WIRCam Throughput
WIRCam Throughput Model

In this context, we consider the throughput to be expressed in any chosen filter waveband and to include all the elements in the optical path, which includes the various optical components of WIRCam, the telescope optics as well as the transmission characteristics of the atmosphere. Therefore we define the throughput as the ratio of the electron flux detected by the WIRCam detectors in a given filter and the incoming flux of photons above the atmosphere intersecting an area equivalent to the surface area of the telescope primary mirror. The throughput is computed using the measured flux of electrons against a model comprised of the transmission of the telescope optics, filter, detector and atmospheric system.

Expressed as a function,
Overall throughput = T_atm * T_mirror * T_Optics * T_filter * Detector QE

The following table provides our current best known values for these transmission coefficients in various WIRcam broadband filters:

Transmission coefficients for the WIRCam broadband filters
FilterCentral λ [μm]Bandpass [μm]T_filter †Detector Q.E. †† T_optics ‡ T_mirror ‡‡
Y 1.020 0.100 0.74 [ ? ] 0.80 0.91
J 1.253 0.158 0.86 0.759 0.70 0.92
H 1.631 0.289 0.975 [ ? ] 0.75 0.94
Ks 2.146 0.325 0.98 0.815 0.69 0.96
Filters were scanned so values are accurate (see filter curves). We assume a constant filter transmission within the cut-off wavelengths.
†† Mean QE values for the four detectors as quoted by Teledyne; only available for the J and Ks filters. Individual QE values for each filter are given in the following table. The quoted values have associated uncertainties as high as 15%, so to be used with caution.
Optics transmission is what was modeled by the designers, INO (Institut national d'optique) including AR coatings based on samples measurements. The optics transmission curves (exluding the telescope mirror and tip-tilt plate) are given here in an Excel file (use the Total column).
‡‡ This is the transmission per surface. WIRCam is at prime focus so counted only once for the primary mirror. Based on measurements closest to the standard star observations of the reflectivity of the mirror at 670 nm and on a curve of the change of reflectivity as a function of wavelength made by D. Salmon.
QEs for the individual WIRCam detectors
Detector #Position in arrayQE J-bandQE Ks-band
52 Bottom, right 0.739 0.813
54 Bottom, left 0.756 0.810
60 Top, left 0.832 0.889
77 Top, right 0.710 0.747
Measured values provided by Teledyne but with uncertainties up to 15%
Throughput measurements

The following table gives the expected zero-points (the magnitude at which the flux is 1 photon/sec) in the Vega and AB systems, computed from these models. Also given are the actual measurements on the sky using standard stars. For these measurements, the electronic gain used is 2.5e-/adu and we also assumed that there is a one-to-one photon/electron conversion. Note that the gain has been corrected for the capacitive coupling measured on our arrays which smooths the noise and causes the traditional transfer curves to overestimates the gain (the correction here was ~13%). The throughput is given for filters J,H,Ks for which standard star magnitudes are published.

Expected and Measured Throughput and Zero Points
FilterExpected Vega Z.P.Expected AB Z.P.Measured Vega Z.P.†Expected ThroughputMeasured Throughput
Y 24.56 25.22 no std star 27% ?
J 25.03 25.98 25.02±0.02 42% 41%
H 25.19 26.58 25.19±0.02 52% 52%
Ks 24.43 26.42 24.45±0.03 52% 53%
LowOH1 21.71 22.40 no std star 21% ?
LowOH2 21.61 22.48 no std star 25% ?
CH4On 23.85 25.31 no std star 48% ?
CH4Off 23.96 25.30 no std star 44% ?
H2 21.78 23.75 no std star 45% ?
Kcont 21.62 23.71 no std star 45% ?
BrGamma 21.58 23.61 no std star 43% ?
This is the mean ZP for the 4 arrays. There is an array to array systematic difference, see next table.

We use 8.4 m2 as the collecting area of the telescope (this deals with central obscuration). We assumed no atmospheric absorption at all (T_atm=1). To compute the flux of photons, we used two magnitude systems:

  • The Vega system of magnitudes where, by definition, Vega has mag=0. We used the models by Kurucz (Teff=9400K, log(g)=3.9, Fe/H=0.00) and the renormalization of the flux (3.44±0.05x10e-9 erg/cm2/s/Ang) by Cohen (1992, AJ 104).
  • The AB system of magnitudes where, by definition, a constant flux of 3720 Jansky represents mag=0.

Caution! it was found by three different teams that the Vega to AB conversions were off by up to 0.2 mag in Ks. The numbers given here will need to be revised. See their numbers on the WIRCam DIET page

Relative Quantum Efficiencies of the 4 arrays through the wide-band filters
FilterMean Vega Z.P.Array #77 (N.-W.) — Mean Z.P.Array #52 (S.-W.) — Mean Z.P.Array #54 (S.-E.) — Mean Z.P.Array #60 (N.-E.) — Mean Z.P.
Y ? +0.07 -0.09 +0.00 +0.01
J 25.02 +0.05 -0.09 +0.00 +0.02
H 25.19 +0.03 -0.05 -0.01 +0.02
Ks 24.45 +0.02 -0.03 -0.02 +0.03

Last update: KT, 14 Nov 2011