Simulations of observations are useful for testing the sensitivity of a specific set of observations to BEAST model parameters. This is useful for quantifying the BEAST performance for proposed and actual observations.

This is done using the beast.observationmodel.observations.gen_SimObs_from_sedgrid function. The script tools/ can be run directly or using the beast simulate_obs command once the beast has been installed. The module uses already created BEAST physics and observation model grids by sampling the full nD prior function that is part of the physics model grid. The observation model grid provides the information on the flux uncertainties and biases as well as the completeness.


The files for the physicsgrid and obsgrid files are required inputs to the script. The output filename is also required. Note that the extension of this file will determine the type of file output (e.g. filebase.fits for a FITS file). The number of observations to simulate is given by the --nsim parameter. The SEDs are picked weighted by the product of the grid+prior weights and the completeness from the noisemodel. The grid+prior weights can be replaced with either grid or prior weights by explicitly setting the --weight_to_use parameter.

$ beast simulate_obs physicsgrid obsgrid outfile --nsim 200

The output file gives the simulated data in the observed data columns identified in the physicsgrid file along with all the model parameters from the physicsgrid file. The simulated observations in each band are given as band_flux in physical units (ergs cm^-2 s^-1 A^-1), band_rate as normalized Vega fluxes (band_flux/vega_flux to match how the observed data are given), and band_vega as vega magnitudes with zero and negative fluxes given as -99.999.

When creating simulated observations, using the standard IMF mass prior will skew your catalog to lower-mass stars. If you wish to have similar weights for stars of all masses, use a flat IMF and a log-flat age prior. To do this, set the mass prior to {'name': 'flat'} and the age prior to {'name': 'flat_log'} in beast_settings.txt before creating the model grid.


The code does not handle the truncheon model at this point. While this model is doable in the BEAST, it has not been done yet due to several potentially complex modeling questions for actually using it that might impact how the model is implemented.


To plot a color-magnitude diagram of the simulated observations, a sample call from the command line may be:

$ beast plot_cmd outfile.fits --mag1 F475W --mag2 F814W --mag3 F475W

where outfile.fits may be the output from simulate_obs. mag1-mag2 is the color, and mag3 the magnitude. By default the figure is saved as outfile_plot.png in the directory of outfile.fits.

Remove Filters

One use case for simulations is to test the impact of specific filters on the BEAST results. One solution is to create multiple physics/observation model grids, create simulations from each set of grids, and then fit the simulations with the BEAST. A quicker way to do this is to create the physics/observation grid set with the full set of desired filters, create the desired simulations, remove filters from the model and simulations as needed, and then fit with the BEAST. This has the benefit of the simulations with different filter sets are exactly the same except for the removed filters.

As an example, to remove the filters F275W and F336W from the simulated observations contained in ‘catfile.fits’ and the ‘physgrid.hd5’/’obsgrid.hd5’ set of models use the following command.

$ python catfile.fits --physgrid physgrid.hd5 \
     --obsgrid obsgrid.hd5 --outbase outbase --rm_filters F275W F336W

New physics/observation model grids and simulated observation files are created as ‘outbase_seds.grid.hd5’, ‘outbase_noisemodel.grid.hd5’, and ‘outbase_cat.fits’.