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chornedsnorkack
2013-Aug-18, 09:52 AM
The boundary between scotopic and mesopic vision over extended environment is said to be 0,01 lx. Well, this is about magnitude -9. Meaning that on starlit ground no colour whatsoever can be seen, and neither on ground lit by narrow crescent Moon - but not only will full moon allow some colour vision, but so will gibbous and half moon.

Also, the line between mesopic and photopic vision, quoted at 3 lx, is well above the brightness of full Moon. So moonlit colours are mesopic, not photopic.

Magnitude -9 is needed to provide mesopic colour vision of illuminated landscape. But how about light source itself?

What is the minimum magnitude for a point source star (whether seen directly in sky or magnified through optics) to be seen in scotopic vision allowing some colour discrimination?

What is the minimum magnitude for the point source to be seen in full colour, photopic vision?

Hornblower
2013-Aug-18, 11:56 AM
In my experience the faintest stars for which I can see color are between 2nd and 3rd magnitude.

chornedsnorkack
2013-Aug-18, 12:43 PM
Thanks!
Listing the second magnitude (between 1,5 and 2,5) M stars:
Scheat - 2,42, M2 or M3. Nearby (alpha Pegasi) comparison - Markab, 2,48, B9
Gruid - 2,14, M5. Nearby (alpha Gruis) comparison - Alnair, 1,74, B6 or B7
Mirach - 2,05, M0. Nearby (alpha Andromedae) comparison - Alpheratz, 2,06, B8
Gacrux - 1,63, M3 or M4. Nearby comparisons (beta and alpha Crucis) - Mimosa (1,25, B0) and Acrux (0,77, B1)

There are just two M stars brighter than 1,5 - Antares and Betelgeuse.