6.8 Long-High Sky Background Observations

As a result of the continual increase in the number of degraded pixels in the IRS Long High module due to the effects of Cosmic Rays, it is strongly recommended that observations of sources with equivalent point source flux densities of 2.0 Jy or less at 25 microns be accompanied by off-source back ground measurements. Spot v14.2 (and later versions) automatically generates an observation of the sky background for LH and SH observations although it is the responsibility of the observer to ensure that the slit is placed on a blank patch of sky. Examination of the properties of Long High warm, or ``rogue" pixels (pixels with abnormally high dark current that vary unpredictably with time), shows that the rogue dark current can be reduced substantially by subtracting a nearby background spectrum taken within a few hours of the science spectrum. Improvements in the signal-to-noise in the extracted spectrum can be substantial, up to 80% when averaged over an entire order of Long High and when the integration time is split evenly between target and background, since for faint sources the dark current in the rogue pixels is the dominant source of noise. SH observations do not suffer as much from rogue pixels but need dedicated backgrounds to be taken if the source flux is comparable to the background to enable accurate flux calibration of the spectrum. This recommendation applies chiefly to those observers using IRS Staring mode. Observers who are mapping their sources may only need one additional background observation (if all their map positions fall on the target of interest), or none (if at least one of the map positions falls on blank sky). If an observer does not want to use the Spot automatically-generated sky background observation for some reason (e.g. they are observing multiple sources in the same region of sky within a 24 hour period and they need only one sky background for all the sources together), then they should be sure they create a separate sky background observation and group it to their main AORs. The cost of splitting the observation into another AOR (i.e. the 215 sec overhead charged to each AOR) will not be charged to the observer. However, the duration of the background measurement must come out of the observer's allocation. The LH background AOR should have no peakup, should replicate the cycles and ramp durations of the LH science AOR, and should be chained to the LH science AOR.