J/A+A/506/999 Photometric Calibration of SNLS (Regnault+, 2009)
Photometric Calibration of the SuperNova Legacy Survey Fields.
Regnault N., Conley A., Guy J., Sullivan M., Cuillandre J.-C., Astier P.,
Balland C., Basa S., Carlberg R.G., Fouchez D., Hardin D., Hook I.M.,
Howell D.A., Pain R., Perrett K., Pritchet C.J.
<Astron. Astrophys. 506, 999 (2009)>
=2009A&A...506..999R 2009A&A...506..999R
ADC_Keywords: Photometry, sensitivity functions ; Photometry, SDSS
Keywords: cosmology: observations - techniques: photometric -
methods: observational
Description:
This is the photometric calibration of the SuperNova Legacy Survey
(SNLS) three year dataset. The SNLS corresponds to the DEEP component
of the larger Canada-France-Hawaii Telescope Legacy Survey (CFHTLS).
The SNLS repeatedly monitors four one square degree fields (labeled
D[1-4]) with the MegaCam wide-field imager, in the g, r, i and z
bands. u-band observations of the same fields are also available,
although not formally part of the SNLS dataset.
The non-uniformities of the MegaCam imager photometric response have
been mapped as a function of the position on the focal plane. The
fluxes, measured on the survey images processed with the CFHT Elixir
pipeline, have been corrected for these non-uniformities in order to
obtain measurements that are uniform at the ∼1% level.
The MegaCam passband transmissions were found to be non-uniform, the
filters being 3 to 6 nanometers bluer on the edges of the camera than
on the center. This result agrees with the filter scans provided by
the filter manufacturer (Sagem/REOSC). This has important
consequences on the definition of the MegaCam magnitudes: the natural
magnitude system depends on the focal plane location where the
observations were made. In order not to break the connection between
calibrated magnitudes and their physical flux counterparts, we chose
to report the *natural magnitudes* of each object, measured in the
MegaCam passbands, at the focal plane location where the object was
observed. We call this system, "Local Natural Magnitudes".
The non-uniformities of the MegaCam effective passbands are small and,
for main sequence stellar objects, they may be accounted for using
linear color corrections. The relation between the Local Natural
Magnitudes of a given star, observed at two locations x_0 and x of the
focal plane are:
g|x = g|x_0 + dkggr(x) * ( (g-r)|x_0 - (g-r)BD+17|x_0 )
...
z|x = z|x_0 + dkziz(x) * ( (i-z)|x_0 - (i-z)BD+17|x_0 )
where the dk(x) are (position dependant) color terms. We provide dk(x)
maps for each MegaCam band (see below).
The SNLS 3 year calibration relies on the (Landolt,
1992AJ....104..340) standard star catalog. Landolt fields are observed
during each photometric night along with the SNLS fields. Zero-points
are derived from these observations. Stable and isolated stars are
detected on the SNLS fields and selected as "tertiary standards". The
calibrated magnitudes of each tertiary standard obtained under
photometric conditions are combined to produce a calibration catalog
for each SNLS field.
To interpret the tertiary standard magnitudes as physical fluxes, we
need a primary standard, i.e. a star with known MegaCam magnitudes and
whose spectral energy distribution has been measured absolutely. The
SNLS uses BD+17 4708 whose SED has been measured in Bohlin &
Gilliland, 2004, Cat. J/AJ/128/3053 using the HST STIS and NICMOS
spectrographs. BD+17 4708 has not been directly observed by SNLS,
however, its MegaCam magnitudes were inferred from its known Landolt
magnitudes (this paper, table 7). The tertiary star Local Natural
Magnitudes are defined so that the associated physical broadband flux
f|x is given by:
f|x = 10{-0.4*(m|x-mref)}*∫[Sref(l)*T(l;x)]dl
where m|x is the tertiary star magnitudes at location x on the focal
plane, mref is the MegaCam magnitude of BD+17 4708 (at the center of
the focal plane, see table 7 of this paper), Sref(l) is the SED of
BD+17 4708 measured in 2004AJ....128.3053B 2004AJ....128.3053B, and T(l;x) is the
effective passband of MegaCam at location x.
Attached is all the necessary information to tie MegaCam observations
to this system. We provide (a) the griz magnitudes of the SNLS
tertiary standards for all four SNLS fields (b) the MegaCam open
transmission and the ugriz MegaCam filter scans at various position
along a diagonal (c) the final uncertainty budget in the form of 3
covariance matrices.
The u-band observations of the SNLS DEEP fields are not formally part
of the SNLS. Nevetheless we give u-band magnitudes for a subset of the
SNLS tertiary stars.
Objects:
-------------------------------------------------
RA (2000) DE Designation(s)
-------------------------------------------------
22 11 31.4 +18 05 34 BD+17 4708 = HIP 109558
-------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table19.dat 153 2546 Tertiary rgiz catalog for the SNLS field D1
table20.dat 153 3878 Tertiary rgiz catalog for the SNLS field D2
table21.dat 153 2827 Tertiary rgiz catalog for the SNLS field D3
table22.dat 153 5294 Tertiary rgiz catalog for the SNLS field D4
d1u.dat 54 1394 D1 tertiary stars u-band magnitudes
d2u.dat 54 2349 D2 tertiary stars u-band magnitudes
d3u.dat 54 714 D3 tertiary stars u-band magnitudes
d4u.dat 54 3051 D4 tertiary stars u-band magnitudes
tabled1.dat 123 9 Full covariance matrix of the BD+17 4708 MegaCam
and Landolt magnitude uncertainties
tabled2.dat 123 9 Covariance matrix of the uncertainties affecting (1)
the differences between the tertiary MegaCam
magnitudes and the MegaCam magnitudes of BD+17 4708
and (2) the Landolt magnitudes of BD+17 4708
tabled3.dat 123 9 Covariance matrix of the uncertainties affecting the
synthetic instrumental magnitudes of BD+17 4708
table13.dat 15 7190 Open transmission of the MegaCam imager (no filters)
table14.dat 95 1545 u_M filter scan (Sagem/REOSC)
table15.dat 95 371 g_M filter scan (Sagem/REOSC)
table16.dat 95 357 r_M filter scan (Sagem/REOSC)
table17.dat 95 446 i_M filter scan (Sagem/REOSC)
table18.dat 95 632 z_M filter scan (Sagem/REOSC)
dkuug.fit 2880 78 dk(x) map (u band)
dkggr.fit 2880 78 dk(x) map (g band)
dkrri.fit 2880 78 dk(x) map (r band)
dkiri.fit 2880 78 dk(x) map (i band)
dkziz.fit 2880 78 dk(x) map (z band)
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Byte-by-byte Description of file: table19.dat table2?.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right ascension, in decimal degrees (J2000)
13- 21 F9.5 deg DEdeg Declination, in decimal degrees (J2000)
25- 30 F6.3 mag gmag ?=-1 g-band magnitude (Vega-like system)
34- 39 F6.3 mag e_gmag ?=-1 Uncertainty on the g-band magnitude
43- 45 I3 --- o_gmag [1,40]?=-1 Number of observations
49- 54 F6.3 --- dkg ?=-1 Grid g-band dk color term for this object
58- 63 F6.3 mag rmag ?=-1 r-band magnitude(Vega-like system)
67- 72 F6.3 mag e_rmag ?=-1 Uncertainty on the r-band magnitude
76- 78 I3 --- o_rmag [1,40]?=-1 Number of observations
82- 87 F6.3 --- dkr ?=-1 Grid r-band dk color term for this object
91- 96 F6.3 mag imag ?=-1 i-band magnitude(Vega-like system)
100-105 F6.3 mag e_imag ?=-1 Uncertainty on the i-band magnitude
109-111 I3 --- o_imag [1,40]?=-1 Number of observations
115-120 F6.3 --- dki ?=-1 Grid i-band dk color term for this object
124-129 F6.3 mag zmag ?=-1 z-band magnitude(Vega-like system)
133-138 F6.3 mag e_zmag ?=-1 Uncertainty on the z-band magnitude
142-144 I3 --- o_zmag [1,40]?=-1 Number of observations
148-153 F6.3 --- dkz ?=-1 Grid z-band dk color term for this object
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Byte-by-byte Description of file: d1u.dat d2u.dat d3u.dat d4u.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.5 deg RAdeg Right ascension, in decimal degrees (J2000)
13- 21 F9.5 deg DEdeg Declination, in decimal degrees (J2000)
25- 30 F6.3 mag umag u-band magnitude
34- 39 F6.3 mag e_umag uncertainty on the u-band magnitude
43- 45 I3 --- o_umag number of observations
49- 54 F6.3 --- dku grid u-band dk color term for this object
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Byte-by-byte Description of file: tabled1.dat tabled2.dat tabled3.dat
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Bytes Format Units Label Explanations
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1- 11 E11.5 --- C1 Covariance matrix coefficient (Δg)
15- 25 E11.5 --- C2 Covariance matrix coefficient (Δr)
29- 39 E11.5 --- C3 Covariance matrix coefficient (Δi)
43- 53 E11.5 --- C4 Covariance matrix coefficient (Δz)
57- 67 E11.5 --- C5 Covariance matrix coefficient (Uref)
71- 81 E11.5 --- C6 Covariance matrix coefficient (Bref)
85- 95 E11.5 --- C7 Covariance matrix coefficient (Vref)
99-109 E11.5 --- C8 Covariance matrix coefficient (Rref)
113-123 E11.5 --- C9 Covariance matrix coefficient (Iref)
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Byte-by-byte Description of file: table13.dat
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Bytes Format Units Label Explanations
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1- 7 F7.1 0.1nm WL Wavelength
9- 15 F7.4 --- OT [0,1] Open Transmission (no filters) (1)
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Note (1): product of (a) the transmission of the optical system, including
the camera window, (b) the mirror reflectivity (c) the average
transmission of the atmosphere above Mauna Kea and (c) the average
quantum efficiency of the MegaCam CCDs.
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Byte-by-byte Description of file: table1[45678].dat
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Bytes Format Units Label Explanations
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1- 5 I5 0.1nm WL Wavelength
9- 14 F6.4 --- T000 [0,1] Transmission at the center of the filter
18- 23 F6.4 --- T023 [0,1] Transmission at 23mm from the center
27- 32 F6.4 --- T047 [0,1] Transmission at 47mm from the center
36- 41 F6.4 --- T070 [0,1] Transmission at 70mm from the center
45- 50 F6.4 --- T093 [0,1] Transmission at 93mm from the center
54- 59 F6.4 --- T117 [0,1] Transmission at 117mm from the center
63- 68 F6.4 --- T140 [0,1] Transmission at 140mm from the center
72- 77 F6.4 --- T163 [0,1] Transmission at 163mm from the center
81- 86 F6.4 --- T186 [0,1] Transmission at 186mm from the center
90- 95 F6.4 --- T210 [0,1] Transmission at 210mm from the center
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Acknowledgements:
Nicolas Regnault, nicolas.regnault(at)lpnhe.in2p3.fr
References:
Guy et al, 2009, in prep. "The Supernova Legacy Survey 3rd Year Sample:
Photometric Properties of 245 high redshift Type Ia Supernovae."
(End) Nicolas Regnault [CNRS/IN2P3/LPNHE], Patricia Vannier [CDS] 27-Aug-2009