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IRS: AOT Examples |
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For more examples, see the Observation Planning Cookbook.
Example #1: High resolution staring mode observation of a single fixed targetIn this example, we will obtain a high-resolution spectrum of a bright target, in order to search for two faint emission lines at 15 and 26 microns. We will use both the SH and LH modules. The target is at RA (J2000) = 15:34:57.4, Dec (J2000) =+23:29:52, and has IRAS 12 and 25 micron flux densities of 0.5 and 7.9 Jy, respectively. By placing this object in the Spot target list, we can check its visibility, showing that there are many windows of opportunity for Spitzer observations. Recall that the orientation (roll angle) of Spitzer is not observer-selectable, and depends on the date of observation. For example, if this target is observed on 01 April 2006, then the SH and LH slits will have positions angles on the sky of 121.4 degrees and 36.6 degrees East of North, respectively. Note that a particular observation date should not be specified unless scientifically essential, because of the resultant constraints imposed on scheduling the observation. This feature of Spot is most useful to simply determine the range of dates during which a particular object could be observed.We would like to obtain at least a 10-sigma detection (or limit) on the two emission lines, which are expected to have fluxes of approximately 5x10-18 W m-2. This target is located at an ecliptic latitude of approximately 41 degrees, corresponding to the MEDIUM background case. The appropriate sensitivity curves indicate that a single, 120 second exposure will provide a signal-to-noise of unity in the SH module for an unresolved emission line having a flux of approximately 6x10-19 W m-2 at 15 microns. Similarly, a 240 sec exposure will provide a signal-to-noise of unity in the LH module for an unresolved emission line having a flux of approximately 6x10-19 W m-2 at 26 microns. If we select two cycles per observation, and the 120 second and 240 second exposure times for the SH and LH slits, respectively, we should obtain a signal-to-noise of approximately 16-17 in the final spectra from both modules for our target line flux of 5x10-18 W m-2. Note that since the default in staring mode is to obtain two spectra per slit, by selecting two cycles per slit we will actually obtain four 120 second exposures in the SH module, and four 240 second exposures in the LH module (combining all four spectra is assumed in the sample S/N value given above). Also, while the target is moderately bright (but below the BSL for SH and LH), there is no problem with saturation in the high-resolution modules, as the flux density in the observable wavelength regime is well below 70 Jy. The observation is shown in the completed AOT in the Figure.
While the science target is a point source, it is too bright in both IRS peak-up filters to serve as the peak-up target (based on its known IRAS flux densities). Therefore, we have selected a fainter, nearby star using the "2MASS selection" routine, and entered this into the AOT for a High accuracy peak-up. Note that we must locate the proper motion values for the selected peak- up star from another source (e.g., the Tycho catalog) as this information -- which is crucial to the success of the peak-up -- is not available from 2MASS. Once the AOT is filled out, the estimated AOR duration should be computed by clicking on the "Calc. Obs. Time" button at the bottom of the form. The total AOR duration in this example is less than the 3-hour limit for IRS AORs.
Example #2: High resolution observations of a fixed cluster targetIn this example, we obtain high-resolution spectra using both the SH and LH modules on a target (NGC 6946) located at RA (J2000) = 20:34:52.34 and Dec (J2000) = +60:06:14.12. In addition, we obtain spectra at several positions forming a cross-like pattern along the cardinal axes at offsets of 180 and 360 from the target coordinates. This is achieved by defining the target type as "Fixed Cluster-offset." Note that the coordinates of the center of the cluster are specified at the top of the target form, and the offset positions are given in the table. East and north offsets (in arcseconds) are given as positive numbers, while west and south offsets are given as negative numbers.The completed AOT for this example is shown below. We requested a Moderate accuracy blue peak-up on a nearby 2MASS star, with 1 cycle of 30 sec integration for the SH module and 1 cycle of 60 sec integration for the LH module. The default behavior in Staring mode is to take two spectra per slit so we will actually obtain two 30 sec exposures in SH and two 60 sec exposures in LH. The Resource Estimates window shows that the total AOR duration for the nine positions is less than the 3-hour limit for IRS AORs. Even though this is an efficient way to obtain spectra for nearby locations without being charged for extra slew overheads, the observer should keep in mind two important restrictions. The first is that all offset positions must lie within a circle of 1 degree radius. The second is that the observing parameters (modules/slits used, duration, peak-up accuracy, and slit position angles) will be the same for every position in the cluster.
Example #3: Spectral Mapping for observations between 7.5 and 14.5 micronsIn this example, we will use the SL IRS module to take spectra of an extended region using Spectral Mapping mode. When planning low-resolution observations, Spot considers each order of the slits separately. Assume for the moment that we are only interested in the spectral range between 7.4 and 14.5 microns covered by the first order of the SL module (SL1). In the example shown in Figure 7.48, we use the "Low-Res 7.4-14.5" (i.e., SL1) sub-slit to make an 8x2 grid of 14 sec exposures on NGC 1068 in the following manner: after the first 14 sec exposure (Position 1), we move the slit in a direction perpendicular to the slit length (white arrow) by 10 and take a second 14 sec exposure (Position 2). We repeat this procedure for a total of 7 times with a fixed step-size of 10 so that in the end (Position 8) we have observed at 8 different positions along a direction perpendicular to the slit. The last pointing of the slit is 70 away from the first position. We then displace the slit 50 in a direction parallel to its long axis, and perform a set of exposures in reverse order from the first eight, but at the new displaced positions (Positions 9, 10, ..., 16). It can be seen from the figure that by the time the final exposure is made (Position 16), the slit has returned to its original position in the perpendicular direction, but is displaced from the starting point (Position 1) by 50 in the parallel direction. The length of the SL 1st order sub-slit is 57 and its width is 3.7, so in this example we have obtained spectra (in the requested sub-slit) over a region of 107 by 70.
The completed AOT form for the map grid of Example #3
is shown below. We have disabled peak-up because the exact
pointing for this large map is less important than if we were obtaining a single
(Staring mode) spectrum across the target.
In Spectral Mapping mode, unlike Staring
mode, we obtain only one spectrum per slit position, and so the total on-source
exposure time for this sequence is 16x14 = 224 sec.
Example #4: High-Resolution Observation of a Moving TargetIn this example, we obtain a high resolution spectrum using the SH module for a moving target, Europa, which is a bright moon of Jupiter. We are interested in observing it with a minimum of stray light from Jupiter. To do this, we will apply timing constraints that specify that we want to observe Europa at or close to the time of greatest elongation from Jupiter, as viewed by Spitzer. We determine the time of greatest elongation using the JPL Horizons tool with Spitzer as the coordinate center (see the Proposal Kit section of the SSC web site for detailed instructions). This tool returns the information that Europa will be within about +/-2 degrees of maximum elongation between 05:20-06:50 on 01 March 2006.
The planned AOR is about 20 minutes in duration.
Consequently, we specify the timing constraints in Spot so that the AOR will
start execution between 05:20 and 06:30 on 01 March 2006.
This specified window is 20 minutes shorter than the window that bounds the
target within +/-2 degrees of maximum elongation. This was done because the
timing window specifies the interval in which the AOR can start, so in this case,
truncating the window by 20 minutes (the duration of the AOR) ensures that
Europa is observed while it is still within +/-2 degrees of maximum elongation,
even if the observation is scheduled to start at the very end of the specified
timing window. The completed AOR for this example is shown.
We requested no peak-up because the target itself is much too bright to be used
for this purpose, and stationary offset targets (i.e., stars) cannot currently be
used to peak-up on moving targets. Because of Europa's brightness, which is
close to the saturation limit, we selected the shortest possible integration time
available for the SH module.
Example #5: Peak-Up Imaging ObservationThis example demonstrates the basic procedure for using PUI to map a region previously mapped with IRAC or MIPS. The field-of-view of the peak-up arrays is considerably smaller than the 5x5 arcminutes covered in a single IRAC or MIPS 24-micron exposure. Nonetheless, observers may choose to perform a 16 micron survey of targets previously observed with IRAC and MIPS, in order to fill in the wavelength gap between the latter instruments. Consider the case of mapping the galaxy NGC 7552 with the blue peak-up imaging array. This galaxy is comfortably observed in a single IRAC or MIPS pointing but requires multiple IRS peak-up array pointings for full spatial coverage. To avoid gaps in the coverage, we want to use PUI mapping in array coordinates and choose steps that are nearly the size of the array. In a 6x4 grid, we can cover most of the required field-of-view. The change in rotation angle between observing dates makes it difficult to match the coverage exactly without substantially more map positions. In order to mitigate bad pixels, we use two positions in the cycling dither pattern.
Another consideration when planning PUI observations is the measurement of the local sky. In this example, we may choose to use the outlying parts of the grid to sample the sky and make additional measurements. However, we might instead add more grid positions to ensure that the outer regions of the target galaxy do not affect the background measurements. Alternately, we could choose to repeat the entire map at a different location. The completed AOT for this example is shown below.
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