SDSS data users, If you want 2.5m photometry that is as close to AB as we currently understand how to make it, apply the following corrections to the magnitudes output by the EDR or DR1 pipelines u(AB,2.5m) = u(2.5m) - 0.042 g(AB,2.5m) = g(2.5m) + 0.036 r(AB,2.5m) = r(2.5m) + 0.015 i(AB,2.5m) = i(2.5m) + 0.013 z(AB,2.5m) = z(2.5m) - 0.002 where, eg, u(AB,2.5m) is the best approximation to an AB magnitude in the 2.5m natural system, and u(2.5m) is the magnitude output by the SDSS photometric pipelines for EDR data (or post-EDR data processed with EDR pipelines) or the about-to-begin DR1 processing with new pipelines. For more information, see details in sdss-calib/845. To explain: It may not be widely recognized that in preparation for DR1, SDSS photometric calibration has moved away (very slightly) from a true AB definition. This is because the calibrators on the survey wanted to use saner equations, but the target selectors wanted backwards-compatibility. This led to a compromise, which could be described as: The 2.5m photometry is presented in a 2.5m natural system, but zeropointed so that the typical star used by NFCALIB has roughly the same 2.5m and USNO magnitudes. With this compromise, DR1 and EDR photometry will be on the *same* system, albeit a slightly non-AB system. (Please do not take this message to be an endorsement of the compromise. Nor as an endorsement of the AB system!) For consideration: Note that the Collaboration *could* decide to apply these corrections *after* processing but *before* public data distribution in DR1. Or we could leave things as-is, and list these corrections in the "documentation". If I have to read the tea leaves, I predict that we will do the latter. Hogg -- David W. Hogg * assistant professor Center for Cosmology and Particle Physics Department of Physics, New York University 4 Washington Pl, New York, NY 10003 212-992-8781 (tel) * 212-995-4016 (fax) david.hogg@nyu.edu * http://physics.nyu.edu/hogg/ DSS DSS DSS DSS DSS Digital Sky Survey DSS DSS DSS DSS DSS DSS DSS DSS This is message 2486 in the sdss-general archive, URL DSS http://www.astro.princeton.edu:81/sdss-general/msg.2486.html DSS The index is at http://www.astro.princeton.edu:81/sdss-general/INDEX.html DSS To join/leave the list, go to DSS http://www.astro.princeton.edu:81/cgi-bin/mailinglists.pl/show_subscription?list=sdss-general DSS To post a message, mail it to sdss-general@astro.princeton.edu DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS