Gnuastro: Simulating The Exposure Map of a Pointing Pattern - Abstract and Intro

9 May 2024


(1) Mohammad Akhlaghi, Centro de Estudios de F´ısica del Cosmos de Aragon (CEFCA), Plaza San Juan 1, 44001, Teruel, Spain {}.

Abstract and Intro

Simulating the Exposure Map

Acknowledgment and References



The pointing pattern is an integral part of designing one’s observation strategy for a certain scientific goal. But accounting for the particular science case or instrument artifacts (like distortion, vignetting or large areas of bad pixels) can make it hard to predict how the exposure map of the final stack will be. To help address this problem, Gnuastro 0.21 includes a new executable program called astscript-pointing-simulate that is fully described in the Gnuastro manual and comes with a complete tutorial. The figures of this research note are reproducible with Maneage, on the Git commit 4176d29.

Keywords: Astronomical methods (1043), Field of view (534), Direct imaging (387), Astronomical techniques (1684), Astronomy software (1855)


Astronomical images are often composed of many single exposures. After completing the observations, when the science topic does not depend on the time of observation, we stack those single exposures into one deep stacked image (also known as coadd). Designing the strategy to take those single exposures is therefore a very important aspect of planning an astronomical observation.

Each exposure has a pointing (the location on the sky that it was targeting). Subsequent exposures are usually taken with different pointings. When the next pointing is ver near (a small fraction of the field of view) to the previous one, it is known as a dither (literally/generally meaning trembling or vibration). But when the distance between subsequent pointings is large (and issues like re-focusing become necessary), the pointing is known as an offset. These terms are sometimes used interchangably by some.

When we only have dithers, most of the central part of the final stack has a fixed depth. Only a thin border becomes shallower (conveying a sense of vibration!). For example see Figures 3 and 4 of Illingworth et al. (2013) which show the exposures that went into the XDF 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. However, it can happen that only offsets are used. For example see Figure 1 of Trujillo et al. (2021), which shows the exposure map for the LIGHTS survey.

The dithering 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 HUDF, CANDELS and other proposals (that constitute the XDF), the scientific target was high redshift (distant) galaxies. When counting the pixels that they cover in relation to the number of pixels in each exposure, these targets are very small objects. Such that within that small footprint (of just 1 arcminute!) we have thousands of such small objects (which may also be stars or nearby galaxies of similar observed size to the high

Figure 1. Sample outputs from the tutorial on designing a pointing pattern in the manual of Gnuastro 0.21. Left: the exposure map of a “+”-shaped pointing (offset) pattern with 5 exposures. Center: The stack of pure noise after applying the same pointing list as the left image. Right: Exposure map of same dither pattern as the left image, but with trimming of the outer pixels in each exposure (this usually happens due to vignetting). The color map used shows the largest values as white/red and the smallest values as black/blue.

redshift galaxies). Therefore for the scientific goal of the XDF, the very small dithers were sufficient to avoid instrument artifacts.

However, the LIGHTS survey is focused on the halos of large nearby galaxies (that can be more than 10 arcminutes wide); thus covering a large fraction of the field of view of the Large Binocular Telescope that was used. In order to do robust calibration of the images (for example to estimate the flat field pattern, or the sky background level) it was necessary to have offsets. This enabled the galaxy and its halo to cover very different pixels from one exposure to the next.

In other cases, the pointings include both offsets and dithers (around each offset). To find the ideal dither pattern for the particular scientific goal, it therefore helps to be able to simulate various strategies. In the next section, an installed script is introduced for this purpose (to simulate the exposure map of a series of pointings) in GNU Astronomy Utilities (Gnuastro Akhlaghi & Ichikawa 2015) version[1] 0.21 and later.

This paper is available on arxiv under CC 4.0 DEED license.

[1] This paper is published shortly before the release of Gnuastro 0.21. If Gnuastro 0.21 is not yet available, please use the latest test (alpha) release.