7.7.3.2 Laying Out the Spectral Map: Pointings and Step Sizes

We must now consider how to lay out the spectral map pointings in order to obtain the desired coverage of the Ring Nebula; namely, we wish to ``slice'' the nebula perpendicularly along its major axis, and obtain both 1st and 2nd order SL spectra of opposite sides of the ``ring.'' The map layout is controlled by the pointing number (``Pointing'') and step size (``Step'') parameters. One pair of these parameters specifies pointings parallel to the long axis of the slit and a second pair specifies pointings perpendicular to the long axis of the slit. In both cases, the step size is the distance between successive slit positions. The following steps outline a method that can be followed in preparing a spectral map layout.

Step #1: Download a 2MASS $K_{\rm s}$ band or DSS image of the target using the ``Images'' pull-down menu. For the Ring Nebula example, we download a $500^{\prime\prime} \times 500^{\prime\prime}$ 2MASS $K_{\rm s}$ image. Center the target in the image by clicking the light blue ``cross-hairs'' icon on the left side of the main Spot window. Adjust the size (by ``dragging'' a corner of the window, or by clicking on the expansion icon in the upper right corner of the image window), magnification (using the plus and minus ``magnifying glass'' buttons on the left frame of the main Spot window), and color scale of the image as desired; the color scale controls are accessed using the small ``rainbow checkerboard'' button on the right border of the image window. Figure 7.6 shows the image window layout with the color table of the 2MASS image set to ``Reverse Grey scale.''

**Figure 7.6:** Main Spot window with an image display window showing the 2MASS $K_{\rm s}$ band image of the Ring Nebula.
$\begin{figure}\centering \epsfig{figure=figs4f/irs_mapping_image_layout, width=3.10in} \end{figure}$

Step#2: If the current AOT entry window is not open, then reopen it for editing using the ``Modify Current AOR'' option under the ``Edit'' pull-down menu, or by clicking on the modify-AOR icon (the sheet of paper labeled ``AOR'' with an arrow), fifth from the left on the taskbar. At this point, only the AOR name and the peak-up settings have been modified - the ``Instrument Settings'' should still have their default values, as shown in Figure 7.4.

Step#3: Since we want to obtain spectra in both 1st and 2nd orders of the SL module, we first examine the possibility of using the ``Low Short Both'' option. Click in the ``On'' boxes next to the ``IRS Slit'' column in the ``Instrument Settings'' so that only ``Low Short Both'' is turned on. Set both of the ``Pointings'' columns to values of 1. Finally, click on ``OK'' in the AOT entry window to store these changes. This will also dismiss the window, but it can be recalled at any time using the instructions in Step #2.

Step#4: In the main Spot window, select the ``Current AOR on current image'' option under the ``Overlays'' pull-down menu. Spot will request a date for the overlay; use ``2004 Oct 30,'' which is one of the dates when the SL module PA is exactly aligned perpendicular to the major axis of the Ring Nebula. Spot will create a projection of the slit positions on the downloaded 2MASS image of the Ring Nebula. In this case, the slit is centered around the target coordinates at the center of the Ring Nebula (see Figure 7.7, left panel). Use of this option will probably not be optimum, since, by coincidence, the size of the Ring Nebula is comparable to the separation of the two orders of the SL slit. Thus, opposite sides of the ``ring'' cannot be centered in the two subslits at the same time. If we were mapping a larger region, then using the ``Low Short Both'' option might be a viable alternative.

**Figure 7.7:** Spot's image display window, showing the 2MASS $K_{\rm s}$ band image of the Ring Nebula with an overlay of (left) the ``Low Short Both'' slit option and (right) the SL 1st order ( $7.4-14.5 \; \mu {\rm m}$ ) option.
$\begin{figure}\centering \epsfig{figure=figs4f/irs_mapping_overlay_lowshortboth,... ... \epsfig{figure=figs4f/irs_mapping_overlay_1storderA, width=2.65in} \end{figure}$

Step#5: Recall the AOT entry window using the instructions in Step #2. Deselect ``Low Short Both'' and turn on the 1st order SL slit (``Low $7.4-14.5 \; \mu {\rm m}$ '') instead. Again, set both corresponding ``Pointing'' columns to values of 1. Now overlay the slit visualization on the 2MASS image using the instructions in Step #4. This is also not optimum since the 1st order slit is now centered on the empty middle of the Ring Nebula instead of the actual ``ring'' as desired (see Figure 7.7, right panel).

Step#6: We now take advantage of a property of IRS Spectral Mapping layout that is described in the IRS chapter of the SOM. When an odd number of steps (in either or both of the parallel or perpendicular directions) is used, then the middle pointing will always be centered on the target coordinates. However, when an even number of steps is used, then no pointing will be exactly centered on the target coordinates. The separation of the midpoints of the Ring Nebula annulus along its minor axis is approximately $51^{\prime \prime }$ , so we reopen the AOT entry window (Step #2) and set the number of parallel pointings to 2, with a step size of $51^{\prime \prime }$ . Re-visualizing the AOR onto the 2MASS image (Step #4) now shows that opposite sides of the nebula annulus are nicely centered in two pointings of the 1st order slit (see Figure 7.8, left panel). The 2nd order slits are also shown, but they are off the target at both pointings.

**Figure 7.8:** Spot's image display window, showing the 2MASS $K_{\rm s}$ band image of the Ring Nebula with an overlay of (left) 2 parallel pointings of the SL 1st order ( $7.4-14.5 \; \mu {\rm m}$ ) subslit, offset by $51^{\prime \prime }$ , and (right) 2 parallel pointings of the SL 1st and 2nd order subslits, offset by $51^{\prime \prime }$ .
$\begin{figure}\centering \epsfig{figure=figs4f/irs_mapping_overlay_1storderB, wi... ...\epsfig{figure=figs4f/irs_mapping_overlay_bothorders, width=2.65in} \end{figure}$

Step#7: We now repeat the procedure in Step #6 with both the 1st and 2nd order subslits of the SL module turned on. This will produce four individual pointings, with two having opposite sides of the Ring Nebula annulus centered in the 1st order subslit, and the other two having opposite sides of the annulus centered in the 2nd order subslit.

The AOR visualization is now somewhat confusing, because there are several overlapping sequential slit positions displayed simultaneously (see Figure 7.8, right panel). The ``Pointings Table'' feature can be used to clarify this situation. Click on the small ``table'' icon, third from the left in the top row of icons on the right of the image window. This will open the Pointings Table window (see Figure 7.9), which contains one row for each slit position currently scheduled in the AOR (notice that it also contains one row for each scheduled peak-up exposure, barely at the edge of the 500 $\times$ 500 arcsecond image).

The corresponding overlays in the image window can be turned on or off by selecting any of these rows (using the check-mark boxes in the ``On'' column). The two ``Animation'' buttons are also useful - these produce cartoon movies showing the sequence in which the pointings will be obtained by Spitzer. Experiment with the Pointings Table, and convince yourself that the current configuration of the Instrument Settings does indeed produce the desired pointings as described at the beginning of this step.

**Figure 7.9:** Pointings Table window for the observation shown in Figure 7.8, right panel.
$\begin{figure}\centering \epsfig{figure=figs4f/irs_mapping_pointingtable, width=4.5in} \end{figure}$

Step#8: We are now ready to configure the second axis of the map, the perpendicular ``slices'' along the major axis of the Ring Nebula. First, reopen the AOT entry window. We would like the map grid to be closely spaced, so set the size of the perpendicular steps for both the 1st and 2nd order subslits to the width of the SL slit, $3\hbox{$.\!\!^{\prime\prime}$}6$ . A more sparsely sampled map using fewer pointings could be obtained by setting the size of the perpendicular steps to be larger than the SL slit width. A few iterations of setting the number of perpendicular pointings and visualizing the slit overlay on the 2MASS image shows that 11 perpendicular pointings completely sample the opposite sides of the Ring Nebula ``parallel'' to the nebula's major axis.

**Figure 7.10:** Complete spectral map coverage of the Ring Nebula. The left panel shows the full extent of the spectral map coverage. The right panel shows one pointing from each of the parallel steps (at different perpendicular steps) to illustrate the positions of individual pointings in the map.
$\begin{figure}\centering \epsfig{figure=figs4f/irs_mapping_completemap, width=5.3in} \end{figure}$

The complete spectral map coverage is shown in Figure 7.10. Notice that many spectra of essentially empty regions around the nebula are obtained. These are a normal result of the fact that 1st and 2nd order spectra of the same slit are always obtained, even if only one of the orders is selected. When mapping a large region, this can often be advantageous; however, the proposer is not penalized for any ``extraneous'' spectra resulting from a carefully designed spectral map. (It may prove advantageous for the Ring Nebula also, since the additional spectral map coverage could reveal as yet unknown, extended IR structure of the nebula, or provide spectra for sky subtraction.)

The right panel of Figure 7.10 shows one pointing from each of the parallel steps (at a different perpendicular step position), in order to clearly illustrate the slit position at each parallel step. The spectra will be obtained starting with the first 2nd order subslit exposure in one corner, then proceeding to the other for the 11 perpendicular steps. Next, the offset to the second 2nd order subslit parallel step is made, and the perpendicular pointings proceed back towards where we started. This process is then repeated for the 1st order subslit pointings. The order in which spectral map pointings are carried out is also described in the IRS chapter of the SOM, and can be demonstrated in Spot using the ``Animation'' buttons in the Pointings Table window.