IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 7.1: Super skyflats for IRAC. These were made by combining the flat fields from the first five years of
operations. The dark s pot in channel 4, near the left side and about half way up, and the dark spot in about
the same place in channel 2, are due to the same speck of contamination on the channel 2/4 pickoff mirror.
The darkest pixels in the s pot are 20% below the surrounding area in channel 2, and 32% in channel 4. Fl atfielding
in the pipeline fully corrects for these dark spots in the data.
The left edges of channels 1 and 3 are vignetted due to misalignment of IRAC optics with the telescope.
The darkest pixels have 50% of the mean throughput in channel 1, and 70% in channel 3. The vignetting
only extends for 10-15 pixels. The vignetting is compensated for by the flat-fielding, and results primarily
in an increase by at most 2 of the noise in the affected pixels.
Finally, one should note that the flat fields are generated from a diffuse, extremely red emission source.
While the resulting flats perfectly flatten the zodiacal background, they are not accurate for compact
objects with different spectral slopes, the most obvious examples being stars. Please see the section on
array location-dependent corrections (Section 4.5).
7.2 Electronic Artifacts
7.2.1 Saturation and Nonlinearity
The IRAC detector pixels are limited in the number of photons (actually, electrons) they can accurately
accumulate and detect. Once this maximum number is reached, the detector pixel is "saturated" and
Data Features and Artifacts 108 Electronic Artifacts