[X] My Assistant

[GregM]

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[edstrick]

Mariner 6 and 7 returned two "primary" image datasets of Mars. Far Encounter and Near Encounter. Far encounter data was narrow angle only, near encounter was alternating wide and narrow angle images. The narrow angle camera was monochrome, with <I think> a yellow filter to "cut haze", while the wide angle camera had a 4 position filter wheel. I think red/green/blue/green.

The spacecraft had dual tape recorders, derived from the Mariner 4 hardware. Unfortunately, it did *NOT* have the capability to recorde the entire dataset in digital form.

All data was recorded on an *ANALOG* tape at full resolution, after being sharpened with a circuit that enhanced fine details. Near encounter analog data was recorded after being passed through an automatic gain control filter <which totally trashed absolute intensity values for the data>. Unfortunately, the taped data was degraded by significant in-flight buildup of tape oxide on the playback <record/playback?> heads, resulting in horizontal streaky noise and banding and occassional analog dropouts in the images.

Every seventh pixel was recorded on the DIGITAL tape recorder without automatic gain control, after having the two most significant bits chopped off, which badly "contoured" the digital data. To make matters worse, the middle 1/4 or 1/5 of each image's digital data was not recorded, spectrometer data being transmitted during that gap <and between the end and beginning of each scan line as well>. BUT..... every 4'th digital pixel in the center gap was trasmitted in real time!....

The efforts to reconstruct the images were described in the Mariner 69 special report issue of the Journal of Geophysical Research, published in 1970 or early 71. They were largely successful, producing 2 versions. One was from the analog data, and was a "Maximum Discriminability" version of the data, best showing fine details. The other was radiometrically decalibrated using preflight and some inflight data on camera background and shading and spectral response, starting from the reconstructed final images. These show the real contrasts of the martian surface better, and the regional shading of the albedo markings.

The wide angle images were taken in overlapping swaths through alternating color filters, and some color information is clearly present in the images. The viewing geometry changed from frame to frame as the field of view was something like 15 degrees. I have never seen attempts to generate color images using the overlapping wide angle close encounter images.

Mariner 69 also returned a "secondary" image dataset... the every 7'th pixel data <missing that central stripe> was transmitted in real time. Most of the far encounter data includes at most slivers of the planet's limb, as the disk was within the central stripe, but Mariner 7 transmitted a unique set of WIDE ANGLE "Late Far Encounter" data with the filter wheel stepping. This data was not recorded for playback. There is a picture floating around made from this data of a half mars <cut off by that central stripe> of the Sinus Meridiani area, in color, with mabye twice or so the resolution of most any earthbased mars imagery from that period.