20.3.  Selecting a Background Sky Image

Once you have your moving target and/or moving target AOR, select "Images" from the menu bar and select the type of image you would like to have used to show the background of the target. The visualisation section for fixed targets (Chapter 19, Fixed Target and AOR Visualisation) describes the types of images that can be used, including a user-provided FITS image, with some important caveats on their suitability for quantitative background estimates. If you are observing a rapidly moving object, such as a comet, or near-Earth asteroid on close pass to Earth, it may be more instructive to select the All Sky Images option (Section 9.2.2), at least until you can determine the extent of the object's path across the sky. For general purpose use, though, requesting the ISSA plates with the default parameters is generally a good starting point for visualisation (Figure 20.1, “ISSA image request dialogue”).