According to Wikipedia's Galileo Solid State Imager (SSI)

The SSI was an 800-by-800-pixel solid state camera consisting of an array of silicon sensors called a charge-coupled device (CCD). Galileo was one of the first spacecraft to be equipped with a CCD camera.[citation needed] The optical portion of the camera was built as a Cassegrain telescope. Light was collected by the primary mirror and directed to a smaller secondary mirror that channeled it through a hole in the center of the primary mirror and onto the CCD.

Usually the secondary mirror of a Cassegrain is convex and a small fraction of the diameter of the primary. In the photo however, the central obstruction appears to be about 60% of the diameter of the entrance aperture, which is roughly how big a flat secondary mirror would have to be to "(channel the light) through a hole in the center of the primary mirror and onto the CCD."

Question: Why was the central obstruction in Galileo's Solid State Imager (SSI) so large? Was the secondary mirror flat?

Galileo Solid State Imager (SSI) [Galileo Solid State Imager (SSI) zoom/crop


I am not 100% sure but I think we are looking at the aperture cover. In the picture below I've pointed out what looks like hinges (top) and a latch (bottom).

enter image description here

There's a cutaway of the scope here which does not show such a large obstruction.

enter image description here

Source: https://dspace.mit.edu/bitstream/handle/1721.1/114747/Carlton%20Extended%20Abstract_Submitted_v2.pdf?sequence=1

This next image appears to confirm that it's a cover.

enter image description here

Image from here https://history.nasa.gov/SP-4231/chapter5.pdf

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  • $\begingroup$ Interesting, I wonder what "deployable aperture cover" means exactly? What might happens when one deploys it? $\endgroup$ – uhoh Sep 17 '19 at 23:53
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    $\begingroup$ I would bet that those hinges are spring loaded and it flips up when the latch is released. $\endgroup$ – Organic Marble Sep 17 '19 at 23:54
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    $\begingroup$ I see, so perhaps the "quartz clear aperture" is there so that if the cover fails to deploy it still works as a telescope? $\endgroup$ – uhoh Sep 17 '19 at 23:56
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    $\begingroup$ Sounds like a suspenders-and-belt approach, very typical! $\endgroup$ – Organic Marble Sep 17 '19 at 23:57

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