IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
distinguish the effect from the internal scattering in channels 3 and 4. The IRAC pipeline does not correct
this effect.
7.2.7 Inter-Channel Crosstalk
We have detected electronic crosstalk between channels only in the brightest sources that have been
observed. All four channels are read out simultaneously, except for the 100/200-second frames in channel
4, for which two/four 50-second frames are taken instead of the long integrations in the other channels,
because IRAC is background-limited in channel 4. When a source falls on a pixel of one array, crosstalk
may occur in the same pixel location in the other arrays, or in the next pixel read out. The crosstalk
appears as a combination of either a positive or a negative offset in the same pixel and the derivative of
the signal in the same or previous pixel. As the source is dithered, the crosstalk follows it, and therefore
crosstalk appears in the mosaics. It is so weak that we have detected it so far only in channel 3, when the
source is in channels 2 and 4, and in channel 4, when the source is in channels 1 and 3. Figure 7.7 shows
parts of the mosaics from the off-beams in a dithered observation of a very bright star. The star was
observed in channels 1 and 3 FOV first, so there are residual images in channels 1 and 3 from the bright
star. The residual images appear as a diffuse glow near the center. This glow is a combination of the
residual images of a very strongly saturated star observed with a Reuleaux dither pattern, thus effective ly
smoothed by outlier rejection. The crosstalk appears in channels 3 and 4 as a partial dark ring with a
bright core.
Figure 7.7: Channels 1 and 2 (top) and 3 and 4 (bottom) showing inter-channel crosstalk (dark s pots near the
center of the lower panels).
Data Features and Artifacts 115 Electronic Artifacts