demo1.yaml

# Copyright (c) 2012-2026 by the GalSim developers team on GitHub
# https://github.com/GalSim-developers
#
# This file is part of GalSim: The modular galaxy image simulation toolkit.
# https://github.com/GalSim-developers/GalSim
#
# GalSim is free software: redistribution and use in source and binary forms,
# with or without modification, are permitted provided that the following
# conditions are met:
#
# 1. Redistributions of source code must retain the above copyright notice, this
#    list of conditions, and the disclaimer given in the accompanying LICENSE
#    file.
# 2. Redistributions in binary form must reproduce the above copyright notice,
#    this list of conditions, and the disclaimer given in the documentation
#    and/or other materials provided with the distribution.
#
#
# Demo #1
#
# The first YAML configuration file in our tutorial about using Galsim config files
# (This file is designed to be viewed in a window 100 characters wide.)
#
# Each of these demo*.yaml files are designed to be equivalent to the corresponding
# demo*.py file.  We expect that the config file usage will be easier for many people
# to get going with GalSim, especially those who aren't very comfortable with writing
# python code.  And even advanced pythonistas may find it more useful for many tasks than
# writing the corresponding python code.
#
# The executable that reads these YAML files is called galsim, which should be installed
# by either `pip install galsim` or `python setup.py install`.  If you used the latter,
# the output should end with a line something along the lines of:
#
#     scripts installed into  /usr/local/bin
#
# telling you which directory they were installed in.  If that directory is not in your
# path, then there should also be a message telling you to add it to your $PATH
# environment variable.  If you used `pip install galsim --user`, then it was probably
# installed into a directory called .local/bin in your home directory.  You can have
# pip tell you where it is installing things by adding the `-v` option.
#
# In any case, you can confirm that `galsim` is in your path by typing
#
#     which galsim
#
# which should show you which executable will be used.  (If nothing shows up, then `galsim`
# is not in your path.)
#
# Then to run this config file, you should be able to type simply:
#
#     galsim demo1.yaml
#
# If you don't have PyYAML installed, you can use JSON files instead.  The directory json has
# JSON configuration files that are exactly equivalent to these YAML files.  The YAML format
# is a superset of the JSON format, so they are quite similar, but the YAML format is easier
# to read and has a few extra nice features.  But if you don't want to install PyYAML for some
# reason, you can use the json/demo*.json instead by typing (for demo1):
#
#     galsim json/demo1.json
#
# See https://github.com/GalSim-developers/GalSim/wiki/Config-Documentation for
# more complete documentation about how to use the GalSim configuration machinery.
#
#
# This first config file is about as simple as it gets.  We draw an image of a single galaxy
# convolved with a PSF and write it to disk.  We use a circular Gaussian profile for both the
# PSF and the galaxy, and add a constant level of Gaussian noise to the image.
#
# In each demo, we list the new features introduced in that demo file.  These will differ somewhat
# between the .py and .yaml versions, since the two methods implement things in different ways.
# (demo*.py are python scripts, while demo*.yaml are configuration files.)
#
# New features introduced in this demo:
#
# - top level fields gal, psf, image, output
# - obj type : Gaussian (flux, sigma)
# - image : pixel_scale
# - image : noise
# - noise type : Gaussian (sigma)
# - output : dir, file_name


# The gal field defines what kind of galaxy profile to use.
gal :
    # One of the simplest profiles is a Gaussian.
    type : Gaussian

    # Gaussian profiles have a number of possible size parameters, but
    # sigma is the most basic one.
    # The others are fwhm and half_light_radius.  At least one of these must be specified.
    sigma : 2  # arcsec

    # The default flux would be 1, but you would typically want to define the flux
    # to be something other than that.
    flux : 1.e5


# Technically, the psf field isn't required, but for astronomical images we always have a PSF
# so you'll usually want to define one.  (If it's omitted, the galaxy isn't convolved
# by anything, so effectively a delta function PSF.)
# We use a Gaussian again for simplicity, but one somewhat smaller than the galaxy.
psf :
    type : Gaussian
    sigma : 1  # arcsec
    # No need to specify a flux, since flux=1 is the right thing for a PSF.


# The image field specifies some other information about the image to be drawn.
image :
    # If pixel_scale isn't specified, then pixel_scale = 1 is assumed.
    pixel_scale : 0.2  # arcsec / pixel

    # If you want noise in the image (which is typical) you specify that here.
    # In this case we use gaussian noise.
    noise :
        type : Gaussian
        sigma : 30  # standard deviation of the counts in each pixel

    # You can also specify the size of the image if you want, but if you omit it
    # (as we do here), then GalSim will automatically size the image appropriately.


# Typically, you will want to specify the output format and file name.
# If this is omitted, the output will be to a fits file with the same root name as the
# config file (so demo1.fits in this case), but that's usually not a great choice.
# So at the very least, you would typically want to specify at least the file_name.
output :
    dir : output_yaml
    file_name : demo1.fits