Paul S. Smith
Steward Observatory, University of Arizona, Tucson, AZ 85721 USA
January 1997; revised March 2001
Photometric calibration of CCD imaging data is a vital, yet time-consuming,
requirement for many science programs.
Observing overheads such as telescope slew time and CCD read-out time make
it important for observers to choose a set of calibration stars
carefully in order to maximize time spent executing their actual science
programs.
This comprehensive atlas of UBVRI standard stars is presented
as an aid to observers wishing to obtain photometrically
calibrated imaging data.
UBVRI photometric standard stars were chosen from Landolt (1992). This atlas contains 258 standard stars in 156 fields each having a 6.8' × 6.8' field of view. The stars listed in the main index to the atlas meet the following criteria:
1.) Declination > -20° ,
2.) observed by Landolt on at least 5 nights, and
3.) is not a variable star or suspected variable.
The main index of the atlas quotes the star's designation (linked to its finding chart), J2000 coordinates, V
magnitude, and B-V, U-B, V-R, and R-I color indices
from Landolt (1992).
The last two columns of the index indicate whether the star has
extreme colors and the chart number(s) (linked to the relevant chart) on which the star appears,
respectively.
A star is designated as BLUE if B-V < 0.000 and RED
if B-V > 1.400.
These stars are particularly useful for accurately determining
the color terms for the photometric transformation equations.
All finding charts have a field of view of 6.8' × 6.8'. The charts were produced using the STScI Digital Sky Survey (© 1994, Association of Universities for Research in Astronomy, Inc.) and are shown with north at the top and east to the left. Just off of the upper left (NE) corner of each chart is the field designation. Along the top of each chart are the J2000 coordinates of the field center. For fields that include several standard stars, the telescope should be pointed at the coordinates of the field center to ensure that most stars are included in the frame.
Standard stars meeting the criteria listed in are labeled in
black on the finding charts.
In addition, many fields include other, ``secondary''
standards from the list
of Landolt (1992) that are not included
in the main index (typically, these stars have not been observed on more
than 4 nights).
These stars are labeled in white. The white labels are difficult
to see in many charts and investigators should use the charts in Landolt
(1992) to verify the identifications of the secondary standard stars if
you wish to include them in the photometric solution.
One advantage of photometry with a CCD is that several standard stars may be observed simultaneously. Choosing fields that include several standards minimizes telescope slews and the number of exposures needed to derive a sound set of photometric transformation equations. Fields that possess a group of standard stars with a large range of colors are particularly useful. A table lists fields in the atlas with direct links to finding charts that should be very useful to observers wishing to include many standards in their photometric calibrations without having to pay the substantial penalty in overhead time inherent in observing single-star fields. The list of recommended fields can also be found from the main index. Information given in the list of recommended fields includes the chart number, J2000 coordinates of the chart center, number (N) of standard stars meeting the selection criteria, number of secondary standard stars (Nsecondary), number of standards with B-V < 0.000 (NBLUE), and number of standards with B-V > 1.400 (NRED). Both NBLUE and NRED include any secondary standard stars that meet these color index criteria.
Associated with each chart is a table that reproduces the same positional and photometric information given in the main index for all standard stars in the field. This includes standards that do not meet selection criteria and these stars are designated with an "S" (secondary) in the "Class" column of the table. Again, if B-V < 0.000, the star is flagged as being BLUE in the last column of the table. Stars with B-V > 1.400 are flagged as being RED.
Proper exposure length for a standard star field is a relatively complex topic
exacerbated by the fact that a large telescope aperture and
sensitive CCD lead to
exceedingly short exposures for many stars (especially late-type
or highly reddened stars).
Short exposure times present the possible problem that slight variations
in the operation of the shutter
mechanism and any shutter correction (the difference
between requested exposure time and the actual time that the
shutter is open) are more significant and may corrupt the photometric
solution.
I thank Nigel Sharp for identifying and correcting an error in the VX software, and thereby allowing labels to be properly added to the charts.
The Digitized Sky Survey is based on photographic data of the National Geographic Society-Palomar Observatory Sky Survey (NSS-POSS) obtained using the Oschin Telescope on Palomar Mountain and on photographic data obtained using the UK Schmidt Telescope. The NGS-POSS was funded by a grant from the National Geographic Society to the California Institute of Technology. The UK Schmidt Telescope was operated by the Royal Observatory Edinburgh, with funding from the UK Science and Engineering Research Council, until 1988 June, and thereafter by the Anglo-Australian Observatory. Original plate material is copyright (©) the Royal Observatory Edinburgh and the Anglo-Australian Observatory. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166.