As next analysis step you will bin the data in a counts cube using ctbin. A counts cube is a 3 dimensional data cube, spanned by Right Ascension (or Galactic longitude), Declination (or Galactic latitude), and energy (typically logarithmically spaced, but this is under user control).
ctbin is executed by typing:
$ ctbin
Input event list or observation definition XML file [events.fits]
First coordinate of image center in degrees (RA or galactic l) (0-360) [83.63]
Second coordinate of image center in degrees (DEC or galactic b) (-90-90) [22.01]
Projection method (AIT|AZP|CAR|MER|MOL|STG|TAN) [CAR]
Coordinate system (CEL - celestial, GAL - galactic) (CEL|GAL) [CEL]
Image scale (in degrees/pixel) [0.02]
Size of the X axis in pixels [200]
Size of the Y axis in pixels [200]
Algorithm for defining energy bins (FILE|LIN|LOG) [LOG]
Start value for first energy bin in TeV [0.1]
Stop value for last energy bin in TeV [100.0]
Number of energy bins [20]
Output counts cube file [cntcube.fits]
The counts cube will be centred on the location of the Crab (Right Ascension 83.63 degrees, Declination 22.01 degrees) and will be aligned in celestial coordinates. A cartesian projection has been selected. The counts cube has 200 x 200 spatial pixels of 0.02 x 0.02 degrees in size, hence it covers a total area of 4 x 4 degrees.
The counts cube will contain 20 maps, which are logarithmically spaced in energy, and which cover the energy range from 0.1 TeV to 100 TeV. In this example, the counts cube will be saved as cntcube.fits in the working directory. In addition to the counts cube, that is stored as the primary image extension, the FITS file also contains an extension named EBOUNDS that defines the energy boundaries that were used, and an extension GTI that defines the Good Time Intervals that have been used. The following image shows the resulting FITS file. The EBOUNDS table has 20 rows, one for each energy bin, while the GTI table has just a single row, indicating the start and stop time of the simulated data.
Counts cube FITS file
An image of the first bin, covering the energy range 100 - 141 GeV, is shown below:
Counts cube for first energy bin
For illustration, the last few lines of the log file ctbin.log are reproduced below:
2015-12-07T20:52:41: +=================+
2015-12-07T20:52:41: | Bin observation |
2015-12-07T20:52:41: +=================+
2015-12-07T20:52:41: === CTA observation ===
2015-12-07T20:52:41: Events in list ............: 22685
2015-12-07T20:52:41: Events in cube ............: 18195
2015-12-07T20:52:41: Event bins outside RoI ....: 0
2015-12-07T20:52:41: Events outside cube area ..: 4490
2015-12-07T20:52:41: Events outside energy bins : 0
From the 22685 events that have been simulated and stored in the events.fits file, 18195 lie within the cube boundaries and are thus put into the resulting counts cube. The counts cube is stored in a cartesian projection in a World Coordinate System (WCS) compliant format.