Daylight Photometry of Bright Stars -- Observations of Betelgeuse at Solar Conjunction

Abstract

Betelgeuse is an important variable star with many observations in the AAVSO database, but there is an annual gap of about four months where Betelgeuse is close to the sun and not observable at night. This gap could be filled with daylight observations. The star is bright enough to be imaged with small telescopes during the day, so photometry is possible when the sun is up. We present V band photometry of Alpha Ori taken with an amateur telescope equipped with an interline-transfer CCD camera and neutral density filter. These data compare favorably with contemporaneous nighttime photometry. The method used is a variation on ensemble photometry (using other bright daytime stars), and involves large stacks of very short exposures. The ensemble method provided V magnitudes of Betelgeuse with calculated errors of 0.020 +-0.008 mag from February to April 2021. From May to July, at the closest distances to the sun, the photometry of Betelgeuse could be continued with mean errors of 0.040 +-0.013 mag.