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 
Filter  Central 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 cutoff 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 tiptilt
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 array  QE Jband  QE Ksband 
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 zeropoints (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 onetoone 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 
Filter  Expected Vega Z.P.  Expected AB Z.P.  Measured Vega Z.P.†  Expected Throughput  Measured 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±0.05x10e9 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 wideband filters 
Filter  Mean 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
