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Standard Star Fields Suitable for UBVRI Photometric Calibration of the WIYN CCD Imager (S2KB)
***DISCLAIMER*** These pages, field-of-view, and exposure times were all developed with the specific intent to assist in observations at the WIYN 3.5-m Observatory with the now retired S2KB imager.
Paul S. Smith
NOAO/Kitt Peak National Observatory
January 1997; revised September 1998
Photometric standard stars chosen from Landolt (1992) are presented in an atlas of 156 charts produced from Digital Sky Survey. Stars are distributed throughout all hours of Right Ascension and most of these UBVRI standards are equatorial. Charts are presented with the same field of view (6.8' x 6.8') as the (now retired) CCD imager employed at the WIYN telescope.
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 with the WIYN telescope.
Selection of Standard Stars
UBVRI photometric standard stars were chosen from Landolt (1992). This atlas contains 258 standard stars in 156 fields corresponding to the WIYN Imager field of view (6.8' × 6.8'). 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 are shown at a scale which corresponds to the field of view of the WIYN Imager (6.8' × 6.8'). The WIYN Imager employs a 2048 × 2048, thinned Tek/STIS CCD with 21 µ m pixels (S2KB). 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 all 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.
There are two tables associated with each finding chart. The first table 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 extremely BLUE in the last column of the table. Stars with B-V > 1.400 are flagged as being extremely RED.
The second table associated with the each chart is a guide to suitable exposure times using the WIYN Imager based on the flux received from the brightest standard star in the field. This table is discussed in the following section.
Proper exposure length for a standard star field is a relatively complex topic exacerbated by the fact that the large aperture of WIYN leads 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. Tests of the WIYN Imager shutter indicate that the shutter correction is < 0.01 s. Therefore, photometric accuracy should be better than 1% if the exposure time is 1 s or greater. However, a more conservative approach may be desired to minimize any systematic effects caused by the operation of the shutter; especially when one considers that the read-out time (~2.5 minutes) of S2KB will typically far exceed the actual exposure time. Therefore, the "recommended" exposure times listed in the atlas are set to at least 10 s, if possible, to ensure that any reasonable shutter correction is not an issue. Of course, at WIYN many of the standard stars are bright enough to saturate (> 65,000 ADU) the peak pixel in 10 s.
The exposure table for each field lists the filter, recommended exposure time, and the exposure time that will likely saturate the peak pixel of the brightest standard star in the field (not including secondary standards). Exposure times for the B, V, R, and I bands were determined empirically using photometric data obtained with the WIYN Imager during 1996 November. These data yield the following transformation equations:
- b-B = (-24.7373 ± 0.0139) + (0.2388 ± 0.0094)AM + (-0.0691 ± 0.0041)(B-V)
- v-V = (-24.9037 ± 0.0118) + (0.1373 ± 0.0079)AM + (-0.0160 ± 0.0030)(B-V)
- r-R = (-25.0757 ± 0.0121) + (0.1043 ± 0.0082)AM + (-0.0106 ± 0.0053)(V-R)
- i-I = (-24.3335 ± 0.0168) + (0.0400 ± 0.0111)AM + (-0.0957 ± 0.0091)(R-I),
where V is the apparent magnitude as determined by Landolt (1992), v is the instrumental magnitude (upper- and lower-case letters are similarly used to distinguish between apparent and instrumental magnitudes for the other filters), and AM is the airmass. The flux received from a star can be calculated from these equations and an appropriate exposure time determined that does not result in saturated pixels, but yields enough signal that the formal uncertainty in the instrumental magnitudes is < 0.01 mag.
Exposure times for the U-band were estimated by first using the "ccdtime" task within IRAF to find the expected ratio between the V and U exposure times that attain the same signal-to-noise ratio in the two bandpasses. The U-band exposure time listed for a field in the atlas was then found by simply scaling this ratio by the empirical result for V.
All estimates of exposure times were made assuming an AM = 1.2 in seeing of 0.8" FWHM. The recommended exposure time should result in ~10,000 ADU in the peak pixel for the brightest standard star in each field. In many cases, longer exposure times are recommended so that fainter standard stars in a field yield more counts without saturating the peak pixel of the brightest star. Note that in some fields, an observer may wish to saturate the brightest star in order to obtain enough signal on fainter standard stars. Also, it should be remembered that the exposure times given in the atlas should be used as a very rough guide. There is no substitute for careful examination of image quality and exposure level of photometric calibration frames by the observer immediately after an image is obtained at the telescope. For example, the number of ADU in the peak pixel is highly dependent on seeing and focus. Exposure times can be quadrupled without encountering saturated pixels if the seeing is 1.6".
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.
References:Landolt, A. U. 1992, AJ, 104, 340
Last modified: 03-Apr-2020 15:25:05 MST
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