A dataset that is ready for scientific analysis is usually composed of many separate exposures and how they are taken is usually known as “observing strategy”. This tutorial describes Gnuastro’s tools to simplify the process of deciding the pointing pattern of your observing strategy.
A “pointing” is the location on the sky that each exposure is aimed at. Each exposure’s pointing is usually moved (on the sky) compared to the previous exposure. This is done for reasons like improving calibration, increasing resolution, expending the area of the observation and etc. Therefore, deciding a suitable pointing pattern is one of the most important steps when planning your observation strategy.
There are commonly two types of pointings: “dither” and “offset”. These are sometimes used interchangeably with “pointing” (especially when the final stack is roughly the same area as the field of view. Alternatively, “dither” and “offset” are used to distinguish pointings with large or small (on the scale of the field of view) movement compared to a previous one. When a pointing has a large distance to the previous pointing, it is known as an “offset”, while pointings with a small displacement are known as a “dither”. This distinction originates from the mechanics and optics of most modern telescopes: the overhead (for example the need to re-focus the camera) to make small movements is usually less than large movements.
In this tutorial, let’s simulate a hypothetical pointing pattern using Gnuastro’s
astscript-pointing-simulate installed script (see Pointing pattern simulation).
Since we will be testing very different displacements between pointings, we’ll ignore the difference between offset and dither here, and only use the term pointing.
Let’s assume you want to observe M94 in the H-alpha and rSDSS filters (to study the extended star formation in the outer rings of this beautiful galaxy!). Including the outer parts of the rings, the galaxy is half a degree in diameter! This is very large, and you want to design a pointing pattern that will allow you to cover as much area, while not loosing your ability to calibrate properly.
Do not start with this tutorial: If you are new to Gnuastro and have not already completed General program usage tutorial, we recommend going through that tutorial before starting this one. Basic features like access to this book on the command-line, the configuration files of Gnuastro’s programs, benefiting from the modular nature of the programs, viewing multi-extension FITS files, and many others are discussed in more detail there.