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