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The message is from John Greaves:
A quick and dirty plot of TASS Mk III Rc versus USNO B1.0 R1, with only a few
hundred data points, suggests the bimodality issue is a consequence of R1 (ie
it crops up again, even though no r'_CMT is be[ing used).
[I've heard of comparative planetology, but this is ridiculous ;^) ]
It should always be remembered that CMC13, USNO B1.0, TASS Mk III, TASS Mk IV,
and others, are all calibrated against Tycho2 BT and VT, which starts breaking
down about mag 10.5 (except it's Tycho1 for TASS Mk III).
This probably means that most of them aren't truly independent re mutual
comparisons, and all of them are faint extrapolations from bright calibrators
for most of their magnitude ranges.
Cheers
John
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