Variable Calibration Standards

We have concluded a visual survey of the photometric consistency of all of the standard stars used in current production to prepare for pipeline 3 reprocessing. The motivation has been to identify standards that are in fact variable and should be excluded from the standards list. Of particular interest are eclipsing binaries, since the number of photometric points can be very small relative to the total number of observations and thus statistically lost in the noise.

A number of variable (and otherwise problematic) stars have been identified in the current standards sample. These were identified by extracting the JHK magnitudes for each standard position from the calibration database, then plotting a variety of mag-mag/col-mag/col-col plots for each field. Light curves are not immediately available for these sources since the UT/Julian day info is only available from the scan database, and extracting this would have slowed down the database extractions.

The variable star standards are listed below, linked to postscript plots of their photometry.  In all cases, the behavior noted for the individual standard is unique to that star and not seen in the other standards in that field (and therefore not related to global calibration problems of the field on various nights).

Eclipsing Binary Variables

These variables are quite obvious in mag-mag plots since they show a big cluster around the summed magnitudes of the two stars, with a small number of points scattered upwards in magnitude in all bands during the periods of eclipse. No significant color variation is usually seen unless the stars are of wildly different spectral types.


Regular Variables

More typical variable stars show up as extended clumps in mag-mag plots with points more uniformly spread through the distribution.


Unusual Glitches

A number of other standards have been identified with odd behaviors.

90009_09 (some J-only attenuation)
90013_37 (some J-only attenuation)
90565_45 (unresolved binary: H band image)
90565_52 (irregular photometry; possible source confusion? H mags show slight positional bias as well: H band image)

Previously Identifed Variables

Information on 4 earlier standards shown to be eclipsing variables (all caused trouble with the photometric solutions for the night) are summarized elsewhere. Reference plots can be found below.


R. Hurt, last updated 11/28/00