4.8 Visualization

We have a completed AOR for one of our targets, determining the background and hence the sensitivity of the instrument, calculating and specifying within Spot the required exposure (frame) time, a dither pattern, and number of repeats. We calculated the total duration of this program, and it is less than the maximum allowed per AOR. So what next?

We will use this AOR as a template for the other targets in the list. However, first, we might want to visualize the observations with an overlay on the optical or infrared sky.

First, we need to obtain or specify an image to use for the overlay. We can obtain ISSA, 2MASS, MSX, or DSS images through Spot. Click on the ``Images'' tab on the toolbar, and, for example, select ``DSS Image.'' You will see a pop-up window, asking you to specify the parameters for the image; a screen capture in shown in Fig. 4.18. After clicking ``OK,'' Spot will work away, and return with an image in the main window.

Figure 4.18: The Spot image menu dialog.

To place the overlay on the image, click on the ``Overlays'' tab, and select ``AORs on current image.'' You will be prompted for an observation date. Choose anything reasonable (this will be more important for constrained observations; however, since we have placed no timing or orientation constraints on the AOR, we can simply choose whatever dates are available to get a visualization of the observations). The resulting overlay (after zooming out) is shown in Fig. 4.19.

Figure 4.19: The overlay of the IRAC mapping observations on the Abell 2218 field.

The overlay shows that the central position has been observed twice, once with the $3.6/5.8 \; \mu {\rm m}$ field of view (large, light blue squares), and once with the $4.5/8.0 \; \mu {\rm m}$ FOV (large, purple squares). There are a few neat visualization aids which we highlight here: Firstly click on the ``Show Side Bar on Current Frame'' under ``Options.'' This will bring up a sidebar with ``IRAC-Mapping'' and ``Base-Image Options'' if it is not already visible. Look under ``IRAC-Mapping'' and you will see four buttons, including one that looks a bit like a tiny grid with red lines. Click that and you will see a pop-up window with options to animate the overlay on an image-by-image basis, animate with a trail, showing the cumulative observations, and the opportunity to select only a subset of the dithered pointings to overlay on the image.

Hint: the visualization tool within Spot is a great, simple way of understanding what all of the AOT options really mean. For example, the various mapping/field of view options described in §4.6.3 can be easily sorted out simply by selecting various combinations of options, and seeing what the resulting visualization looks like. Combined with the detailed duration tool from within the AOT entry window, you are presented with a unified picture of how the sky is mapped with your observations, and what depth, per pixel, you are obtaining.

Hint: in the visualization, you will notice a few small squares with containing X's, approximately one arcminute from the main fields of view. These are the scattered light boxes, highlighting locations where bright point sources will scatter light into the IRAC FOVs. If a bright star falls in one of those boxes, it is recommended that the observations be adjusted to move the object out of the troublesome regions by e.g., constraining the date of the observations. Well-dithered observations will also mitigate scattered light problems.

A similar exercise can be done to examine the local IR sky at the target position. Instead of selecting a DSS image from the ``Image'' pulldown tab, select 2MASS or ISSA instead.

For another way to get a feel for the IR background around the target position, we can place the ISSA image on the DSS image, viewing both simultaneously. To do this, click on the ``Overlays'' pulldown tab, and follow ``Image Overlays'' to ``ISSA Image overlaid on current image....'' Bringing up a 1 degree image, at 12 microns, and overlaying the current AOR on the image, gives the result shown in Figure 4.20.

Figure 4.20: The overlay of the IRAC mapping observations on the Abell 2218 field. The checkerboard pattern is the ISSA image.