Through the “zero point”, we are able to give physical units to the pixel values of an image (often in units of “counts” or ADUs) and thus compare them with other images (as well as measurements that are done on them). The zero point is therefore an important calibration of pixel values (as astromerty is a calibration of the pixel positions). The fundamental concepts behind the zero point are described in Brightness, Flux, Magnitude and Surface brightness. We will therefore not go deeper into the basics here and stick to the practical aspects of it.
The purpose of Gnuastro’s
astscript-zeropoint script is to obtain the zero point of an image by considering another image (where the zero point is already known), or a catalog.
The operation involves multiple lower-level programs in a standard series of steps.
For example, when using another image, the script will take the following steps:
In the sections below we have prepared two tutorials on the use of this script. The first uses an image as a reference (Zero point tutorial with reference image) and the second uses a catalog (Zero point tutorial with reference catalog). Afterwards, in Invoking astscript-zeropoint, the details of all the options and how to run this script are provided.
Stars have an almost identical shape in the image (as opposed to galaxies for example), using confirmed stars will produce a more reliable result.
For a complete tutorial on aperture photometry, see Aperture photometry.
For a tutorial on matching catalogs, see Matching catalogs).