Selecting IACT observations¶

What you will learn

You will learn how to select observations from a dataset.

As next step you have to select observations from a dataset and to build an observation definition XML file for the analysis.

Selecting observations¶

First run csfindobs to select the observations of interest. The most common search for observations is by pointing direction on the sky, and you do this by typing

$ csfindobs
Path were data are located (NONE uses $VHEFITS environment variable) [NONE] data
Name of FITS production (Run csiactdata to view your options) [prod-name] hess_dl3_dr1
Right ascension of selection region centre (deg) [83.63]
Declination of selection region centre (deg) [22.51]
Search radius of selection region (deg) [2.5]
Output runlist file [runlist.lis]

This creates an output ASCII file named runlist.lis that contains the identifiers for all observations with pointing directions located within 2.5 deg around the position of the Crab nebula. The content of the file is

hence four observations were found in the production. An excerpt of the log file csfindobs.log of the run is shown below:

2019-04-05T13:22:08: +===================+
2019-04-05T13:22:08: | Find observations |
2019-04-05T13:22:08: +===================+
2019-04-05T13:22:08:  Expression ................: ANGSEP(83.63,22.51,RA_PNT,DEC_PNT)<=2.5&&QUALITY<=0
2019-04-05T13:22:08:  Observations ..............: 4
2019-04-05T13:22:08:  Observation 1 .............: 23523
2019-04-05T13:22:08:  Observation 2 .............: 23526
2019-04-05T13:22:08:  Observation 3 .............: 23559
2019-04-05T13:22:08:  Observation 4 .............: 23592

Note

You may add further selection criteria using the hidden expression parameter. You may use any FITS selection expression that is supported by the cfitsio row filtering specification. Column names that can be used in the expression can be found here or by simply browsing the observation index file with a FITS viewer. For example

$ csfindobs expression="ZEN_PNT < 30 && LIVETIME > 1600"
Path were data are located (NONE uses $VHEFITS environment variable) [data]
Name of FITS production (Run csiactdata to view your options) [hess_dl3_dr1]
Right ascension of selection region centre (deg) [83.63] NONE
Output runlist file [runlist.lis]

will select all observations that have a zenith angle less than 30 deg and a livetime larger than 1600 seconds. Note that pointing selection was omitted by specifying NONE for the Right Ascension.

Note

By default, csfindobs only selects data of highest quality (i.e. QUALITY=0). You may overwrite this default by specifying the hidden parameter min_qual. For example, min_qual=1 selects all data with a looser quality criteria.

Create an observation list¶

Now you can convert the runlist into an observation definition XML file. You do this conversion with the csiactobs script:

$ csiactobs
Path were data are located (NONE uses $VHEFITS environment variable) [NONE] data
Data storage name [fits-prod-name] hess_dl3_dr1
Input runlist file [runlist.lis]
Number of free parameters per background model [1] 2
Output model definition XML file [bkgmodels.xml]
Output observation definition XML file [obs.xml]

The csiactobs script will create two output files: the observation definition XML file obs.xml and an output model definition XML file bkgmodels.xml. To generate obs.xml, csiactobs has used the IACT data storage and extracted the relevant file names. bgmodels.xml is a file that is used for background modeling, where each observation will have its own independent background model. In the example above, you have set the number of free parameters per background model to one, hence the normalisation of the background model for each observation will be a free parameter that is later adjusted by the maximum likelihood fit.

Here the observation definition file that was created

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<observation_list title="observation list">
  <observation name="Crab Nebula" id="23523" instrument="HESS">
    <parameter name="EventList" file="data/data/hess_dl3_dr1_obs_id_023523.fits.gz[events]" />
    <parameter name="EffectiveArea" file="data/data/hess_dl3_dr1_obs_id_023523.fits.gz[aeff]" />
    <parameter name="PointSpreadFunction" file="data/data/hess_dl3_dr1_obs_id_023523.fits.gz[psf]" />
    <parameter name="EnergyDispersion" file="data/data/hess_dl3_dr1_obs_id_023523.fits.gz[edisp]" />
    <parameter name="Background" file="" />
  </observation>
  <observation name="Crab Nebula" id="23526" instrument="HESS">
    <parameter name="EventList" file="data/data/hess_dl3_dr1_obs_id_023526.fits.gz[events]" />
    <parameter name="EffectiveArea" file="data/data/hess_dl3_dr1_obs_id_023526.fits.gz[aeff]" />
    <parameter name="PointSpreadFunction" file="data/data/hess_dl3_dr1_obs_id_023526.fits.gz[psf]" />
    <parameter name="EnergyDispersion" file="data/data/hess_dl3_dr1_obs_id_023526.fits.gz[edisp]" />
    <parameter name="Background" file="" />
  </observation>
  <observation name="Crab Nebula" id="23559" instrument="HESS">
    <parameter name="EventList" file="data/data/hess_dl3_dr1_obs_id_023559.fits.gz[events]" />
    <parameter name="EffectiveArea" file="data/data/hess_dl3_dr1_obs_id_023559.fits.gz[aeff]" />
    <parameter name="PointSpreadFunction" file="data/data/hess_dl3_dr1_obs_id_023559.fits.gz[psf]" />
    <parameter name="EnergyDispersion" file="data/data/hess_dl3_dr1_obs_id_023559.fits.gz[edisp]" />
    <parameter name="Background" file="" />
  </observation>
  <observation name="Crab Nebula" id="23592" instrument="HESS">
    <parameter name="EventList" file="data/data/hess_dl3_dr1_obs_id_023592.fits.gz[events]" />
    <parameter name="EffectiveArea" file="data/data/hess_dl3_dr1_obs_id_023592.fits.gz[aeff]" />
    <parameter name="PointSpreadFunction" file="data/data/hess_dl3_dr1_obs_id_023592.fits.gz[psf]" />
    <parameter name="EnergyDispersion" file="data/data/hess_dl3_dr1_obs_id_023592.fits.gz[edisp]" />
    <parameter name="Background" file="" />
  </observation>
</observation_list>
<observation_list title="observation list" />

and here the model definition file

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<source_library title="source library">
  <source name="bkg_23523" type="CTAAeffBackground" instrument="HESS" id="23523" tscalc="0">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="1" error="0" scale="1e-14" min="0.01" max="100" free="1" />
      <parameter name="Index" value="-2" scale="1" min="-5" max="5" free="1" />
      <parameter name="PivotEnergy" value="1" scale="1000000" free="0" />
    </spectrum>
  </source>
  <source name="bkg_23526" type="CTAAeffBackground" instrument="HESS" id="23526" tscalc="0">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="1" error="0" scale="1e-14" min="0.01" max="100" free="1" />
      <parameter name="Index" value="-2" scale="1" min="-5" max="5" free="1" />
      <parameter name="PivotEnergy" value="1" scale="1000000" free="0" />
    </spectrum>
  </source>
  <source name="bkg_23559" type="CTAAeffBackground" instrument="HESS" id="23559" tscalc="0">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="1" error="0" scale="1e-14" min="0.01" max="100" free="1" />
      <parameter name="Index" value="-2" scale="1" min="-5" max="5" free="1" />
      <parameter name="PivotEnergy" value="1" scale="1000000" free="0" />
    </spectrum>
  </source>
  <source name="bkg_23592" type="CTAAeffBackground" instrument="HESS" id="23592" tscalc="0">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="1" error="0" scale="1e-14" min="0.01" max="100" free="1" />
      <parameter name="Index" value="-2" scale="1" min="-5" max="5" free="1" />
      <parameter name="PivotEnergy" value="1" scale="1000000" free="0" />
    </spectrum>
  </source>
</source_library>

Note

In case that you have already a model definition XML file that describes a model of the celestial source distribution (a.k.a. a sky model), you may provide this sky model to csiactobs using the hidden inmodel parameter. The output model definition XML file will then contain the sky model and the background model, and you can use the model directly in a maximum likelihood fit.

$ csiactobs inmodel=$CTOOLS/share/models/crab.xml
Path were data are located (NONE uses $VHEFITS environment variable) [data]
Data storage name [hess_dl3_dr1]
Input runlist file [runlist.lis]
Number of free parameters per background model [2]
Output model definition XML file [bkgmodels.xml] models.xml
Output observation definition XML file [obs.xml]

Select events¶

The final step needed is the selection of events, to define in particular the region of interest and energy range for the analysis. You do this by typing

$ ctselect
Input event list or observation definition XML file [events.fits] obs.xml
Radius of ROI around pointing or specified RA/DEC (degrees) (0-180) [3.0] 2.0
Start time (UTC string, JD, MJD or MET in seconds) [NONE]
Lower energy limit (TeV) [0.1] 0.5
Upper energy limit (TeV) [100.0] 10.0
Output event list or observation definition XML file [selected_events.fits] obs_selected.xml

which will select all events within a radius of 2 degrees around the pointing direction with energies between 500 GeV and 10 TeV.

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Selecting IACT observations¶

Selecting observations¶

Create an observation list¶

Select events¶