11.5.3.4 Shadow Observations

We are not necessarily done yet! If we expect to not have enough sky in the images we obtain, or if we expect a very patchy background, or if we are attempting to distinguish our moving object from extragalactic confusions levels, or if we expect the object to be extended and diffuse (e.g., a comet) and therefore need a really good background subtraction to meet our science goals, we can obtain so-called ``shadow'' observations. Shadow observations are repeated observations of this same field at a later time, without the object (or at least with the object having moved with width of several PSFs), so as to better subtract off the background. If we request this option, Spitzer follows exactly the same track as it does for the actual observations we have planned of Pholus, just after (or just before) Pholus has already moved on (or gotten there). When using shadow constraints, it is easier to schedule observations if your shadow (background) observation comes AFTER the main observation. If it doesn't matter to your science whether your shadow observation comes before or after the object, put it after. Here we put it after.

Shadow observations are a separate form of constraint, like a group/follow-on constraint. In order to set up a shadow constraint, first we need two identical AORs to link via this constraint. This is true for ALL AORs, including IRS observations of a moving target with peak-up; DO NOT disable the peakup in a moving target shadow observation - the Spot software will take care of this behind the scenes. We select our MIPS AOR with all three wavelengths, and then copy it (via the Edit menu or the toolbar icon). Then, having done that, from the Tools menu, we select ``Group/Follow-on Constraints'' to pull up a dialog box. We select the ``shadow'' tab to pull up the right options. We then choose ``Add Shadow.'' We want to make these observations sufficiently later in time that Pholus has moved at least the width of several PSFs, but as close as possible in time to minimize changes in the Zodiacal background and the instrument calibration. If making Spitzer IRS observations, it is also advisable to select the shortest viable interval between shadow and primary observations to increase the chances of being able to use the shadow observation to calibrate the stongly time-variable rogue pixels.

It is also important to try to specify a shadow interval that is (preferably much) less than a typical instrument campaign - this is good both scientifically and for our scheduling process. For MIPS, campaigns are currently 2 weeks on average. So, we ask for shadow observations to be taken some time between 16 hrs and 3 days after our original observations; see Figure 11.11. We can also choose to have the shadow observations taken before the main observations; see Figure 11.11.

Figure 11.11: The options for Shadow constraints.

To accept these parameters, we click OK, and then click-and-drag the AOR with all three wavelengths and its new copy into this constraint. We could choose to also add the second 160 micron observation into this constraint, but this will obviously take more observing time, so we have to be sure that we justify this in our proposal (because it will also make it harder to schedule our observation).

Figure 11.12: The enacted Shadow constraints.

To enact the constraint, we click ``OK.'' Having done this, now we see in the AOR list that the Follow-on constraint box (``F'') is checked.