GNU Astronomy Utilities

10.8.1 Overview of the PSF scripts

To obtain an extended and non-parametric PSF, several steps are necessary and we will go through them here. The fundamental ideas of the following methodology are thoroughly described in Infante-Sainz et al. 2020. A full tutorial is also available in Building the extended PSF. The tutorial will go through the full process on a pre-selected dataset, but will describe the logic behind every step in away that can easily be modified/generalized to other datasets.

This section is basically just a summary of that tutorial. We could have put all these steps into one large program (installed script), however this would introduce several problems. The most prominent of these problems are:

Therefore, following the modularity principle of software engineering, after several years of working on this, we have broken the full job into the smallest number of independent steps as separate scripts. All scripts are independent of each other, meaning this that you are free to use all of them as you wish (for example, only some of them, using another program for a certain step, using them for other purposes, or running independent parts in parallel).

For constructing the PSF from your dataset, the first step is to obtain a catalog of stars within it (you cannot use galaxies to build the PSF!). But you cannot blindly use all the stars either! For example, we do not want contamination from other bright, and nearby objects. The first script below is therefore designed for selecting only good star candidates in your image. It will use different criteria, for example, good parallax (where available, to avoid confusion with galaxies), not being near to bright stars, axis ratio, etc. For more on this script, see Invoking astscript-psf-select-stars.

Once the catalog of stars is constructed, another script is in charge of making appropriate stamps of the stars. Each stamp is a cropped image of the star with the desired size, normalization of the flux, and mask of the contaminant objects. For more on this script, see Invoking astscript-psf-stamp After obtaining a set of star stamps, they can be stacked for obtaining the combined PSF from many stars (for example, with Stacking operators).

In the combined PSF, the masked background objects of each star’s image will be covered and the signal-to-noise ratio will increase, giving a very nice view of the “clean” PSF. However, it is usually necessary to obtain different regions of the same PSF from different stars. For example, to construct the far outer wings of the PSF, it is necessary to consider very bright stars. However, these stars will be saturated in the most inner part, and immediately outside of the saturation level, they will be deformed due to non-linearity effects. Consequently, fainter stars are necessary for the inner regions.

Therefore, you need to repeat the steps above for certain stars (in a certain magnitude range) to obtain the PSF in certain radial ranges. For example, in Infante-Sainz et al. 2020, the final PSF was constructed from three regions (and thus, using stars from three ranges in magnitude). In other cases, we even needed four groups of stars! But in the example dataset from the tutorial, only two groups are necessary (see Building the extended PSF).

Once clean stacks of different parts of the PSF have been constructed through the steps above, it is therefore necessary to blend them all into one. This is done by finding a common radial region in both, and scaling the inner region by a factor to add with the outer region. This is not trivial, therefore, a third script is in charge of it, see Invoking astscript-psf-unite.

Having constructed the PSF as described above (or by any other procedure), it can be scaled to the magnitude of the various stars in the image to get subtracted (and thus remove the extended/bright wings; better showing the background objects of interest). Note that the absolute flux of a PSF is meaningless (and in fact, it is usually normalized to have a total sum of unity!), so it should be scaled. We therefore have another script that will calculate the scale (multiplication) factor of the PSF for each star. For more on the scaling script, see Invoking astscript-psf-scale-factor.

Once the flux factor has been computed, a final script is in charge of placing the scaled PSF over the proper location in the image, and subtracting it. It is also possible to only obtain the modeled star by the PSF. For more on the scaling and positioning script, see Invoking astscript-psf-subtract.

As mentioned above, in the following sections, each script has its own documentation and list of options for very detailed customization (if necessary). But if you are new to these scripts, before continuing, we recommend that you do the tutorial Building the extended PSF. Just do not forget to run every command, and try to tweak its steps based on the logic to nicely understand it.