GNU Astronomy Utilities

10.6 Pointing pattern simulation

Astronomical images are often composed of many single exposures. When the science topic does not depend on the time of observation (for example galaxy evolution), after completing the observations, we stack those single exposures into one “deep” image. Designing the strategy to take those single exposures is therefore a very important aspect of planning your astronomical observation. There are many reasons for taking many short exposures instead of one long exposure:

In the jargon of observational astronomers, each exposure is also known as a “dither” (literally/generally meaning “trembling” or “vibration”). This name was chosen because two exposures are not usually taken on exactly the same position of the sky (known as “pointing”). In order to improve all the item above, we often move the center of the field of view from one exposure to the next. In most cases this movement is small compared to the field of view, so most of the central part of the final stack has a fixed depth, but the edges are shallower (conveying a sense of vibration). When the spacing between pointings is large, they are known as an “offset”. A “pointing” is used to refer to either a dither or an offset.

For example see Figures 3 and 4 of Illingworth et al. 2013 which show the exposures that went into the XDF survey. The pointing pattern can also be large compared to the field of view, for example see Figure 1 of Trujillo et al. 2021, which show the pointing strategy for the LIGHTS survey. These types of images (where each pixel contains the number of exposures, or time, that were used in it) are known as exposure maps.

The pointing pattern therefore is strongly defined by the science case (high-level purpose of the observation) and your telescope’s field of view. For example in the XDF survey is focused on very high redshift (most distant!) galaxies. These are very small objects and within that small footprint (of just 1 arcmin) we have thousands of them. However, the LIGHTS survey is focused on the halos of large nearby galaxies (that can be more than 10 arcminutes wide!).

In Invoking astscript-pointing-simulate of Gnuastro’s Installed scripts is described in detail. This script is designed to simplify the process of selecting the best pointing pattern for your observation strategy. For a practical tutorial on using this script, see Pointing pattern design.