Subject: converting pipeline mags to AB mags
From: David Hogg
Submitted: Tue, 13 Aug 2002 18:02:35 -0400
Message number: 845
(previous: 844,
next: 846
up: Index)
Conversion from SDSS natural 2.5m "pipeline" magnitudes to an AB system
-----------------------------------------------------------------------
David W. Hogg (NYU)
2002 August 13
Summary: to get from the 2.5m natural magnitudes output by the SDSS
pipelines--denoted, eg, r(2.5m)--to our best guess at a "true" AB
system--denoted, eg, r(AB,2.5m)--use the following transformations:
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 the uncertainties in these corrections are probably at the
few-percent level at least, but also positively covariant. Note that
current DR1 (u-g) colors are off the AB system by about 8 percent.
-----
Details:
The following from Smith, Tucker et al, 2002, AJ, 123, 2121
Here is the (nearly) AB photometry for BD+17 in the USNO system
(denoted by, eg, "r'")
BD+17
r': 9.350 +/- 0.001
(u'-g'): 0.920 +/- 0.001
(g'-r'): 0.290 +/- 0.001
(r'-i'): 0.100 +/- 0.001
(i'-z'): 0.020 +/- 0.001
From the above I compute that BD+17 has, in the USNO system:
u' = 9.350+0.290+0.920 = 10.560
g' = 9.350+0.290 = 9.640
r' = 9.350 = 9.350
i' = 9.350-0.100 = 9.250
z' = 9.350-0.100-0.020 = 9.230
The following from http://www-sdss.fnal.gov:8000/edoc/jeg_photometric_eq.html
Here is how to convert USNO magnitudes (denoted by, eg, "r'") into
the natural 2.5m magnitudes output by the SDSS survey pipelines
(denoted by, eg, "r(2.5m)").
To convert from u'g'r'i'z' to 2.5m ugriz:
u(2.5m) = u' - b25(u)*( (u'-g')-(u'-g')_zp ) + zpOffset25(u)
g(2.5m) = g' - b25(g)*( (g'-r')-(g'-r')_zp ) + zpOffset25(g)
r(2.5m) = r' - b25(r)*( (r'-i')-(r'-i')_zp ) + zpOffset25(r)
i(2.5m) = i' - b25(i)*( (r'-i')-(r'-i')_zp ) + zpOffset25(i)
z(2.5m) = z' - b25(z)*( (i'-z')-(i'-z')_zp ) + zpOffset25(z)
where
b25(u) = 0.000
b25(g) = -0.060
b25(r) = -0.035
b25(i) = -0.041
b25(z) = 0.030
(u'-g')_zp = 1.39
(g'-r')_zp = 0.53
(r'-i')_zp = 0.21
(i'-z')_zp = 0.09
zpOffset25(u) = 0.000
zpOffset25(g) = 0.000
zpOffset25(r) = 0.000
zpOffset25(i) = 0.000
zpOffset25(z) = 0.000
Plugging in, I get, for BD+17, the SDSS pipeline magnitudes:
u(2.5m) = 10.560 - 0.000*( 0.920 - 1.390 ) + 0.000 = 10.560
g(2.5m) = 9.640 + 0.060*( 0.290 - 0.530 ) + 0.000 = 9.626
r(2.5m) = 9.350 + 0.035*( 0.100 - 0.210 ) + 0.000 = 9.346
i(2.5m) = 9.250 + 0.041*( 0.100 - 0.210 ) + 0.000 = 9.245
z(2.5m) = 9.230 - 0.030*( 0.020 - 0.090 ) + 0.000 = 9.232
These are the magnitudes BD+17 WOULD BE GIVEN BY THE SDSS PIPELINES
if it could be observed by the 2.5m without saturating the detector.
Daniel Eisenstein computes, for BD+17, plus 2.5m sensitivities, plus
1.2 atmospheres of typical extinction:
u(AB,2.5m) = 10.518
g(AB,2.5m) = 9.662
r(AB,2.5m) = 9.361
i(AB,2.5m) = 9.258
z(AB,2.5m) = 9.230
I.e., our best bet to estimate "true" AB magnitudes is to take the
SDSS pipeline outputs and make the following adjustments:
u(AB,2.5m) = u(2.5m) - 0.042 +/- 0.050
g(AB,2.5m) = g(2.5m) + 0.036 +/- 0.030
r(AB,2.5m) = r(2.5m) + 0.015 +/- 0.030
i(AB,2.5m) = i(2.5m) + 0.013 +/- 0.030
z(AB,2.5m) = z(2.5m) - 0.002 +/- 0.030
where the uncertainties are optimistic, educated guesses about the
quality of the spectrophotometry. On the other hand, if the
spectrophotometry errors are correlated over wavelengths, there will
be positive covariances among neighboring bandpasses, and maybe
overall. This means that although the u- and g-band adjustments are
only about one sigma individually, the (u-g) adjustment might be
many sigma.
Douglas Tucker points out that there is a definite contribution to
the uncertainties in the AB offsets from the uncertainties in the
correct values of the "b" coefficients:
(0.000 +/- 0.003)
(0.060 +/- 0.001)
(0.035 +/- 0.003)
(0.041 +/- 0.001)
(0.030 +/- 0.005)
but this is sub-dominant at present
Please email me with comments or questions.
--
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 845 in the sdss-calib archive, URL
DSS http://www.astro.princeton.edu:81/sdss-calib/msg.845.html
DSS The index is at http://www.astro.princeton.edu:81/sdss-calib/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-calib
DSS To post a message, mail it to sdss-calib@astro.princeton.edu
DSS
DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS DSS