Spitzer Documentation & Tools
IRS Instrument Handbook

3.2  Mapping-Mode Spectroscopy

The Spectral Mapping AOT allowed an observer to configure a grid of map positions around a central target position and obtain spectra at each position. The observer specified the desired slits, each with different ramp durations and cycles, for single or cluster targets. The telescope was moved in a direction perpendicular to the long axis of the slit for the desired number of steps before moving parallel to the slit for the desired number of steps. By default, each map position produced a single ramp (unlike Staring Mode, described in Section 3.1, which automatically obtained spectra at two nod positions along the slit).  However, the observer could set a larger number of ramp cycles to be obtained at each map position or repeat the entire mapping sequence multiple times. The individual pointings were arranged so that the science target defined the center of the map. The orientation of the map on the sky was determined by the date and time of observation.

The observer could choose any combination of IRS slits in Spectral Mapping Mode.  For example, if the "Low 5.2-8.7" and "Low 7.4-14.5" options were selected, then you will get two maps, one centered on the 2nd order SL slit and one centered on the 1st order SL slit. In this case there would have been many extraneous spectra taken in both orders since there is only one physical SL slit (so spectra were always obtained in both orders - with one order offset from the target coordinates - even when only the 1st or 2nd order subslit was selected).  In addition, for efficient mapping of extended sources, there were two more module choices in Spectral Mapping mode: Low Short Both and Low Long Both. These used the vignetted region between the subslits of the indicated modules as the reference point, rather than the center position along each subslit. If maps were desired in both 1st and 2nd orders, this option offered a significant savings in time over mapping in each subslit separately.  Regardless of which module was selected, the observer must have specified the step size in arcseconds and the number of steps for each leg of the map. When observing a cluster with more than one slit in Spectral Mapping Mode, all targets were observed first in one slit, then all targets were observed in the second slit, and so on. This yielded better control of the pointing accuracy while observing in the narrow slits, and reduced the overhead necessary to switch from one IRS module to the next.

The observer could choose to map a given target (or cluster of targets) more than once using the "Number of Map Cycles" parameter. This invoked a peak-up (if requested), repositioned the telescope to the beginning of the first map leg of the first target, and repeated the entire map, starting with the narrowest slit requested. Mapping large areas did not typically require high pointing accuracy, but observers were reminded that the telescope pointing suffered random positional offsets of order 0.2 arcsec for every commanded movement of the telescope. Spectral mapping of multiple sources with multiple slits had the potential to generate a very large number of telescope movements, with significant associated AOR duration overheads if high positional accuracy was requested. When peak-up target acquisitions were requested, they were carried out only once per map cycle and, as noted above, only at the beginning of each map cycle of cluster targets. For example, a request for high-accuracy peak-ups for three multi-slit spectral maps of 20 cluster targets would have yielded a total of three peak-ups during the course of the AOR.

The CUBISM tool was designed to reconstruct and extract full spectral cubes from spectral mapping data sets.