Selecting event data

What you will learn

You will learn how to use the ctselect tool to select relevant events from the simulated event data.

As next step you may want to select some events from the simulated event data. You may want to restrict the events to a region of interest (ROI) of 3 degrees around the pointing direction, you may want to extract a 30 minutes time slice from the data, and you may want to limit the energy range of the events to 0.1 - 100 TeV. You do this event selection using the ctselect tool as follows:

$ ctselect
Input event list or observation definition XML file [events.fits]
Radius of ROI around pointing or specified RA/DEC (0-180) [3.0]
Start time (UTC string, JD, MJD or MET in seconds) [NONE] 2020-01-01T00:10:00
Stop time (UTC string, JD, MJD or MET in seconds) [NONE] 2020-01-01T00:40:00
Lower energy limit (TeV) [0.1]
Upper energy limit (TeV) [100.0]
Output event list or observation definition XML file [selected_events.fits]

This created a new event list FITS file selected_events.fits that contains the selected data. Below an excerpt of the ctselect.log log file that illustrates the event selection:

2019-04-02T12:52:50: +===================+
2019-04-02T12:52:50: | Input observation |
2019-04-02T12:52:50: +===================+
2019-04-02T12:52:50: === GObservations ===
2019-04-02T12:52:50:  Number of observations ....: 1
2019-04-02T12:52:50:  Number of models ..........: 0
2019-04-02T12:52:50:  Number of observed events .: 206048
2019-04-02T12:52:50:  Number of predicted events : 0
2019-04-02T12:52:50:
2019-04-02T12:52:50: +=================+
2019-04-02T12:52:50: | Event selection |
2019-04-02T12:52:50: +=================+
2019-04-02T12:52:50: === CTA observation (id=000001) ===
2019-04-02T12:52:50:  Input filename ............: events.fits
2019-04-02T12:52:50:  Event extension name ......: EVENTS
2019-04-02T12:52:50:  GTI extension name ........: GTI
2019-04-02T12:52:50: === Event selection ===
2019-04-02T12:52:50:  Time range (MJD) ..........: 58849.0077451852 - 58849.0285785185 days
2019-04-02T12:52:50:  Time range (UTC) ..........: 2020-01-01T00:10:00 - 2020-01-01T00:40:00
2019-04-02T12:52:50:  Time range (MET) ..........: 631109469.184 - 631111269.184 seconds
2019-04-02T12:52:50:  Selected energy range .....: 0.1 - 100 TeV
2019-04-02T12:52:50:  Requested RoI .............: Centre(RA,DEC)=(83.63, 22.51) deg, Radius=3 deg
2019-04-02T12:52:50:  RoI of data ...............: Centre(RA,DEC)=(83.63, 22.51) deg, Radius=5 deg
2019-04-02T12:52:50:  Selected RoI ..............: Centre(RA,DEC)=(83.63, 22.51) deg, Radius=3 deg
2019-04-02T12:52:50:  cfitsio selection .........: TIME >= 631109469.18399978 && TIME <= 631111269.18400002 && ENERGY >= 0.10000000 && ENERGY <= 100.00000000 && ANGSEP(83.630000,22.510000,RA,DEC) <= 3.000000
2019-04-02T12:52:50:  FITS filename .............: /var/tmp/tmp.0.vWKyrl[EVENTS][TIME >= 631109469.18399978 && TIME <= 631111269.18400002 && ENERGY >= 0.10000000 && ENERGY <= 100.00000000 && ANGSEP(83.630000,22.510000,RA,DEC) <= 3.000000]
2019-04-02T12:52:51:
2019-04-02T12:52:51: +====================+
2019-04-02T12:52:51: | Output observation |
2019-04-02T12:52:51: +====================+
2019-04-02T12:52:51: === GObservations ===
2019-04-02T12:52:51:  Number of observations ....: 1
2019-04-02T12:52:51:  Number of models ..........: 0
2019-04-02T12:52:51:  Number of observed events .: 22708
2019-04-02T12:52:51:  Number of predicted events : 0

From the 206048 events in the input event list 22708 events were selected.

Note

Event selection by ctselect in a given dimension can be avoided by specifying NONE (or INDEF, UNDEF or UNDEFINED) for one of the selection parameters. This is recommended rather than specifying the same selection previosuly applied to the data. The following run for example only performs a selection by time but no selection by ROI or energy:

$ ctselect
Input event list or observation definition XML file [events.fits]
Radius of ROI around pointing or specified RA/DEC (degrees) (0-180) [3.0] NONE
Start time (UTC string, JD, MJD or MET in seconds) [2020-01-01T00:10:00]
Stop time (UTC string, JD, MJD or MET in seconds) [2020-01-01T00:40:00]
Lower energy limit (TeV) [0.1] NONE
Output event list or observation definition XML file [selected_events.fits]