By definition, the throughput is a fraction that expresses the flux of electrons detected on the detectors divided by the incoming flux of photons above the atmosphere intersecting the surface area of the telescope mirrors. Thus, the measured flux of electrons must be compared to a model of the transmission of our telescope + optics + filter + detector + atmosphere system.

Throughput = T_atm * QE * T_Optics * T_mirror * T_filter

Here is a table of our best guess values for each of these transmissions for each filter:

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

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.