IRAC Instrument Handbook - IRSA - California Institute of Technology

Table 4.6: Color corrections for NGC 7023 (PAH-dominated) s pectrum.
Band
F K
ν 0
1 17.3 2.21 38.3
2 30.3 1.21 36.6
3 169 1.40 237
4 1021 0.59 599
quot
Fν 0
IRAC Instrument Handbook
For observations of sources dominated by spectral lines, the quoted flux densities should be converted
into fluxes using
quot
Fν
νλ
0 0
F
R λ
∆
= (4.14)
where λ is the wavelength of the spectral feature and R l is the spectral response at that wavelength.
Both λ 0 and effective width ∆ ν are in Table 4.2. The formalism used for continuum sources is
inappropriate for spectral-line sources because it is likely that
Calibration 42 Array Location-Dependent
Photometric Corrections for
Compact Sources with Stellar
Spectral Slopes
l
F and K = ∞ . It is important that the
normalization of R used to determine ∆ ν and R l is the same. In Table 4.2, the column ∆ ν (effective
width) was calculated with the same normalization of response function as on the documentation website
so it is the appropriate one to use. The maximum response, R max is also given in that table, so the fluxes
of lines in heart of the waveband can be estimated by simply multiplying the quoted flux densities by
∆ ν / R , which is listed in the table in the column “Width.”
max
4.5 Array Location-Dependent Photometric Corrections for Compact Sources
with Stellar Spectral Slopes
Point source photometry requires an additional correction that arises from the way in which the data are
flat fie lded. Flat-fielding is a way of removing pixe l-to-pixel gain variations. The IRAC flatfie ld is
derived by imaging the high surface brightness zodiacal background. The way the IRAC flatfield is
derived has a few consequences on making photometrical measurements using IRAC data.
First, the zodiacal background is extended and essentially uniform over the 5.2’x5.2’ IRAC field of view.
The vast majority of objects seen by IRAC are not like this. Many are compact, being either stars
or background galaxies. IRAC has significant scattering as well as distortion. As a result, the extended
ν 0