[vsnet-chat 5484] Re: [AAVSO-DIS] GCPD vs Tycho2

["Sebastian Otero" <varsao@fullzero.com.ar>]

Mati, Thom and all,

> One can approach the issue of sequences from two standpoints:
> 1).  The photometric purist, for whom standardization and the second or
> third decimal place is _always_ important. And rightly so for someone
> performing their own photometry.
> 2)  The visual observer, who notes the second decimal place in a V
> magnitude, but in daily use has to round off to one decimal. The second
> decimal becomes academic.  The active visual observer values a sequence
more
> for its consistency.

This generalization doesn't apply.
An accurate sequence is a consistent sequence. You can't be sure of the
magnitudes of Tycho-2 and so there is no consistency secured when sometimes
a star in your sequence can be 0.8 mag. off randomly.
That happens a lot even for stars as bright as 10.5. There was a disucssion
on vsnet about the comparison star sequence of NSV 6160 based on Tycho-2.
One star's magnitude was totally off and its color was similar to other
stars in the sequence.

Furthermore, I think the goal is the possibility to compare different sets
of data, so the photometric purists may well be us, visual photometric
purists. I wouldn't do this if I didn't want to do it the best I can and the
more useful possible. The lightcurve I presented for HR Car includes my
visual observations to 2 decimal places because a lot of information is lost
when rounded off to 0.1 mag. It is not important for a mira but it is
important for stars of lower amplitude.

> A sequence star which, though photometrically correct, just doesn't fit in
> with the rest of the sequence, becomes a nuisance.
> We have all come across these examples at one time or another.

Photometrically correct is simply correct for me.
Another point: a sequence with gaps of 0.7 mag. is not useful for me.
Difference smaller than 0.3 mag. are very much more useful because you can
improve the accuracy by detecting small brightness differences.


> My sequence for HR Car, while it includes some Tycho-2 data, is ultimately
> aimed at the visual observer. That is why there are 11 stars in the
> sequence. An experienced photometrist can get by with one comparison star
> and a check star (or two).

I would say there are only 4 stars, since the brightness of HR Carinae
hasn't been fainter than 8.3 for several years and never goes fainter than
8.8.



> GCPD vs Tycho-2.
> I have been using the GCPD (in various iterations) since 1979, so I am
> fairly wise to its strengths and limitations. As a compilation of
> ground-based UBV data, it is virtually 100% complete for stars down to
mag.
> 6.5.  And stars down to 7.0V are also well served, but below that level
> there are enormous gaps.  The GCPD data is good, the problem being that
> there is just not enough of it (usually in the sky regions of particular
> interest). In comparison, Tycho-2 V mags, while being of diminishing
> accuracy at the faint end, is quite satisfactory for _visual_ observers,
> down to mag. 10.0 or slightly fainter.  It is a one-stop shop for the
whole
> sky.

But I don't get the point in throwing away better data... If you don't have
anything else, that's okay, but you are not winning anything by using bright
Tycho-2 magnitudes, regardless you are a pepper, a visual observer or
whatever. You are using Tycho-2 magnitudes converted to Johnson's V, so they
SHOULD be the same (that's the point in converting them) as Johnson's V. If
not this is a problem of the catalogue, of the convertion formulae, of the
detector or whatever. There is no need for it to be my problem too.... Some
are equal to V , others no. So this is not consistency. The fact that the
magnitudes are published together or in the same catalogue doesn't mean they
are good.
PEP magnitudes are alwayas better. I would ony agree of using a different
source if this was in another system, but it sounds a little strange not to
use actual Johnson's V magnitudes but using JOHNSON'S V magnitudes from
Tycho-2 instead!!


> As a matter of interest, I took my HR Car sequence (11 stars) and looked u
p
> each star in the GCPD.  The GCPD returned UBV data for only 3 stars out of
> the 11. They are HD 89740 (V= 6.922), HD 90087 (7.780) and HD 90313
(8.38).
> There is a lot of useful published UBV data which has never made it into
the
> GCPD, for various reasons (stars marked only on charts, no coordinates
> published, no HD or DM nos. given, or whatever).

You can get something more from VizieR for fainter stars, and, of course for
Hipparcos, with an accuracy compared to V from the ground, when talking
about bright stars.
Don't forget that this disucussion is about HR Car and this is a bright
variable nowadays, so we DO have PEP magnitudes for this one.

> Over a couple of decades, I have followed a simple rule : NEVER  accept
> published data blindly, always look for confirmation/verification from a
> different source.  In this way typos and misidentifications are detected,
as
> well as other discrepancies.

Mati, we follow the same rule. That's why I don't use Tycho-2 magnitudes for
bright stars.
The GCPD not only gives you mean UBV values, the Geneva, uvby, WBVR,
Straizys, and other data is very useful and can be used to check mgnitudes
if properly used. Hipparcos data are pretty accurate if porperly converted
to V. I simply can't understand how it can be even compared with messy Tycho
data (as Thom pointed out) for the same star...

I think we all look to make the most out of our observations. But we have
different ways of approaching the issue.
That will be ok as long as you get good results. Useful data.

Regards,
Sebastian.


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