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The image below shows a part of a backup satellite for Telstar 1 and 2. In addition to the many small and highly segmented solar panels, there are what look like individual elements of the same kind of solar cells placed on special mounts. The first image shows four of them, the second shows one.

In each instance (both the large solar panels and the individual cells) the cell is further subdivided into five segments.

Question: What is the purpose and function of these separated, individual solar cells? Why were they "special" yet so similar to all the others in the larger panels?

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Backup spacecraft for Telstar, the world’s first active communications satellite. Telstar 1 began an era of live international television. After its launch on July 10, 1962, it relayed television images between the United States and France and England.

Source: cropped (some zoomed and sharpened) from Smithsonian Air and Space Museum page Telstar

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2 Answers 2

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The small separated cells had several functions.

Six individual units, the "solar aspect cells", were each oriented and attached as if they were on the sides of a cube to identify the spacecraft's attitude. (This is probably the 2nd and 4th pictures above) This was a time when onboard electronics, and particularly computation, was expensive, so these were mounted to give a direct readout of angles (or more specifically, the cosine of angles) from their power levels.

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The group of four separate-but-close units were a radiation damage experiment. (2nd and 3rd pictures above) Three were covered by 20, 25 and 30 mils of sapphire, while one was left unshielded (like all the other ones) and served as one of the solar aspect cells described above. Like the other cells, these were separately read out for telemetry back to Earth.

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The not-quote-round assembly next to the radiation damage solar cells (center picture) is a related experiment to test damage to NPN transistors.

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Source: "The Spacecraft Radiation Experiments", W.L. Brown, T.M. Buck, L.V. Medford, E.W. Thomas, H.K. Gummel, G.L. Miller and F.M.Smits, published in NASA SP-32 p.899 1963.

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    $\begingroup$ Wow! Another gold mine answer! (as is this one). $\endgroup$
    – uhoh
    Commented Sep 1, 2019 at 4:39
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I just ran across Scott Manley's September 30, 2023 video How The First TV Satellite Changed The World - Communications Satellites Part 2 and noticed that there are images of the data from these solar cells and from test transistors with the various sapphire shielding layers discussed in @BobJacobsen's answer

First: solar cell current ratio vs time (log scale), Second: transistor gain vs time (log scale). Click for full size:

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The video quotes the following four NASA documents - I am not sure from whence these images come.

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