The Config Module

The basic configuration method is to use a dictionary which can be parsed in python. Within that structure, each field can either be a value, another dictionary which is then further parsed, or occasionally a list of items (which can be either values or dictionaries). The hierarchy can go as deep as necessary.

Our example config files are all yaml files, which are read using the executable galsim. This is a nice format for config files, but it is not required. Anything that can represent a dictionary will do. For example, the executable galsim also reads in and processes json-style config files if you prefer.

If you would like a kind of tutorial that goes through typical uses of the config files, there are a series of demo config files in the GalSim/examples directory. See Tutorials for more information.

For a concrete example of what a config file looks like, here is demo1.yaml (the first file in the aforementioned tutorial) stripped of most of the comments to make it easier to see the essence of the structure:

gal :
    type : Gaussian
    sigma : 2  # arcsec
    flux : 1.e5  # total counts in all pixels

psf :
    type : Gaussian
    sigma : 1  # arcsec

image :
    pixel_scale : 0.2  # arcsec / pixel
    noise :
        type : Gaussian
        sigma : 30  # standard deviation of the counts in each pixel

output :
    dir : output_yaml
    file_name : demo1.fits

This file defines a dictionary, which in python would look like:

config = {
    'gal' : {
        'type' : 'Gaussian',
        'sigma' : 2.,
        'flux' : 1.e5
    },
    'psf' : {
        'type' : 'Gaussian',
        'sigma' : 1.
    },
    'image' : {
        'pixel_scale' : 0.2,
        'noise' : {
            'type' : 'Gaussian',
            'sigma' : 30.
        }
    },
    'output' : {
        'dir' : 'output_yaml',
        'file_name' : 'demo1.fits'
    }
}

As you can see, there are several top level fields (gal, psf, image, and output) that define various aspects of the simulation. There are others as well that we will describe below, but most simulations will want to include at least these four.

Most fields have a type item that defines what the other items in the field mean. (The image and output fields here have implicit types Single and Fits, which are the default, so may be omitted.) For instance, a Gaussian surface brightness profile is defined by the parameters sigma and flux.

Most types have some optional items that take reasonable defaults if you omit them. E.g. the flux is not relevant for a PSF, so it may be omitted in the psf field, in which case the default of flux=1 is used.