[vsnet-chat 2206] Re:[vsnet-alert 3325]

[Mati Morel <morel@ozemail.com.au>]

Berto asked how the USNO A1.0/A2.0 catalogues could be so wrong in measuring the
b and r mags. Well, they are derived from scans of Schmidt plates, which are not
always (hardly ever?) exposed on the same night. If the r plate was taken at
ESO,
Chile, and the b plate at Siding Spring, Australia, the epoch of observation may
differ by many months. An attempt to derive colours for variable stars will
obviously be futile.

Regards,
/Mati

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