Sky brightness
during the eclipse – Miguel Rodríguez - Madrid
My photographic haul is rather poor,
but I took advantage of the phenomenon to carry out an experiment inspired by
an article published in Volume 34, no. 1 of the Journal of the AAVSO. I tried
leaving the telescope fixed, without drive, staring at the sky, with the CCD
camera attached and working, so that it would take images of the sky, each of 4
minutes exposure, while I was outside. I have reduced the data, starting with
the reprocessing of the 92 useful images ((raw image-dark)/flat), followed by
the calculation, in each case of the mean of the image pixels, assuming that it
is indicative of the sky brightness and unaffected by stellar contributions. I
thought that even if successful, the results would not be very spectacular.
However, on the contrary, representing the average value for each image against
the time, like a normal light curve, the decline in the brightness of the sky
background is spectacularly shown as the Moon was submerged in the penumbra and
then the umbra. During totality the values remain almost unchanged apart
changes Due to high cloud and sky transparency. At shadow egress the increase
in brightness of the sky is palpable. In theory the curve should be symmetrical
although two factors must be taken into account: first, the Moon’s altitude
changes as the eclipse progresses, which is a factor influencing in the
variation of the sky background. Secondly, the conditions of sky transparency
improved during the night, due to which the sky background decreased
independently of the eclipse. As a corollary, when the egress from the penumbra
was completed, which is when data taking ended, the level of counts did not
return to the level found at the start of the run, some six hours earlier.