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The differences in reporting and vetting of datasets in vsnet and
AAVSO seem to arise mainly from what you might call cultural differences.
The problems with amateur observers (notably CCD'ers) having a poor or just
incorrect understanding of formal photometric systems has its cause not
in how things are reported by the different groups, but instead can be
blamed on ineffective communication by professionals (or those that _do_
know about photometry) as to what's desired and required. Rather than
casting aspersions about AAVSO, or coming to its defense when in fact the
system needs an overhaul, the leaders should be looking to see why these
problems are coming up. Both set-ups need work in my opinion: the AAVSO
seems to be very slow to react to things, though is thorough after-the-fact
without being very transparent about what goes on behind the scenes;
vsnet is fast on its feet but is an irresponsible youth, and should be more
careful nurturing beginning observers (i.e. probably 80 percent or more of
its contributors).
The silly idea of the AAVSO separating CCD V-band observations from
single-channel photoelectric V (and coding them as though they are different)
has led to the widespread notion that somehow CCD is "better" (or the reverse!)
and that they need to be kept separate (and the old-time photoelectric guys
won't talk to the new-fangled CCDers). The photometric scales are defined
independent of the detectors, so if data from two devices don't match,
then at least one of them is wrong.
A comparable charge can be lodged against the lack of vetting in
the vsnet data. It ultimately means that if you want to use datasets,
you have to do the culling of bogus data yourself. I can imagine there
are cases where you would end up with _zero_ useable data because _all_
of it has some systematic problems or its provenance is unrecoverable.
Finally you either take new data yourself, or identify a couple of
first-rate _visual_ observers you can rely on, and exclude everyone else
(that 80 percent mentioned above).
I'm in the camp of wanting folks to use standard filter passbands, and
encourage amateur variable-star observers to do so as well. Yes, there
are some applications where sheer signal-to-noise/time-resolution matters, and
an unfiltered system will work. But ultimately you need to place your data
on a standard system, and despite the loss of limiting magnitude, you must use
filters (and the correct ones, as Ulisse Munari pointed out!). The old
American phrase is that "there's no free lunch", and it applies everywhere
in photon-starved astronomy. You match the capabilities of your
telescope/detector to the stars that are available and the time you have.
Don't bother taking crappy data just because it's some amazing faint object---
CCDs are not magical, and the GIGO law still applies.
A V-band magnitude should consist of an observation using a V filter
against stars in the field (speaking just of differential photometry here)
that also have V magnitudes to ~0.01 precision/accuracy. In principle, you
should in fact be using _two_ filters to get V, so that always-present color
term can be assessed. (Just because you have a V filter doesn't mean a simple
observation is on the standard V system!) But in practice the V-only filter
is a lot better than nothing. If you are adjusting unfiltered CCD data to V,
these will have systematic errors, and will be rejected ultimately. If you are
using a long-pass filter, as Munari indicates some do, that's no good either.
If you are using the magnitudes from an old chart (typical older AAVSO chart,
for example), this is also not V, because the chart magnitudes are given to
only 0.1-mag precision and most of the time there are systematic errors in
zero-point, in scale, or comp-star color---or all three(!). It is possible to
do strictly differential photometry with a V filter using a single star from
one of those charts, which you hope will have standard V somewhere downstream,
but these data need to be reported only as magnitude-differences, not on an
absolute scale, since it will inevitably be wrong. Doubtless there will be
special cases, but I would tend to purge from the database any results not
taken in the first manner (or B, R, I, or other standard passbands). All the
'unfiltered' stuff would go whether transformed or not, all the "orange" stuff
would go, all the etc. In any publishable analysis it would all be omitted
anyway, so don't keep it, and certainly don't encourage people to waste time
taking/reducing such data. There are enough problems with data nominally taken
on standard systems that dealing with non-standard results is a waste of time.
Another source of problems I've noticed is that folks are believing
without question whatever numbers come out from a piece of commercial
software. (It's the "it comes from a computer, it must be right" syndrome.)
Uncertainties seem to be underestimated by factors of several as well. You can
test the software of course by measuring known constant standard stars and
seeing how close you get to "truth", and work on figuring out why errors
occur---is it the software or your observing? By doing such tests on many
nights (every night you observe), you'll get a feeling for what your errors
really are, and learn a lot in the process.
\Brian
Return to the Powerful Daisaku
vsnet-adm@kusastro.kyoto-u.ac.jp
