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:

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.

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

Last update: KT, 14 Nov 2011