[vsnet-chat 2321] Re: (fwd) Hipparcos/Tycho photometry

[Roger Pickard <rdp@star.ukc.ac.uk>]

The Guide7 software package available from Project Pluto (pluto@gwi.net) 
includes the B and V magnitudes from the Tycho Catalogue (i.e. those 
corrected to Johnson B and V).  This was done at the suggestion of 
Richard Miles, an amateur photometrist from the UK.
Roger Pickard



On Mon, 27 Sep 1999, Taichi Kato wrote:

> (fwd) Hipparcos/Tycho photometry
> 
> Date: Sun, 26 Sep 1999 13:21:14 -0700 (MST)
> From: Brian Skiff <bas@lowell.edu>
> 
>      Maciej Reszelski asked about the differences between the Hipparcos/Tycho
> photometry and the standard Johnson V and B systems.  There _are_ differences,
> and in some instances confusion can arise as it pertains to variable-star
> observing.
>      The Hipparcos "Hp" magnitudes result from essentially "unfiltered" data
> taken with the detectors on the spacecraft.  The sensitivity curve has a very
> wide passband across the visible part of the spectrum, and the magnitudes
> have been adjusted in an approximate way to conform to V.  However, there is
> a significant color term in these data, such that they should not be used for
> any other than internal comparisons for the _same_star_ (i.e. not even
> differentially), and color changes in, for instance, Miras will yield
> additional problems.  The 5th/95th percentile ranges given for the Hipparcos
> stars merely represent the span of magnitudes within which 90 percent of the
> measurements fall.  This gives some indication of variability when combined
> with knowledge of the scatter for certifiably constant stars of similar
> brightness, and allows for statistical tests, etc.  Even for bright stars, this
> range is typically a few percent, and for stars fainter than about mag. 8, the
> per-observation scatter becomes rapidly larger (the errors are about 0.4 mag.
> by mag. 10!).
>      The Tycho part of the instrument observed in two filter passbands that
> were similar to B and V, but again there is a substantial color term, and in
> the published Tycho catalogues these raw "instrumental" data have been
> transformed to the standard system and are shown explicitly.  The "raw" data
> are coded as "VT" and "BT" for the yellow- and blue-light observations---these
> are the ones to avoid!  The corrected magnitudes are specified as Johnson
> B and V.  I don't understand why the consortium bothered to show the "VT" and
> "BT" magnitudes.  The idea was, I gather, to preserve whatever internal
> precision there is in the data, which any transformation will inevitably
> reduce.  This seems in my opinion to be a misplaced emphasis, since the
> internal errors are already 0.1 mag. at V mag. 8.0 (i.e. lousy photometry),
> and 90 percent of the Tycho stars are fainter than this.  Another result has
> been that those unfamiliar with photometric systems (including professional
> astronomers) have confused the two sets of data, leading to all sorts of
> problems.  
>      Although many star-chart software packages have the H/T data incorporated,
> I am unsure about which sets of magnitudes are used.  It is easy to look up
> data for individual stars using the Strasbourg 'VizieR' utility on the Web:
> 
> http://vizier.u-strasbg.fr/cgi-bin/VizieR
> 
> There are mirror copies in the US (which I find slower than the French one) and
> in Japan.  Ask for catalogue "i/239" in item 1 of the form this URL brings up.
> It's best to have accurate (~0'.1) coordinates handy for this.
> 
> \Brian
> 
>

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