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.