[vsnet-chat 5481] GCPD vs Tycho2

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

My recent posting of the V sequence for HR Car [vsnet-sequence 176,
vsnet-chat 5475], attracted some comment from Thom Gandet and Sebastian
Otero.  One can approach the issue of sequences from two standpoints:
1).  The photometric purist, for whom standardization and the second or
third decimal place is _always_ important. And rightly so for someone
performing their own photometry.
2)  The visual observer, who notes the second decimal place in a V
magnitude, but in daily use has to round off to one decimal. The second
decimal becomes academic.  The active visual observer values a sequence more
for its consistency.
A sequence star which, though photometrically correct, just doesn't fit in
with the rest of the sequence, becomes a nuisance.
We have all come across these examples at one time or another.

My sequence for HR Car, while it includes some Tycho-2 data, is ultimately
aimed at the visual observer. That is why there are 11 stars in the
sequence. An experienced photometrist can get by with one comparison star
and a check star (or two).

GCPD vs Tycho-2.
I have been using the GCPD (in various iterations) since 1979, so I am
fairly wise to its strengths and limitations. As a compilation of
ground-based UBV data, it is virtually 100% complete for stars down to mag.
6.5.  And stars down to 7.0V are also well served, but below that level
there are enormous gaps.  The GCPD data is good, the problem being that
there is just not enough of it (usually in the sky regions of particular
interest). In comparison, Tycho-2 V mags, while being of diminishing
accuracy at the faint end, is quite satisfactory for _visual_ observers,
down to mag. 10.0 or slightly fainter.  It is a one-stop shop for the whole
sky.
As a matter of interest, I took my HR Car sequence (11 stars) and looked up
each star in the GCPD.  The GCPD returned UBV data for only 3 stars out of
the 11. They are HD 89740 (V= 6.922), HD 90087 (7.780) and HD 90313 (8.38).
There is a lot of useful published UBV data which has never made it into the
GCPD, for various reasons (stars marked only on charts, no coordinates
published, no HD or DM nos. given, or whatever).
Over a couple of decades, I have followed a simple rule : NEVER  accept
published data blindly, always look for confirmation/verification from a
different source.  In this way typos and misidentifications are detected, as
well as other discrepancies.

Regards,
Mati Morel
morel@ozemail.com.au
http://vsnet.ozemail.com.au/~morel

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