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From the Wikipedia page about LUVOIR design:

To enable the extreme wavefront stability needed for coronagraphic observations of Earth-like exoplanets, the LUVOIR design incorporates three principles. [...] . Third, the telescope is actively heated to a precise 270 K (26 °F) to control thermal disturbances.

I always thought the chiller a telescope mirror, the better. Why is LUVOIR heated then?

seldon
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The main reason appears to be to prevent any icing up and contamination of the mirrors and other optical coponents with water and other "non-volatile residues", which would be extremely detrimental to the UV performance.

Emissivity at 270K is not too much of an issue since the instrument only works out to 2 microns.

The issue is briefly discussed in section 8.1.10 of the LUVOIR final report.

ProfRob
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