[vsnet-chat 3012] (fwd) TYC 8659 00702 1 (Greaves)

[Taichi Kato <tkato>]

(fwd) TYC 8659 00702 1 (Greaves)

Date: Wed, 31 May 2000 07:56:07 +0000
From: JG <jgts@jgws.totalserve.co.uk>
Subject: TYC 8659 00702 1

The Tycho1 Vj magnitude for this star is derived from only 42 photometric
transits in VT & BT, with a fairly low signal to noise ratio of around 8.
Vj 10.55 is near the accuracy limit of Tycho1, anyway, though 10.45 from
Tycho2 is still a mag shy of that limit. There are still only 42 transits
available to Tycho2, of course [normal number 150 - 200].

The scatter noted in the Tycho1 catalogue is not particularly large for a
star of this magnitude as per that catalogue, as instrumental effects also
play a part.

Epoch photometry in the VT and BT passband is available in Epoch Photometry
Annex B : such data is available upon email request from the CDS Strasbourg
[http://cdsweb.u-strasbg.fr , then questions@simbad or similar link: give
full Tyc1 ID, & mention annex _B_].

NOTE that it can range from extremely difficult to impossible to
distinguish instrumental effects from true variability using the data in
Annex B, and full guidelines as per Tycho1 Catalogue Volume 1 should at
least be read before making use of it.

VERY IMPORTANT : whether epoch photometry is available in Annex B or not
for any particular star is flagged in the Tycho1 catalogue.  People _should
not_ hassle the simbad folk unless they know data is actually extant in
annex B!  Annex A data comes distributed with the Tycho1 catalogue CDROMs,
& is obtained in that way [?possibly also via ESTEC webpages?].  NOT _all_
Tycho stars have epoch photometry available, so annex A and B are a subset
of the full number of Tycho stars! It is not the SIMBAD staff's job to find
out what Tycep annex a star is in, but only to provide data for any
particularly requested star(s) known to be in Annex B.


Never has data been so continually abused as the photometric results from
the "bonus" astrometric Tycho experiment on the astrometric Hipparcos
satellite.  This is somewhat uncalled for, as the catalogues' compilers
went out of their way to include relevant secondary data useful in
assessing the quality of the primary data, and to note all problems in the
documentation. Tycho photometric data can be very good if used critically.


Cheers

John

John Greaves
UK

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