pictures a resolution of five kilometers. The resolution was to have been achieved by using a television system that recorded 160,000 (400 by 400) picture elements per frame. The whole system of two cameras would have weight about 50 pounds.

When the Television system had to be redesigned for Mariner C, we were limited to about 30 pounds, including the tape recorder needed for data storage. Because this reduñed the data-storage space to about 10 percent of the space originally available, we were obliged to settle for] one camera and a television system that recorded only 40,000 (200 by 200) picture-elements per frame. [The camera selected had a focal length intermediate between the focal lengths of the two cameras originally planned and could resolve surface features of one or two kilometers.] The [specific] focal length selected, 12 inches, was determined by the fact that it was assumed [for planning purposes] that the distance between the spacecraft and Mars when the pictures were being taken would be between 12,000 and 15,000 kilometers [(7,500 and 9,300 miles).]

The other, characteristics of the camera system followed from the resolution desired, the focal length and the sensi­tivity of the [Vidicon] television picture tube. The shutter speed had to be held to a fifth of a second or less in order to limit blurring of the image caused by the spacecraft motion of four or five kilometers per second with respect to Mars. The light sensitivity of the television picture tube then established that the aperture had to be about an eighth of the focal length, [giving a focal ratio of f/s.] To obtain the optical system [we needed within the prescribed limits of weight and size – and with optical components that had proved themselves in space flight] we selected a reflecting telescope [of the Cassegrain type] with an aperture of 1,5 inches [(see top illustration at right). The development of the entire television system was handled by engineers of the Jet-Propulsion Laboratory.]

To transmit the pictorial data back to the earth [various signalling schemes were considered. One sophisticated scheme involved data compression, in which only a change of intensity from one picture element to the next would be transmitted. We also had to decide whether to transmit the signal in analogue or digital form. (Ordinary television signals are transmitted in analogue form.)

Experience had shown that the best way to send a weak radio signal through, space in the presence of background noise is to uses a signalling method known as pulse-code modulation. In this signal-coding method the output of an electrical device, whether it be a thermometer or a television camera, is coded into a sequence of "bits", or binary digits made up of 0's and 1's, that represent a particular level of intensity. Accordingly] the output of the Mariner IV televi­sion camera was translated into a six-bit code that iden­tified the brightness of each picture element [on a scale that had 64 steps from full black tî full white. The 64 steps of the sequence ran from 0 to 63.] A sequence of six l’s rep­resented full black, or no light at all; a sequence of six 0’s represented full white, or maximum-light. To encode the information contained in 40,000 picture elements therefore required 240,000 binary digits. These were transmitted back to the earth at the rate of 81/3 bits a second. The total trans­mission time for a single picture should have been eight hours; [in actuality 82/3 hours were required because a small amount of extra information, such as that required for synchronization, had to be sent with each picture.

In an effort to obtain information about the surface col­oration of Mars we designed the television system to take overlapping pairs of pictures, with one member of each pair being taken through a green filter and the other through a red filter. A wheel carrying four filters, alternately red and green, was arranged to rotate 90 degrees after each expo­sure, thus producing a sequence of pictures alternately red and green. To have recorded all these pictures, however, would have used up all the data-storage capacity long before the television scan path had crossed the planet. To stretch out the sequence and yet have same pairs of overlapping colored pictures, every third picture was omitted from the stored sequence. Hence the overlapping pairs of pictures followed the sequence green-red, red-green, green-red and so on.

Although] (5) the system was provided with automatic gain control to adjust for changes in the brightness of the Martian surface, [the gain adjustment could function only after the first picture had been recorded on the face of the Vidicon tube and had been scanned electronically. Further­more, in order to keep the gain control simple sand not run the risk of a large error in correction between pictures, the gain correction was made only on the basis of the green image and then was limited to a gain change, up or down, of only one step, representing a factor of three. This meant that we had to estimate the exposure rather accurately for the first picture or several of our precious pictures might he wasted before the gain-control system could make the corrections.

Even after we had gone through the calculations several times we could not feel satisfied until the system had been (tested on some real object, illuminated by the sun itself, outside the laboratory. The moon seemed an ideal subject; its reflectivity, compared with that of Mars, is known with considerable certainty. Accordingly we attached the Mariner IV camera-and-television system to the 60-inch telescope on Mount Wilson and checked its operation as we scanned a less than full moon from its bright side across the terminator to the dark side. The test not only confirmed our calculations but also increased our confidence in the operation of the whole system.


Date: 2015-04-20; view: 773

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