/ ciao4.16 / dictionary / virtualcolumns.html
This page is for the obsolete CIAO 4.16 release. Please see the CIAO home page for the latest release.
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)
/ ciao4.16 / dictionary / virtualcolumns.html