Simulations¶
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/simulate_obs.py provides a commandline interface. 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.
Toothpick¶
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 filter to use for the completeness function is given by the
--compl_filter parameter. By default, the SEDs are randomly chosen, and
weighted by their grid+prior weights; this can be changed with the
--weight_to_use parameter.
$ python simulate_obs.py physicsgrid obsgrid outfile \
--nsim 200 --compl_filter F475W
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 names of the observed columns are
band_rate and the units are normalized Vega fluxes (to match how
the observed data are given).
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 datamodel.py before creating the model grid.
Truncheon¶
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.
Plotting¶
To plot a color-magnitude diagram of the simulated observations, a sample call from the command line may be:
$ python plot_cmd.py outfile.fits --mag1 F475W --mag2 F814W --magy F475W
where outfile.fits may be the output from tools/simulate_obs.py.
mag1-mag2 is the color, and magy 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 remove_filters.py 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’.