The u'g'r'i'z' Standard Star Network

TL;DR

The paper establishes a 158-star u'g'r'i'z' standard network to calibrate SDSS photometry, anchored to AB magnitudes via synthetic photometry of BD+17 4708 and tied to two additional fundamental stars. It details the instrument setup, filter system, observing strategy, and a MTPIPE-based reduction pipeline that derives nightly zeropoints and extinction terms, including a correction for a known red leak in the u' band. The results include calibrated five-band magnitudes and colors for the standard stars, plus transformations to the UBVRcIc system and comprehensive astrometric data. Looking ahead, the authors plan to extend the network to redder stars and the southern hemisphere and to broaden calibration ties with other surveys, aiming for robust, sky-wide photometric standards for SDSS and future surveys.

Abstract

We present the 158 standard stars that define the u'g'r'i'z' photometric system. These stars form the basis for the photometric calibration of the Sloan Digital Sky Survey (SDSS). The defining instrument system and filters, the observing process, the reduction techniques, and the software used to create the stellar network are all described. We briefly discuss the history of the star selection process, the derivation of a set of transformation equations for the UBVRcIc system, and plans for future work.

Figures (15)

• Figure 1: The linearity curve for the TK1024 CCD used on the USNO 1.0-m telescope for this program (solid line). $DN_{\rm meas}$ is the raw, bias-subtracted value of the signal; $DN_{\rm true}$ is the value that would have been measured if the CCD were completely linear. Note the "knee" at $DN \approx 15,200$ ADU. The dashed line acts as a reference for what a fully linear relation would look like.
• Figure 2: The $u'g'r'i'z'$ system filter bandpasses convolved with a typical coated CCD. The curves represent the expected total quantum efficiencies of the camera plus telescope on the sky. Solid curves indicate the response function without atmospheric extinction; dashed curves include extinction at 1.2 airmasses at the altitude of the U.S. Naval Observatory's Flagstaff Station.
• Figure 3: The (normalized) responses of the $u'g'r'i'z'$ system bandpasses (at 1.2 airmasses of extinction) compared those of the the Johnson-Morgan-Cousins ($UBVR_{\rm c}I_{\rm c}$) system. (Filter curves for the Johnson-Morgan $UBV$ filters and for the Cousins $R_{\rm c}I_{\rm c}$ filters were obtained from The General Catalogue of Photometric Data at http://obswww.unige.ch/gcpd/gcpd.html; MMH97.)
• Figure 4: The distribution of the primary standards in right ascension and declination. Clearly seen are the clustering of stars near the celestial equator and the relative dearth of standards in the southern hemisphere. Most of the equatorial fields contain multiple stars therefore, though there are 158 stars in the system, there are not as many individual points on this plot. The three fundamental standards --- BD+17$\arcdeg$4708, BD+26$\arcdeg$2606, and BD+21$\arcdeg$0607 are indicated by the filled symbols.
• Figure 5: The photometric zeropoints calculated for each night of data used in developing the standard star network. The mirrors were re-aluminized twice during our program --- once after the second month of observations and again prior to the last four months of observations (denoted by the dotted vertical lines). The first re-aluminization is clearly visible as a break in the zero point values while the second break is less obvious. The two large gaps ($\approx$mjd5100--51100 and mjd51350--51450) correspond to the two monsoon seasons (summers) in northern Arizona.