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While link-clicking for Where did Herschel Space Telescope go in 2013? I ran across the 2009 Time Magazine article Two Telescopes to Measure the Big Bang which shows the file photo below with the caption:

Herschel's 3.5-m diameter mirror is checked with ultraviolet light at Europeís Spaceport in Kourou, French Guiana T VALLEE/AFP/Getty

According to Wikipedia's Herschel Space Observatory the telescope covers a wavelength range from far-infrared to sub millimeter waves; 55–672 µm.

Why then check the mirror's figue with a wavelength 200 times smaller in the ultraviolet? Even visible light would have been more convenient, and there should be several IR bands where coherent sources and imaging detectors work even in the humid French Guiana climate.

Herschel's 3.5-m diameter mirror is checked with ultraviolet light at Europeís Spaceport in Kourou, French Guiana T VALLEE/AFP/Getty

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    $\begingroup$ speculation: they were looking for contaminants which show up in UV light. $\endgroup$
    – Hobbes
    Commented Jun 4, 2019 at 9:41
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    $\begingroup$ Indeed, this ESA press release says, "During the cleaning process the mirror was inspected a number of times under UV light with other illumination in the clean room turned off. This is common practice when examining the cleanliness of a surface and provides the ability to pick up particles which fluoresce under UV." $\endgroup$
    – Peter Erwin
    Commented Jun 4, 2019 at 14:59
  • $\begingroup$ @PeterErwin biologicals? (fingerprints, sneezes, etc.) Anyway, I think you can probably write an answer based on that. $\endgroup$
    – uhoh
    Commented Jun 4, 2019 at 21:57
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    $\begingroup$ Semi-related: Effects of surface contamination on the infrared emissivity and visible-light scattering of highly reflective surfaces at cryogenic temperatures (most recent archived version) $\endgroup$
    – try-catch-finally
    Commented Jan 1, 2020 at 19:57
  • $\begingroup$ @PeterErwin biologicals? (fingerprints, sneezes, etc.) Anyway, two years later I think you still can certainly write even a short answer based on that. $\endgroup$
    – uhoh
    Commented Aug 20, 2021 at 23:24

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