Using imSim
imSim is implemented as a set of modules that are executed by the galsim program. In order to run imSim you create a YAML file which imports and configures the modules. imSim builds on basic GalSim functionality so when using imSim you will configure both GalSim and imSim functionality. In this section a simple config file is demonstrated. You can find a more complete description of the config system option in the Config System.
imSim is packaged with some YAML config files for it’s own use which you can look at if you would like to see some examples. You can see some in the “config” and also the “examples” directory.
To get started copy the two files imsim-user.yaml and example_instance_catalog.txt to a working directory. Edit the imsim-user.yaml file to point to your local copy of the example_instance_catalog.txt file by removing the two instances of $os.environ.get('IMSIM_HOME')+ from the file.
Now try to run the file with the command:
galsim imsim-user.yaml
GalSim will process the YAML file and open and read the example instance catalog file, generating a FITS file which corresponds to the listed sources.
imSim has more than one way to specify sources. Instance catalogs are simple text files best suited to making small handcrafted inputs. For legacy and compatibility purposes, They follow the format of the PhoSim program inputs which are documented on PhoSim Web Site. For more complex and large area simulations, imSim utilizes an API based system known as skyCatalogs. By querying the skyCatalog via its API it can return a list to imSim of all the objects at that position in the the sky at that time. skyCatalogs can return both static and time dependent sources and can be configured to serve objects from both synthetic sky maps and true lists of sources such as from Gaia. The skyCatalog can also serve as a source of truth information when later analyzing simulated data.
It is also important to specify MetaData to imSim which supplies necessary information such as the location, pointing, filters being used and time that the exposure was taken. This information can be manually specified in the YAML files, given as part of a text based instance catalog, or retrieved from a Rubin Observatory OpSim file which is the simulated output of a Rubin Schedular execution.
Almost every element of the system from the atmosphere to optics, to details of the electronics readout can be specified through the config system in the YAML files. Here, we will give an example of how to construct a very simple instance catalog which will image a singe bright, magnitude 16 star. The photons from this bright star will be raytraced through the atmosphere and optics of the telescope and then read out in the LSSTCam sensors.
Each YAML file begins by importing the imSim code which is located in a module. It then reads a template yaml file that configures many of the of the imSim classes. Everything in your user file will modify or add to those values. The file is located in config/imsim-config.yaml and is an excellent reference.
The rest of the file configures the input and output options. You should make a copy yourself.
modules:
- imsim
template: imsim-config-instcat
input.instance_catalog.sed_dir: $os.environ.get('SIMS_SED_LIBRARY_DIR')
input.instance_catalog.file_name: 1-star.txt
input.opsim_data.file_name: 1-star.txt
input.tree_rings.only_dets: [R22_S11]
output.dir: output
output.det_num:
type: List
items: [94]
output.nfiles: 1
This file contains the needed metadata and a single star.
rightascension 0.0
declination 0.0
mjd 59797.2854090
altitude 0.0
azimuth 0.0
filter 2
rotskypos 0.0
rottelpos 0.0
dist2moon 90.0
moonalt -90.0
moondec 0.0
moonra 0.0
moonphase 0.0
nsnap 2
obshistid 1
seed 57721
seeing 1.0
sunalt -50.0
vistime 33.0
seqnum 0
object MS_567_8a 0.0 0.0 16.0 starSED/phoSimMLT/lte033-4.5-1.0a+0.4.BT-Settl.spec.gz 0 0 0 0 0 0 point none CCM 0.0635117705 3.1
After creating these files you can:
galsim 1-star.yaml
In the ‘output’ directory (that you specified in the YAML file), you will find three files an eimage FITS file (which contains the true electron informationon the sensor), an amp FITS file which contains the full electronic readout of each amplifier on the sensor (each in its own HDU), and a text centroid file with truth information on where the source was located. The FITS files are viewable with a program such as ds9.