[vsnet-chat 1329] Henden magnitude sequences

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

     Thanks to work from Arne Henden at USNO-Flagstaff, I have been able to
make a substantial addition to my collection of photometry for faint field
stars for use in calibrating sky survey images.  At the request of several
variable-star observers and organizations, Arne recently observed over two
dozen fields to obtain BV magnitude sequences.  Apart from a couple of
exceptions the fields were observed on at least two nights each.  The original
files total nearly 2.5Mb, involving around 9000 stars ranging from about
mag. 12 to 19.  Henden's source data are available for the moment at:

http://ftp.nofs.navy.mil/pub/outgoing/aah/sequence

Be sure to look at the file 'readme.1st' for details of the observations.
(Because of ongoing repairs following a lightning strike, this address may not
always be reachable at present.)
     For the purposes of calibrating wide-field CCD images, only a handful of
stars are needed in any single field, and because of the way Arne reduced the
data, quite a lot of the stars have very large photometric uncertainties.  Thus
some editing of the data is called for.  I trimmed the file down to about 790
stars as follows.  In each field I first sorted the stars to omit all those
with errors in V _and_ B-V larger than 0.030 magnitudes.  After sorting the
remainder in order by V magnitude, I selected more-or-less randomly three or
four stars in each magnitude bin from bright to faint so that the stars fall
at intervals of around 0.3 or 0.4 mag.  If a bit of color range was available,
I sometimes added a star or two; if one star among several with similar
brightness had smaller errors, I chose it over the others.  In any case, I have
not examined any of the stars for crowding, relying instead on the small
photometric errors to omit the worst cases.  The files include RA/Dec in
degrees, and these were converted to ordinary units at one-arcsecond precision.
Another change is that V and B-V are given to just two decimals rather than
three as in the source data.  Finally, it is worth noting that the stars in 
each field were not selected to be appropriate for visual variable-star
observers, and the sequences chosen for that purpose will likely consist of an
entirely different set of stars.
     The trimmed list has been larded into the file of BVRI photometry at:

http://ftp.lowell.edu/pub/bas/starcats/loneos.stds

...which now comes to about 290Kb and contains 4000 stars all over the sky.
It is worth reiterating that only a few of the stars in this file are bona-fide
standards; nearly all are merely observed by somebody, although I have taken
some care to avoid poor work in the literature (there's plenty of it!).  The
list does include many of the fainter, well-observed Landolt equatorial
standards, and high-quality data from elsewhere.  The list is appropriate for
getting magnitude zero-points to within ~0.05 mag. from wide-field CCD data.

\Brian

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