IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
increased until the box length is many hundreds of pixels. For reference the solid line shows the expected
linear relation.
4.11.5.2 Small Scales
There is a discrepancy between the expected linear behavior and the data at short binning length scales of
just a few pixels (mosaics only). This discrepancy occurs because we have correlated noise on a
mosaicked image on small pixe l scales (a few pixe ls), so the noise does not bin down appropriately.
4.11.5.3 Medium Scales
On 5”−30” scales much of the extra noise is due to sources in the image. The first level of masking used
the SExtractor segmentation map as a mask. The resulting noise properties are shown with asterisks.
Increasing the size of the masks to 1.5 (2.0) times the SExtractor-determined object radii produced noise
properties shown with a square (triangle) symbol. Further increases in mask size are inconsequential. The
discrepancy between the observed and expected behaviors in this binning length regime is dominated by
noise from the wings of galaxies that are improperly masked. Even after increasing the mask sizes, extra
sources of noise remain which prevent detection of ultra-low surface brightness. There appears to be a
floor to the noise at roughly 0.0005 MJy/sr at 3.6 μm and 0.0008 MJy/sr at 4.5 μm.
4.11.5.4 Large Scales
Some of the large-scale noise is caused by the mapping pattern used for the observations. On scales of
roughly half a fie ld of view there are differences in the total exposure time and hence the total number of
electrons detected (not a dominant source of noise). There are remaining sources of noise on the large
scales, both instrumental and astronomical, which are very hard to disentangle. Uncertainties in the flatfielding
and removal of the first frame effect are two instrumental effects that are contributing to the noise
on large scales. The first frame effect has a column-wise dependence that requires special calibration data
to measure. Astrophysically, there is real structure in the zodiacal light and Galactic cirrus. There is also
documented diffuse intracluster light in the Virgo cluster itself, and a small signal from the extragalactic
background light that are both adding to the noise at low levels. There is potentially also noise due to the
blue infrared color of intracluster light, while the zodiacal light from which the flats are made is red in
near-IR (see Section 4.2). Differentiating between all of these sources of noise is difficult.
4.11.5.5 Increasing exposure time
The IRAC dark field was used to study whether the noise decreases with the square root of exposure time,
as expected. The dark field has extremely low zodiacal light and low Galactic diffuse emission. Using all
the warm mission dark calibration data through 2010, a mosaic was made from 300 dark frames (each
with 100 second frame time). Object masking was made with the SExtractor segmentation image. The
Calibration 62 Extended Source Photometry