[Last Change: 15 Feb 2012 (rev 6)]

Specifying Source Positions

Source positions may be specified in a number of different ways, but they must ultimately resolve into a position relative to the Chandra optical axis. The standard way of passing source positions to the raytrace is via the position position argument to an extent function. The position argument takes a Lua table as its value.

Mirror Spherical Coordinates

The Mirror Spherical Coordinate system is a polar coordinate system (θ, φ) fixed to the telescope reference frame. It is centered on the optical axis.

Coordinates for an object in an existing Chandra observation are most easily obtained by using the CIAO dmcoords command to determine the theta and phi coordinate values.

As an example, here's a close-up of an HRC-I observation of NGC 2516 (OBSID 27). I've decided to simulate the indicated object and use ds9 to determine its SKY coordinates.

dmcoords provides the MSC theta (in arcminutes) and phi (in degrees) coordinates:

% dmcoords infile=/data/hrc/i/00027/analysis/hrcf00027_evt2.fits.gz \
           asolfile=/data/hrc/i/00027/primary/pcadf066989205N003_asol1.fits.gz \
           x=17560 \
           y=15729 \
           option=sky
% pget dmcoords theta phi
2.956520430906318
302.0712805942328

The position is provided to SAOTrace as a Lua table given as the position argument to an extent function:

point{ position = { theta = 2.956520430906318, phi = 302.0712805942328 }, ... }

SAOTrace defaults to units of arcminutes for θ and degrees for φ. To specify other units, see here.

Celestial Coordinates

Source positions may also be specified in equatorial celestial coordinates. This is more involved than specifying positions in Mirror Spherical Coordinates, as the orientation of the telescope with respect to the celestial coordinate frame is required.

The following information is required:

1. The source position.
2. The optical axis position. For an existing observation this is available from
   • the RA_NOM and DEC_NOM keywords in the header of an event file; or
   • via the CIAO dmcoords command, by determining the center of the field in celestial coordinates.
3. The telescope roll. This is available for an existing observation from
   • the ROLL_NOM keyword in the header of an event file

In the above example using NGC2516, the object has

RA	= 7:57:47.348
Dec	= -60:47:15.46
RA_NOM	= 1.1953549452648E+02
DEC_NOM	= -6.0763660833014E+01
ROLL_NOM	= 3.3121684998055E+02

The position is provided to SAOTrace as a Lua table given as the position argument to an extent function:

point{
    position = { ra = '7:57:47.348',
                 dec = '-60:47:15.46',
                 ra_aimpt = 1.1953549452648E+02,
                 dec_aimpt = -6.0763660833014E+01
               }, ... }

Note that the RA or Dec may be provided either in sexagesimal notation (as strings) or as floating point numbers (default units are degrees). To specify other units, see here.

The telescope roll is specified separately using the roll() function, which takes as argument the roll in degrees.

roll( 3.3121684998055E+02 )

The default unit is degrees. For a complete example of this simulation, see How To Raytrace ASingle Point Source.