Synphot Data User's Guide

3.3 Non-HST Photometric Systems

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This section provides references and some further discussion on various non-HST photometric systems that are characterized in the Calibration Database (CDBS).

The Cousins R and I throughputs are taken from Bessell (1983), Table AII, and have been transformed into photon-counting form.

The throughput data for the Johnson UBV bands have been obtained from MaÌz ApellÀniz (2006), while the Johnson RIJK are from Johnson (1965), Table A1.

The Landolt photometric system is defined as the Johnson UBV bands plus the Cousins RI bands.

The Sloan Digital Sky Survey (SDSS) ugriz filter throughputs were provided by Sebastian Jester on behalf of the SDSS team. They are described in the Photometry White Paper by Gunn et al. (2001). See further discussion below. Figures showing the filter response can be seen at:

 http://www.sdss.org/dr1/instruments/imager/index.html#filters.

The filter throughputs for the StrÆmgren system are taken from MaÌz ApellÀniz (2006).

The GALEX FUV and NUV throughputs were provided by Tom Barlow on behalf of the GALEX project. They are described in Morrissey et. al. (2007). They were measured on the ground in units of effective area, and were divided by the full area of the GALEX primary mirror (1963.495 cm²) to convert them to the dimensionless transmission values required by synphot. These curves therefore represent the true total throughput, including obscuration by the secondary mirror, reflectivity of the mirrors, sensitivity of the detector, etc.

3.3.1 Comparing synphot results with observed non-HST photometry

There are two issues that are sometimes overlooked when comparing synthetic and observed photometry.

Firstly, one should be careful whether the throughput data have been defined for a photon-counting or an energy-integrating detector. Synphot always assumes that throughputs are of the first type. In particular, some authors in the past have defined throughput curves for photomultipliers as if these detectors were energy integrators, which they are not. Such curves have to be converted into photon-counting form before they can be correctly used by synphot (MaÌz ApellÀniz 2006). Using the wrong definition can lead to errors of a few percent for broad-band filters.

Secondly, many systems (e.g. Johnson UBV) use Vega as a reference spectrum, but have been calibrated using secondary standards, leading to the existence of finite zero points. In some systems (e.g. StrÆmgren), those zero points are not even close to 0.0 for some filters. We provide in Table 3.1: Zeropoint Corrections for Groundbased Filter Systems some recent measurements of zero points collected from the literature.

These values should be added to the vegamag synphot magnitudes before they are compared with the observed data.

These values should be added to the vegamag synphot magnitudes before they are compared with the observed data.

The existence of these issues has led us to divide the non-HST photometric systems into supported and non-supported. The first category refers to those systems for which there are recent analyses in the literature that deal with these issues and, therefore, we can be reasonably confident that the possible systematic errors in the synphot results are small. The systems in that category are Cousins RI, Johnson UBV (but not RIJK), Landolt UBVRI, SDSS ugriz, and StrÆmgren uvby. The second category (non-supported filter systems) includes the rest of the filter systems in this section.

Table 3.1: Zeropoint Corrections for Groundbased Filter Systems

Filter/Color/Index (mag)	Zero point	Reference
Johnson/Landolt V	0.026	1
Johnson/Landolt B-V	0.010	2
Johnson/Landolt U-B	0.020	2
Cousins/Landolt V-R	-0.012	3
Cousins/Landolt V-I	-0.002	3
StrÆmgren y	0.038	3
StrÆmgren b-y	0.007	2
StrÆmgren m_1	0.154	2
StrÆmgren c_1	1.092	2

1. Bohlin & Gilliland (2004).

2. MaÌz ApellÀniz (2006).

3. Holberg & Bergeron (2006).

3.3.2 Further detail on the SDSS filter curves

The throughput data for the SDSS ugriz bands give the system photon response to point sources of the 2.5m SDSS survey telescope, including extinction through an airmass of 1.3 at Apache Point Observatory (to which all SDSS photometry is referenced). Originally, the ugriz system was intended to be identical to the u'g'r'i'z' system described in Fukugita et al. (1996)and defined by the standard star system in Smith et al. (2002). However, in the course of processing the SDSS data, the unpleasant discovery was made that the filters in the 2.5m telescope have significantly different effective wavelengths from the filters in the USNO telescope, which was used to observe the u'g'r'i'z' standards (the difference originates from the USNO filters being exposed to ambient air, while the survey-telescope filters live in the vacuum of the survey camera). Therefore, it became necessary to distinguish between the primed and unprimed SDSS filter sets.

The response curves in r and i are slightly different for large extended sources (larger than about 80 pixels in size) because the extended infrared scattering wings in these bands, which do not affect the photometry of point sources, begin to be included. The modified curves are available from the SDSS web site:

 http://www.sdss.org/dr3/instruments/imager/#filters. 

The SDSS photometry is intended to be on the AB system (Oke & Gunn 1983), by which a magnitude 0 object should have the same counts as a source of Fnu = 3631 Jy (except that it used the so-called asinh magnitudes defined by Lupton, Gunn, & Szalay (1999) instead of conventional "Pogson" magnitudes). However, this is known not to be exactly true, such that the photometric zeropoints are slightly off the AB standard. The SDSS team continues to work to pin down these shifts. Their present estimate, based on comparison to the STIS standards of Bohlin, Dickinson, & Calzetti (2001) and confirmed by SDSS photometry and spectroscopy of fainter hot white dwarfs, is that the u band zeropoint is in error by 0.04 mag, u_AB = u_SDSS - 0.04 mag, and that g, r, and i are close to AB. These statements are certainly not precise to better than 0.01 mag. The z band zeropoint is not as certain at the time of this writing (Jan. 2005), but there is mild evidence that it may be shifted by about 0.02 mag in the sense z_AB = z_SDSS + 0.02 mag.

The reader is referred to Holberg & Bergeron (2006) for a calibration of SDSS magnitudes using Vega as a reference spectrum.

Further information about SDSS photometric calibration and the asinh magnitude system can be found at:

 http://www.sdss.org/dr3/algorithms/fluxcal.html

3.3.3 Unsupported bandpass systems

As of March 2006, the following non-HST bandpass systems were deprecated, for the reasons described in Section 3.3.1. We have no plans to remove them from the system, for backwards compatibility; but we also have no plans to attempt to update them.

The ANS system is a set of ultraviolet filters used by the Astronomical Netherlands Satellite, described in van Duinene et al. (1975).

The Baum filter set is a set of 15 broadband and intermediate-band filters that are copies of some of the onboard WF/PC-1 filters and were used as part of a ground-based observing campaign to determine calibrations for the WF/PC-1 instrument. In order to match the response of the WF/PC-1 flight passbands as closely as possible, the throughputs for the Baum filters have been multiplied by the spectral response curve of the ground-based CCD (measured in the laboratory) and twice by the spectral reflectance of aluminum (see Harris et al., 1991, for details).

The Bessell JHK filter curves are taken from data in TableIV of Bessell and Brett (1988). These curves include mean atmospheric transmission equivalent to 1.2 air masses of a standard KPNO atmosphere.

The ESO band throughput tables were received from Jan Koornneef in 1990. There are 530 of these bandpasses; interested users should use the obsmode task to obtain the complete listing.

The KPNO JHK filter curves are taken from tracings of the Simultaneous Quad Infrared Image Device (SQIID) filter set, which were provided by Richard Joyce (KPNO).

The Steward Observatory JHK filter curves are from data provided by Marcia Rieke (Steward Observatory).

The throughput data for the Walraven bands are from Lub and Pel (1977), Table 6.