Directory Configuration

The “built-in” Sources and Spectra in SNCosmo depend on some sizable data files. These files are hosted remotely, downloaded as needed, and cached locally. This all happens automatically, but it is helpful to know where the files are stored if you want to inspect them or share a common download directory between multiple users.

By default, SNCosmo will create and use an sncosmo subdirectory in the AstroPy cache directory for this purpose. For example, $HOME/.astropy/cache/sncosmo. After using a few models and spectra for the first time, here is what that directory might look like:

$ tree ~/.astropy/cache/sncosmo
├── models
│   ├── hsiao
│   │   └── Hsiao_SED_V3.fits
│   └── sako
│       ├── S11_SDSS-000018.SED
│       ├── S11_SDSS-001472.SED
│       └── S11_SDSS-002000.SED
└── spectra
    ├── alpha_lyr_stis_007.fits
    └── bd_17d4708_stisnic_005.fits

You can see that within the top-level $HOME/.astropy/cache/sncosmo directory, a particular directory structure is created. This directory structure is fixed in the code, so it’s best not to move things around within the top-level directory. If you do, sncosmo will think the data have not been downloaded and will re-download them.

Configuring the Directories

What if you would rather use a different directory to store downloaded data? Perhaps you’d rather the data not be in a hidden directory, or perhaps there are multiple users who wish to use a shared data directory. There are two options:

  1. Set the environment variable SNCOSMO_DATA_DIR to the directory you wish to use. For example, in bash:

    export SNCOSMO_DATA_DIR=/home/user/data/sncosmo

    If this environment variable is set, it takes precedence over the second option (below).

  2. Set the data_dir variable in the sncosmo configuartion file. This file is found in the astropy configuration directory, e.g., $HOME/.astropy/config/sncosmo.cfg. When you import sncosmo it checks for this file and creates a default one if it doesn’t exist. The default one looks like this:

    $ cat ~/.astropy/config/sncosmo.cfg
    ## Directory containing SFD (1998) dust maps, with names:
    ## 'SFD_dust_4096_ngp.fits' and 'SFD_dust_4096_sgp.fits'
    ## Example: sfd98_dir = /home/user/data/sfd98
    # sfd98_dir = None
    ## Directory where sncosmo will store and read downloaded data resources.
    ## If None, ASTROPY_CACHE_DIR/sncosmo will be used.
    ## Example: data_dir = /home/user/data/sncosmo
    # data_dir = None
To change the data directory, simply uncomment the last line and set it to the desired directory. You can even move the data directory around, as long as you update this configuration parameter accordingly.