[vsnet-chat 2387] Re: SN1999em sequence problem (Ripero)

[Hitoshi YAMAOKA <yamaoka@rc.kyushu-u.ac.jp>]

Dear all,

  Generally speaking, the GSC magnitudes for equatorial and southern
stars, say Decl. < +3 deg, is near to B mag, as Prof. Skiff noted.
The GSC magnitudes are something 'raw' one measured on the
photographic plates.  The northern part of the sky is covered by
'photovisual' plates, so the GSC magnitudes for the northern stars are
sometimes appropriate to be used as comparison sequences at the early
phase when the precise photometries has not been available.  The
southern part is, however, taken by UK schmidt with 'J', whose
response is near to B colorband.  GSCmag for them should not be used
for visual or CCD magnitude estimates if the other source comes to be
available. 

  On the other hand, the USNO catalog gives the red magnitudes (mag)
and the blue magnitudes (bmag), which are the converted values from
the 'raw' magnitudes measured from some photographic plates with
different emulsions/filters.  The internal error for the conversion
and the errors arose from the plate-to-plate differences are,
unfortunately, not small.  The VSNET sequence, which is based on
USNO-A1.0 magnitudes and converted to visual magnitudes, is also
intended for the temporal use, I guess.

  For SN 1999em case, Prof. Henden had kindly given us the photometric
sequence of this region (vsnet-chat 2382 -- 2384).  After this, we
should use this sequence for comparison.  The visual observer should
use V sequence, and the user of CCD with no filter can be use V or R
sequence, according the response of the chip.  If it is possible, the
magnitude estimates reported previously should be re-calibrated with
Henden's sequence.  

  The following is a cross-identifications of GSC/VSNET/AAVSO/Henden
sequence, omitting the brightest stars.  The order is roughly south to
north.

GSC_ID    VSNET AAVSO   V     B-V    V-R    R-I
4739-0111 1240  134   12.854  0.780  0.460  0.511
4739-0434 1262  124   12.348  1.225  0.743  0.716
4739-0235 1470  149   14.827  0.804  0.518  0.530
 --       1539  156   15.607  0.693  0.457  0.522
4739-0125 1260  130   12.966  1.169  0.709  0.660
4739-0012 1401  139   14.280  0.667  0.460  0.469
4739-0155 1344  144   13.748  0.848  0.586  0.544
4739-0459 1330  142   13.566  0.876  0.622  0.611
4739-0598 1388  138   13.865  0.717  0.523  0.531

Prof. Henden wrote:
>There seems to be considerable difference on a number of the stars.

  The GSCmags for AAVSO star 134, 142 and 144 are 13.48, 14.22 and
14.38, respectingly.  It is probable that AAVSO mag for these stars
were extracted from GSC, which can differ from V.  

Sincerely Yours,
Hitoshi YAMAOKA, Kyushu Univ., Japan
yamaoka@rc.kyushu-u.ac.jp

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