Universal time of observations for all drawings in the dataset. The vertical gap in 1996–1997 corresponds to period when the time of observations was not included in the scanned images. These data will be updated in later versions of the dataset. A second vertical gap corresponds to 2004–2007, when the sunspot drawings program was shut down. Annual variations in the earliest time of observation are due to change in the sunrise time during the year. Two horizontal gaps seeing in the time of observations correspond to a shading of the coelostat mirror tracking the Sun by the support pillar and secondary mirror (which reflects light to the main telescope mirror) in early morning hours (about 15:00 UT) and the break for the lunch hour at the MWO (about 20:00 UT). The lunch break was a formal event at MWO until about 1984, when the operations of solar telescopes were transferred from Carnegie observatories to UCLA. Early morning horizontal gap (15:00 UT) seems to disappear after 1970, which suggests that at that time, the morning observations may have been taken with the first flat mirror moved to the east position, and thus having a strongly oblique angle between the two mirrors.