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What is the purpose of the 4 vents at the aft section of the Hubble Space Telescope? The spacecraft is operating in vacuum and the interior of the telescope is devoid of any gas, so venting seems pointless.

Venting might have played some role during ascent to orbit (when the telescope had to get rid of the air inside the instrument section), but whether this is the case I'm not sure. Can there be other reasons behind that?

Hubble Telescope aft section

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From the HST Service mission 2 Media Guide (large PDF)

The shroud and bulkhead support a gas purge system used to prevent contamination of the science instruments before launch. All vents used to expel gases are light-tight; that is, no stray light can enter the OTA focal plane.

From this description, I suspect that the interior of the telescope was filled with e.g. nitrogen under a slight overpressure before launch. During the launch, as the ambient pressure decreased, this nitrogen was expelled via these vents.

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