The following questions have got me thinking.

A long time ago I think I wrote an answer, or maybe just a series of comments in a different SE site. I think it was Biology SE, and I described ways that people used algae or mold or moss or something in a petrie dish to record a projected image; it photosynthesized faster and grew faster in the brighter areas of the projection.

That's too slow for recording an astronomical image from a telescope.

I don't think the image of the moon from an amateur telescope would be bright enough to burn itself into a small, smooth piece of wood, but I remember something about secret messages written in lemon juice that revealed themselves upon gentle warming.

Is there any way to permanently record a nighttime astronomical image without using a digital imager or photographic emulsion?

  • 5
    $\begingroup$ Pen, pencil and paper? $\endgroup$
    – JohnHoltz
    Jan 21, 2021 at 17:04
  • 2
    $\begingroup$ xkcd what if has an article about using moonlight to burn (it can't happen for entropy reasons) what-if.xkcd.com/145 That would also rule out the lemon juice (which works by charring) $\endgroup$
    – James K
    Jan 21, 2021 at 20:44
  • $\begingroup$ @JamesK thanks for the reference! It cites conservation of étendue which is discussed in my answer to Can a telescope ever increase the apparent luminance of an extended object?. Conversely, a new answer posted to that question citing your link is sorely needed. $\endgroup$
    – uhoh
    Jan 21, 2021 at 22:58
  • 1
    $\begingroup$ @JohnHoltz That's certainly how Galileo did it. $\endgroup$
    – Michael C
    Jan 25, 2021 at 10:08
  • $\begingroup$ @MichaelC and how Schiaparelli documented Mars' canali. $\endgroup$
    – uhoh
    Jan 25, 2021 at 14:22


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