[vsnet-chat 864] Re: Magnitude Estimates for Current SNe

[bas@lowell.Lowell.Edu (Brian Skiff)]

     People once again seem to be forgetting that in the south (specifically
south of +3 Dec) the GSC magnitudes are more nearly _blue_ magnitudes, _not_ V,
and _not_ R-like magnitudes that appear in the catalogue in the northern
hemisphere.  This is why charts for NGC 4462 and its supernova are screwed up.
Right now I'm betting Berto's estimates for the comp stars are more nearly
correct than stuff based on the GSC.
     As a for instance, here are the V magnitudes and the GSC magntiudes
for the fainter stars in GSPC sequence S506 mentioned by Berto:

star    V      GSC 
C     11.09   11.76
D     12.04   12.41
E     13.34   13.96
F     14.14   14.84
G     14.58   15.36

These stars happen to have been observed on two to four nights each for the
GSPC, and this series has been shown to be quite reliable by photometrists at
SAAO.  Thus if you are observing visually, the supernova is half to three-
quarters of a magnitude brighter than indicated by magnitudes based on the GSC.
Likewise if you are adjusting CCD magnitudes using the GSC data to set the
zero-point, you'll come out way too faint because of this offset (and implied
color term) in the southern GSC system.
     This is why the magnitude estimates for the M81 supernova were
inconsistent (the GSC is off by -0.5 mag. in that region).  This is why, to
give another example, Pluto has been erroneously reported to be several tenths
of a magnitude fainter than predicted---those CCD observations were based on
differential photometry with respect to the GSC.  Now that Pluto is well south,
the observers were reporting in effect a blue magnitude rather than what they
were assuming was V or R---even though the GSC magnitudes, besides being a
hodge-podge, are in fact not on _any_ of these standard systems.
     In brief, don't use the GSC magnitudes for anything.  No, not anything.
     As for the USNO-A1.0 catalogue, I'll repeat again the first sentence of
the photometry "read me" file composed by Dave Monet, who oversees that
project and is responsible for the results:

     "The photometric calibration of USNO-A1.0 is about as poor as one can
have and still claim that the magnitudes mean something."

In his sarcastic manner, the guy who conceived of the project is telling you
the magnitudes are crap.  If you want more evidence, read the FAQ item at
the PMM Web page (http://psyche.usno.navy.mil/pmm/) that is called "The
photometry in the southern sky is hosed!".
     I realize everybody is desperate for getting photometry of these various
fields of interest (CVs, SNe, etc.), but is anybody out there willing to do the
work required to provide some calibrated photometry on short notice?  With so
many observers using CCDs, how come not a single person is doing this?  Instead
of trying to be first to spot an outburst of a CV, you could gain endless
appreciation from the astronomical community by spending your time setting up
comparison sequences.  Not all of you are at crummy sites, and with a modest
amount of effort, you can make a big difference.  Such results are publishable
(and should be!), so this is not wasted effort:  well-done sequences have
wide use far beyond the immediate needs of the supernova or other transient
target.
     ...sorry for the ranting, but it seems like folks are flailing around
without taking some action on the problem.

\Brian

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