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[vsnet-chat 887] Re: Magnitudes, sequences, etc



   Returning to the original topic, the main issue of (at least temporarily,
until good sequences become available) using GSC (or USNO) as visual
comparison stars is the inhomegeneity of the GSC between the northern
and southern hemispheres.  Northern GSC magnitudes are usually based on
'Quick-V' exposures, which roughly approximates the V response, while the
southern part is from the existing blue survey plates.  Scatter and zero-point
error of northern GSC sequence is usually as good as (or even better) AAVSO
preliminary charts, when used carefully (reject too bright -- often close
double -- stars etc.).  I once surprised to see my CCD measures and GSC
magnitudes agreeing within 0.2 mag for almost every star measured (later I
learned it's a lucky case; some fields are substantially worse).   From
visual observer's standpoint, northern GSC looks usually enough for initial
guesses for "new" objects without available charts.  People even tend to
continue using GSC magnitude even some "preliminary degree" charts are made
available (Nova Sco 1997 magnitudes are mostly based on GSC sequence).
I presume the time for an observer is too limited to continuously prepare
materials before observation.  If some printout from a star mapping software
looks reasonable, it would be no wonder if the observer has no motivation
to spend addtional time in gathering and plotting new photometric data.

   The recent SNe misfortunes seem to arise from the southern extension
of the "received availability" of the northen GSC as temporary visual
standard.  If only 'Quick-V' survey were employed in producing the GSC...

Regards,
Taichi Kato

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