7.7.3.3 Ramp Cycles vs. Map Cycles

For this example, we select a basic ramp duration of 60 seconds. However, in order to increase the S/N of the resultant spectra and provide redundancy against cosmic ray hits, we will repeat each ramp two times (to obtain a total exposure time of 120 seconds at each pointing in the spectral map). There are two means of repeating ramps. The first is to set a number of ``Cycles'' for each ramp. This will repeat the selected ramp in succession, without moving the telescope. The second is to set a number of ``Map Cycles.'' This will repeat the ramp at each pointing only after finishing all of the pointings in the preceding cycle through the spectral map.

Multiple ramp and map cycles can be used in combination (as long as the total AOR time does not exceed the 3-hour limit for IRS AORs). Because the absolute positions of pointings will not exactly repeat from one map cycle to another (due to the small, cumulative, and random errors associated with all movements of Spitzer), the best choice in this example is to set the number of ramp cycles to 2, and the number of map cycles to 1. This will produce the following result: at each map pointing, the IRS makes 2 successive exposures without moving the telescope, then moves to the next map pointing and makes 2 successive exposures, and so on, until the complete map has been observed once. This ensures that the two spectra obtained at each pointing sample exactly the same spatial region of the nebula. If we were interested in exploring time variability in the spectra (rather than achieving the highest S/N spectrum at each map pointing), then obtaining one ramp at each pointing over a number of map cycles might have been a better strategy.

Observers who are planning SL observations where the sky background saturates the peak-up arrays (e.g. 12.5 MJy/sr in 240s), should consider reducing their ramp time (and increasing the number of cycles) to avoid saturation. If the peak-up arrays are saturated, the droop correction for the entire array, including the region with the spectral orders may be affected as described in Section 7.2.5.1 of the SOM.