Top Level Fields

At the top level, there are 6 basic fields:

  • psf defines what kind of PSF profile to use.

  • gal defines what kind of galaxy profile to use.

  • stamp defines parameters related to building the postage stamp image of each object.

  • image defines parameters related to the full images to be drawn.

  • input defines any necessary input files or things that need some kind of initialization.

  • output defines the names and format of the output files.

None of these are technically required, although it is an error to have _neither_ psf nor gal. (If you don’t want to draw anything but noise, you need to let GalSim know that this is intentional by using type: None for one of these.) But the most common usage would be to use psf, gal, image and output. It is not uncommon for there to be no input files, so you will often omit the input field. And sometimes you will omit the gal field to draw an image with just stars. Most simulations will use the default stamp type (called ‘Basic’), which involves drawing a galaxy convolved by a PSF (or just a PSF image if gal is omitted) on each postage stamp, so this field will very often be omitted as well.

We will go through each one in turn. As we do, some values will be called float_value, int_value, etc. These can either be a value directly (e.g. float_value could just be 1.5), or they can be a dict that describes how the value should be generated each time (e.g. a random number or a value read from an input catalog). See Config Values for more information about how to specify these values.

In addition each value will have one of (required) or (optional) or (default = _something_) to indicate whether the item is required or if there is some sensible default value. The (optional) tag usually means that the action in question will not be done at all, rather than done using some default value. Also, sometimes no item is individually required, but one of several is.

psf:

The psf field defines the profile of the point-spread function (PSF). Any object type is allowed for the psf type, although some types are obviously more appropriate to use as a PSF than others. For a list of all the available object types, see Config Objects.

If this field is omitted, the PSF will effectively be a delta function. I.e. the ideal galaxy surface brightness profiles will be drawn directly on the image without any convolution.

gal:

The gal field defines the profile of the galaxy. As for the psf field, any object type is allowed for the gal type, although some types are obviously more appropriate to use as a galaxy than others. For a list of all the available object types, see Config Objects.

Technically, the gal field is not fundamental; its usage is defined by the stamp type. One could for instance define a stamp type that looked for a gal_set field instead that might give a list of galaxies to draw onto a single stamp. However, all of the stamp types defined natively in GalSim use the gal field, so it will be used by most users of the code.

If this field is omitted, the default stamp type = ‘Basic’ will draw the PSF surface brightness profiles directly according to the psf field. Other stamp types may require this field or may require some other field instead.

stamp:

The stamp field defines the relevant properties and parameters of the stamp-building process. For a list of all the available stamp types, see Config Stamp Field.

This field is often omitted, in which case the ‘Basic’ stamp type will be assumed.

image:

The image field defines the relevant properties and parameters of the full image-building process. For a list of all the available image types, see Config Image Field.

If this field is omitted, the ‘Single’ image type will be assumed.

input:

The input field indicates where to find any files that you want to use in building the images or how to set up any objects that require initialization. For a list of all the available input types, see Config Input Field.

This field is only required if you use object types or value types that use an input object. Such types will indicate this requirement in their descriptions.

output:

The output field indicates where to write the output files and what kind of output format they should have. For a list of all the available output types, see Config Output Field.