Why is LUVOIR actively heated? 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?
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.