Twilight sky brightness
The surface brightness B of the moonless sky dims by a factor ~ 10 million between day
and the start of astronomical night (typically ~ 70 - 80 mins after sunset).
B (photons/sec/pixel) was
measured as a function of time T (mins) after sunset (or before sunrise), from
INT WFC images taken during 2010 - 2011.
For 10 < T < 35 mins, the dependence is well fitted by a straight line in log10(B)
log10 (B) = A - 0.103 * T
where A is
5.4 for observing through a U filter (ING filter #204),
6.1 for B (#191),
5.6 for g (#220),
5.5 for V (#192),
6.1 for r (#214),
6.1 for i (#215) and
5.6 for Z (#195).
E.g. a 1-sec exposure with INT WFC, through an r filter, 20 mins after sunset,
should yield ~ 10000 counts/pixel/sec (corresponding
to a sky brightness ~ 13 mag/arcsec2).
The slope of the above relation implies that the sky dims by a factor
of 2 every 2.9 minutes (i.e. by ~ 30% every minute),
at least for the range 10 < T < 35 over
which the measurements were made, with no evidence of any dependence on wavelength.
The measurements for each band were typically obtained over 2 or 3 well-separated
nights, and suggest that night-to-night variations in A (resulting e.g.
from changes in dustiness) are <~ 0.1.
For WHT ACAM/AUXCAM or WHT PF/WHTWFC, with larger telescope
aperture, but smaller pixels (0.25 instead of 0.33 arcsec),
add 0.2 to the above values of A.
The above measurements were made by Mansour Karami of the
Iranian National Observatory,
while he was working as a student support astronomer at the INT (project
supervised by Javier Mendez and Chris Benn).
Last updated: 10 December 2013