IRAC Instrument Handbook - IRSA - California Institute of Technology

solving for F, we find
ΣI
P
F =
ΣP
i i
2
i
IRAC Instrument Handbook
Now we derive the uncertainty in the flux. Using the well-known theorem for propagation of errors
2 ⎛ dF ⎞
σ ⎜
⎟
F = Σ ,
⎝ dIi
⎠
2
and applying it to the result above, we find that
σ F
⎛
2 Pi = Σ⎜ ⎝
2
ΣPi ⎞
⎟
⎠
2
σ 2 = ΣP 2 2
i σ σ2
2
2 = 2
ΣP ΣP
i
i
,
( )
1
or, equivalently, σF = σ N where N = , which is the definition of noise pixels.
2
ΣPi
There are two columns for the full width at half-maximum (FWHM) of the PRF in Table 2.1. The mean
FWHM is from observations of a star at 25 different locations on the array. The FWHM for “centered
PRF” is for cases where the star was most closely centered in a pixel. The fifth column in Table 2.1 is the
fraction of the flux in the central pixel for a source that is well centered in a pixel. It was determined from
the images of the focus star (after the telescope was focused) that were the most symmetric and
concentrated. These values for the flux in the central pixel can be used in the saturation predictions (see
Section 2.4 below). The flux in the central pixel for a random observation is lower, because the Spitzer
PRF is rather undersampled at the IRAC pixel scale.
2.2.3 Spectral Response
The IRAC system throughput and optical performance is governed by a combination of the system
components, including the lenses, beamsplitters, filters, mirrors, and detectors. The system response is
based on measurements of the final in-flight system, including the beamsplitter, filter, ZnS & ZnSe
coating transmissions, mirror reflectance, BaF2 and MgF2 coating transmissions, and detector quantum
efficiency.
At each wavelength, the spectral response curve gives the number of electrons produced in the detector
per incoming photon. While the curves provided are best estimates of the actual spectral response, it is
recommended that the curves are used in a relative sense for color corrections and the supplied
photometric scaling (implicit in Level 1 products [“BCDs”] and described in Reach et al. 2005, PASP,
117, 978, [22]) is used for absolute photometric calibration. Tests during IOC/SV showed that the out-of-
Instrument Description 8 Description of Optics