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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:

• Modern astronomical telescopes have very high precision (with pixels that are often much smaller than an arc-second or 1/3600 degrees. However, the Earth is orbiting the Sun at a very high speed of roughly 15 degrees every hour! Keeping the (often very large!) telescopes in track with this fast moving sky is not easy; such that most cannot continue accurate tracking more than 10 minutes.
• For ground-based observations, the turbulence of the atmosphere changes very fast (on the scale of minutes!). So if you plan to observe at 10 minutes and at the start of your observations the seeing is good, it may happen that on the 8th minute, it becomes bad. This will affect the quality of your final exposure!
• When an exposure is taken, the instrument/environment imprint a lot of artifacts on it. One common example that we also see in normal cameras is vignetting; where the center receives a larger fraction of the incoming light than the periphery). In order to characterize and remove such artifacts (which depend on many factors at the precision that we need in astronomy!), we need to take many exposures of our science target.
• By taking many exposures we can build a stack that has a higher resolution; this is often done in under-sampled data, like those in the Hubble Space Telescope (HST) or James Webb Space Telescope (JWST).
• The scientific target can be larger than the field of view of your telescope and camera.

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.

• Invoking astscript-pointing-simulate