11.2 Target Selection and Suitability: Pholus

As with all other observations, we need to first check the ROC using Leopard. We find that Pholus observations with MIPS have already been taken by another observer. Had this been a real proposal, we could then check the Duplicate Observations Rule to see if the specific proposal idea we have in mind qualifies as a new observation - perhaps we would like to go deeper than previous observations, in which case our observation might not be considered a duplicate. Or perhaps the Solar System target we are planning to observe is known to be time variable, and we feel we could make a good scientific case for why it would need to be observed more than once during the Spitzer mission. However, for purposes of this example only, we will ignore the potential duplication, and continue planning a simple MIPS photometry observation of Pholus.

Another thing that is important to check is the known positional accuracy of your target at the time you propose. Although Spitzer's pointing accuracy is extremely good (typically less than an arcsecond), the position of your Solar System target as a function of time (the ephemeris) may have a relatively large error. This could either be because the target is newly discovered, and has had relatively few observations made to determine its orbital parameters, or because the object has not been seen for quite some time and/or it is subject to unpredictable, non-gravitational forces, as is the case for comets close to perihelion. It is especially important to check for any available positional accuracy information when planning observation with the IRS, which has relatively small measurement apertures. However, even observations with MIPS and IRAC may have to pay attention to positional errors.

Positional error information for ephemeredes can be obtained for many objects, especially those likely to have large positional errors such as comets and NEOs, using Horizons. Instructions on how to do this can be found in the Horizons Tips document in the Solar System area of the website; there is additional information on this issue in the SOM. We do this for Pholus, and discover that it has 3-$\sigma$ positional error ellipse of no more than $\sim$1.3 arcsecond throughout 2004. For our planned MIPS observations with a 5$\times$5 arcmin field, this positional error is insignificant, so we do not need to worry about it. Causes for concern would include 3-$\sigma$ radial errors that are more than half of the linear size of the measurement aperture, e.g., 2.5 arcminutes, in this case, or a lack of any available error information. However, even with large errors or no available error information, we could still have proposed for this target, on the proviso that we clearly note in the proposal that the object had large errors or no available error information at the time of proposal.