On three recent occasions, we updated the comparison star database to improve the photometry. The first update was JD 2454397 (Oct 24, 2007) when we updated 21,826 comparison stars whose photometry differed by less than 0.2mag from the existing chart labels. A second similar update of 6,451 stars took place on JD 2454596 (May 10, 2008).

This can make small shifts in the mean levels of some stars. In addition, CCD observers may have started using the precise photometry for their comparison stars rather than the tenth-magnitude rounded values on the charts, so there may be a systematic shift of a tenth or two for those observers. The more important update was on JD 2454690 (August 12, 2008), when we updated approximately 4,500 stars whose photometry differed by larger amounts; in some cases, the photometry differed by several magnitudes from the chart labels. This is primarily due to systematic errors in the chart magnitudes, usually at the faint end of a sequence, and preferentially on long period variable charts.

When analyzing your dataset, be aware that there may be a transition from the old magnitude system to the new one that occurs near these dates. You can use the JD's as a guide, but also look at the Chart ID: if it is a new-format ID (not yymmdd), then the photometry will be the revised values.