Generating a sky map from the events

Once the observations are selected, the first thing that you want to do is to visualize the content of the observations by generating a sky map from the events. You do this with the :ref:`ctskymap` tool by typing:

$ ctskymap
Input event list or observation definition XML file [events.fits] obs.xml
First coordinate of image center in degrees (RA or galactic l) (0-360) [83.63] 0.0
Second coordinate of image center in degrees (DEC or galactic b) (-90-90) [22.01] 0.0
Projection method (AIT|AZP|CAR|MER|MOL|STG|TAN) [CAR]
Coordinate system (CEL - celestial, GAL - galactic) (CEL|GAL) [CEL] GAL
Image scale (in degrees/pixel) [0.02]
Size of the X axis in pixels [200] 500
Size of the Y axis in pixels [200] 250
Lower energy limit (TeV) [0.1]
Upper energy limit (TeV) [100.0]
Background subtraction method (NONE|IRF) [NONE]
Output skymap file [skymap.fits]

We generated here a sky map centred on the Galactic Centre in Galactic coordinates using a cartesian projection. The sky map is 10° wide and 5° high, with an image scale of 0.02° per pixel. All events between 0.1 and 100 TeV were collected in the skymap. The :ref:`ctskymap` tool wrote the sky map into the FITS file skymap.fits that was created in the my_first_analysis folder. The FITS file can be displayed using for example ds9 (see below):

The sky map shows a wide-spread distribution of events with a number of sources superimposed. Most of the events are due to irreducable background that hampers the recognition of the gamma-ray sources. To describe the irreducable background in the CTA data, templates of the background event distribution are shipped together with the :ref:`Instrument Response Functions <glossary_irf>`. These template can be used by :ref:`ctskymap` to subtract the background contribution. To enable the background subtraction you should re-run :ref:`ctskymap` with the background subtraction method set to IRF as follows:

$ ctskymap
Input event list or observation definition XML file [events.fits] obs.xml
First coordinate of image center in degrees (RA or galactic l) (0-360) [83.63] 0.0
Second coordinate of image center in degrees (DEC or galactic b) (-90-90) [22.01] 0.0
Projection method (AIT|AZP|CAR|MER|MOL|STG|TAN) [CAR]
Coordinate system (CEL - celestial, GAL - galactic) (CEL|GAL) [CEL] GAL
Image scale (in degrees/pixel) [0.02]
Size of the X axis in pixels [200] 500
Size of the Y axis in pixels [200] 250
Lower energy limit (TeV) [0.1]
Upper energy limit (TeV) [100.0]
Background subtraction method (NONE|IRF) [NONE] IRF
Output skymap file [skymap.fits]

The figure below shows the resulting sky map. The irreducable background has been subtracted and the sources of gamma-ray emission are now clearly descernable.