Subject: correcting SDSS magnitudes onto the AB system

From: David Hogg

Submitted: Wed, 14 Aug 2002 14:48:28 -0400

Message number: 2486 (previous: 2485, next: 2487 up: Index)

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/

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