Chandra X-Ray Observatory
	(CXC)
Skip to the navigation links
Last modified: 13 March 2017

Virtual Columns


Virtual columns - such as RA and DEC - do not physically exist in the event file; they are defined by the WCS information attached to another column, e.g. SKY.

For example:

unix% dmlist acisf01838N002_evt2.fits"[cols sky]" cols
 
--------------------------------------------------------------------------------
Columns for Table Block EVENTS
--------------------------------------------------------------------------------
 
ColNo  Name                 Unit        Type             Range
   1   sky(x,y)             pixel        Real4          0.50:     8192.50    sky coordinates
 
--------------------------------------------------------------------------------
World Coord Transforms for Columns in Table Block EVENTS
--------------------------------------------------------------------------------
 
ColNo    Name
1:    EQPOS(RA ) = (+278.3860) +TAN[(-0.000136667)* (sky(x)-(+4096.50))]
           (DEC)   (-10.5899 )      (+0.000136667)  (   (y) (+4096.50)) 

There clearly are columns named "x" and "y." Due to the presence of the "World Coord Transforms" information, you can access the virtual columns RA and DEC as well:

unix% dmlist acisf01838N002_evt2.fits"[cols ra,dec][#row=1:5]" data
 
--------------------------------------------------------------------------------
Data for Table Block EVENTS
--------------------------------------------------------------------------------
 
ROW    EQPOS(RA,DEC)
 
     1 (      277.9936762709,      -10.6687399176)
     2 (      278.0801911056,      -10.6038671997)
     3 (      278.4390511376,      -10.5561325327)
     4 (      278.4338783089,      -10.5913628326)
     5 (      278.4249249501,      -10.5603907662)


Last modified: 13 March 2017
Smithsonian Institute Smithsonian Institute

The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.   Email:   cxchelp@head.cfa.harvard.edu Smithsonian Institution, Copyright © 1998-2017. All rights reserved.