[vsnet-chat 880] Re: Magnitudes, sequences, etc

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

     To determine V magnitudes (say), one needs to observe in two filters,
V and some adjacent one (B or R usually) in order to correct for the color
term in the system (there's no such thing as a set-up with a zero color
term).
     Mike Bessell has published in several places (PASP, and the erstwhile
CCD Astronomy magazine) recipes for filters that very nearly establish the
bandpasses using the transmission of the filters alone---one says the
response of the system is "filter-defined".  Thus one needn't be concerned
about the varying response of different CCDs, as John suggests.  Any 
differences at V and R at least will be minor, and would have to calibrated
out in the color term in any case, since the sensitivity of commercial CCDs
is going to vary as a function of temperature (different in summer than in
winter).  
     I think John is also misinterpreting what we mean by "standards", if
he is referring to the earlier discussing about problems with the GSC, etc.
The GSC magnitudes were determined from only a single photographic plate, and
simply are not anything like "standards" or even reference stars.  By
"standards", we are referring to those data on stars that _define_ the system
of magnitudes and colors in different bandpasses.  For the common broadband
UBVRI system (where R and I are on the Cousins system), these consist of the
equatorial standards determined by Landolt and stars in the Harvard 'E' regions
at -45 Dec observed by Cousins himself and his younger colleagues at the
South African Astronomical Observatory.  There are slight systematic
differences between the two series, but I challenge any CCD observer to actually
detect them in all-sky photometry---the differences are microscopic for any
ordinary observing.  There is no real diagreement about the photometric system
these stars define, at least above the few-thousandths of a magnitude level.
     On the related subject of which two colors, I would argue that V and R
are the most practical.  The V-R color of star is not as diagnostic as either
B-V or V-I (say), but it the two colors have distinct advantages for amateur
observers and also for professionals following transient events.  We want V
simply because there are a lot of visual observers, and it is the familiar
magnitude everyone doing visible or near-IR astronomy reckons by.  Most amateur
CCD observers are running unfiltered, and in practice the resulting 'natural'
magnitudes are fairly close to Cousins R, at least for stars of ordinary color.
Thus being able to provide an R magnitude, and a V-R color in order to correct
for color terms is important so that results from all observers can be
compared.  Most CCDs reach their peaek sensitivity across the V and R colors,
so this means exposures are minimzed for varying objects, and one is able to
reach the faintest possible magnitudes for a given maximum exposure.  These
attributes are thus most advantageous for those at less than wonderful sites.

\Brian

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