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IRS: AOT Description |
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For the IRS, the available AOTs are: Staring, Spectral Mapping, and
Peak-Up Imaging. An IRS AOR can have a total duration (including all
overheads) of no longer than six hours.
Staring ModeStaring mode is the basic "point and shoot" operating mode of the IRS. In this mode, science targets are placed on one or more of the IRS slits for a specified integration time. The target parameters, desired spectral wavelengths (slits), and method of target acquisition (i.e., peak-up) are input to the AOT form by the observer. An estimate of the total observation time, the overhead times, the observation visibility windows and slit position angles (PAs) for each window, and a means to save the planned observation in ASCII format are all provided by the Spot software.The IRS Staring mode AOT can be configured as either Standard or Peak-Up Only by selecting the appropriate tab in the Instrument Settings section. Standard Staring ModeIn the Instrument Settings section of the Standard Staring mode AOT, the observer specifies the spectra to be obtained with the IRS. Each IRS slit corresponds to a particular wavelength region and a particular spectral resolution. There are six "slits" (or "modules") to choose from since SL and LL have two sub-slits each. The observer must enable a slit for an AOT by turning it "on" in Spot. Note that Spot uses the formal names for the modules. For each slit, the observer must select from one of four ramp durations, and then enter the number of cycles of that ramp duration to be performed. The ramp duration is the time between the first and last non-destructive reads of the array, and corresponds to the "effective exposure time." The shortest ramp duration for all of the slits is six seconds. The longest ramp duration for the slits varies from 120 seconds to 480 seconds depending upon the module. The ramp durations have been chosen by the IRS Instrument Team to provide the highest dynamic range, while minimizing the number of routine calibrations required in orbit. Note that the IRS SL and LL modules reach background-limited performance in about 40 seconds and about 10 seconds, respectively. Longer ramp durations have smaller overhead times but may be affected more severely by cosmic ray hits. The "cycles" parameter is the number of times a given spectrum is repeated before moving on to the next slit position or the next target. Total "integration time" is determined by both ramp duration and number of cycles.Standard Staring mode successively places the target at the two nominal slit locations for each requested slit, located 1/3 and 2/3 of the way along the length of the slit, to provide redundancy against cosmic rays and detector artifacts. The automatic "nodding" that occurs in Standard Staring mode means that, for example, the choice of a 480 second ramp for one cycle in SH would produce two spectra, with each having an exposure time of 480 seconds, yielding a total on-source integration time of 960 seconds. The same exposure time with two cycles would produce four 480-second exposures, for a total on-source integration time of 1920 seconds.
Peak-Up Only Staring ModeThe Peak-Up Only observation type is intended to provide "early target acquisition" functionality to the IRS Standard Staring mode. In a Peak-Up Only observation, the peak-up algorithm is run in its normal fashion to acquire the science target but no spectroscopic data are taken. This may be useful in cases where long spectroscopic observations are needed but no obvious peak-up target presents itself from other ground-based or space-based data, so it is desirable to confirm the success of the peak-up before exposing the spectrum. The peak-up images and the position of the target selected by the software are sent to the ground in this mode. Since the peak-up software runs its normal course here, the overheads (including the initial slew overhead and the settling times within the peak-up process) are significant. In addition, the peak-up itself will have to be repeated at a later time in conjunction with the actual spectroscopic exposure. The DCS images obtained during a Peak-Up Only observation will be dark-current-subtracted, cleaned of cosmic rays, and flat-fielded on-board (as is done for all peak-up data) and these images will be made available to the observer for planning future IRS spectroscopic observations.Peak-Up Only is intended to test the peak-up algorithm, not to obtain science images, which can be accomplished with the Peak-Up Imaging AOT.
Spectral Mapping ModeThe Spectral Mapping AOT allows an observer to configure a grid of map positions around a central target position and obtain spectra at each position. The observer may specify any combination of slits, each with different ramp durations and cycles, for single or cluster targets (provided the 3- hour AOR maximum is not exceeded). The telescope is 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 produces a single ramp (unlike Staring mode, which automatically obtains spectra at two nod positions along the slit), but the observer can 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 are arranged so that the science target defines the center of the map. The orientation of the map on the sky is determined by the date and time of observation.The observer can choose any combination of IRS slits in Spectral Mapping mode. The general information regarding slit selection above applies here as well. For example, if you click on the "Low 5.2-8.7" and "Low 7.4-14.5" buttons, 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 will be many extraneous spectra taken in both orders since there is only one physical SL slit (so spectra are always obtained in both orders -- with one order offset from the target coordinates -- even when only the 1st or 2nd order subslit is selected). In addition, for efficient mapping of extended sources, there are two more module choices in Spectral Mapping mode: Low Short Both and Low Long Both. These use the vignetted region between the subslits of the indicated modules as the reference point, rather than the center position along each subslit. If maps are desired in both 1st and 2nd orders, this option offers a significant savings in time over mapping in each subslit separately (yet see below for possible dangers in using the "Both" option). Regardless of which module is selected, the observer must specify 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 are observed first in one slit, then all targets are observed in the second slit, and so on. This yields better control of the pointing accuracy while observing in the narrow slits, and reduces the overhead necessary to switch from one IRS module to the next. The observer can choose to map a given target (or cluster of targets) more than once using the "Number of Map Cycles" parameter. This will invoke a peak-up (if requested), reposition the telescope to the beginning of the first map leg of the first target, and repeat the entire map, starting with the narrowest slit requested. Mapping large areas will typically not require high pointing accuracy, but observers are reminded that the telescope pointing suffers random positional offsets of order 0.2 arcsec for every commanded movement of the telescope. Spectral mapping of multiple sources with multiple slits can generate a very large number of telescope movements, with significant associated AOR duration overheads if high positional accuracy is requested. When peak-up target acquisitions are requested, they are carried out only once per map cycle and, as noted above, only at the beginning of each map cycle of cluster targets. For example, requesting High accuracy peak-ups for three multi-slit spectral maps of 20 cluster targets will yield a total of three peak-ups during the course of the AOR. See the SOM for a detailed discussion of peak-up target acquisition, and for a discussion of potential pitfalls in spectral mapping. Click here for an in-depth look at how to design IRS spectral maps. Peak-Up Imaging ModeThe IRS Peak-Up arrays provide a science-quality imaging capability in the wavelength range that falls between that covered by IRAC and MIPS, and as such may be of interest for imaging alone. For Cycle-2, a new AOT was made available to allow Peak-Up Imaging (PUI) observations. Both a red (18.5-26.0 microns) and a blue (13.3-18.7 microns) filter are available. The red filter provides images over a similar wavelength window as does the MIPS 24 micron mode. The main motivation for the new AOT is to provide photometric imaging in the 16 micron (blue) filter, thereby filling the wavelength gap between IRAC and MIPS.Parallel red and blue peak-up images are obtained simultaneously. The field-of- view seen by each filter is 54x81 arcsec, and they are separated by a 33 arcsec wide vignetted zone (see Figure 7.6 in the SOM). The images are 30x45 pixels in size and have a plate scale of ~1.8 arcsec/pixel. The position of the two images on the SL detector is an accurate representation of their position on the sky; that is, the two fields-of-view together with the vignetted region see a contiguous 54x195 arcsec area. A single World Coordinate System (WCS) can be used to describe the combined red and blue regions within the 128x128 pixel detector. PUI observations consist of an optional dither pattern superposed on an optional rectangular map pattern. The exposure time and dither/map patterns are defined in a single dialog box in Spot. When mapping and dithering are selected, the required dialog fields open within the main box. PUI observations may be requested for any of the standard target types (see the SOM Section 7.2.3.4.5.3 for an exception to this for PUI Mapping). An example PUI observation is described in the examples section. Examples of IRS AOTsSee also the Observation Planning Cookbook.
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help@spitzer.caltech.edu http://ssc.spitzer.caltech.edu/irs/aotintro.html
