Measure Grating Dispersion Distance

Important note for multiobi users

If you are working with a multiobi dataset (not sure? Read the why topic), the ROLL_NOM value in the merged evt2 file may not produce truly accurate results from this script. For the most reliable grating scale, run tg_scale_reg.sl on each evt1a dataset. It is also possible to use the command line method, substituting the ROLL_PNT header keyword value where ROLL_NOM is used in the example.

Download the script

This thread uses the tg_scale_reg and tg_scale_reg.sl scripts (tg_scale_reg is an slsh wrapper that allows tg_scale_reg.sl to be run from the command line). The most recent version of these scripts is v1.3 (04 August 2003) and v1.4 (12 December 2005), respectively:

unix% grep Id `which tg_scale_reg`
% $Id: tg_scale_reg,v 1.3 2003/08/04 16:56:42 dburke Exp $

unix% grep Version $ASCDS_CONTRIB/share/slsh/local-packages/tg_scale_reg.sl
% Version: 1.4 (12 December 2005)

Note that $ASCDS_CONTRIB/share/slsh/local-packages/ is the default path in the standard CIAO scripts installation; see the Scripts page for more information. Please check that you are using the most recent version before continuing. If you do not have the scripts installed or need to update to a newer version, please refer to the Scripts page.

Optional: Using the script in Sherpa

The script is written with general S-Lang commands and may be run on the command line - as shown in this thread - or in either Sherpa or ISIS.

The function can be made available to Sherpa using the evalfile command:

sherpa> evalfile("tg_scale_reg.sl")
1

Executing the script without any options will print a usage message. For information on using scripts in Sherpa see the Sherpa and Scripts and Customizing Sherpa with a Resource File threads.

FITS region file as input

The REGION block appended to acisf01010N001_evt2.fits is used to create the output ruler file. The default scale and values are used.

unix% punlearn tg_scale_reg
unix% pset tg_scale_reg infile="acisf01010N001_evt2.fits"
unix% pset tg_scale_reg outfile="wave_1010.reg"
unix% tg_scale_reg 
Name of FITS file with obs info (or blank) (acisf01010N001_evt2.fits): 
Name of output file (wave_1010.reg): 

        ** Found 1 source.

        Output region file "wave_1010.reg" has been created

To view the region file, open the event list in ds9:

unix% ds9 acisf01010N001_evt2.fits &

The extraction regions should be loaded automatically; if they aren't, use "Region-> Load Regions-> acisf01010N001_evt2.fits[REGION]" to do so.

To add the scale, use the "Region -> Load Regions..." menu to select the file. Figure 1  shows the finished image, binned to "block 4."

Supplying the parameter values

There are several pieces of information that need to be obtained before using this method. As mentioned in the Basic Script Syntax section, we need the source position, rotation angle, grating arm, and instrument.

• source position: the simplest way to find the sky coordinates of the source is to examine the event file in ds9. Figure 2  shows a cross marking the approximate center of the source; double-clicking on the region brings up the region information box from which the position may be read.

  The source position is (x,y) = (4101,4089).

• rotation angle: the roll angle for the observation is recorded in the ROLL_NOM header keyword. If you are working with a multiobi observation you should use the ROLL_PNT value instead, as explained in the important note for multiobi users.

  unix% dmkeypar acisf01010N001_evt2.fits ROLL_NOM echo+
  262.65257360233
  

  The rotation angle is 262.65257.

• grating arm: although most users will know by this point what transmission grating was used, it is also stored in the GRATING header keyword:

  unix% dmkeypar acisf01010N001_evt2.fits GRATING echo+
  HETG
  

  For this example, we choose to use only the MEG arm of HETG.

• instrument: if you are unsure which instrument was used for your observation, check the INSTRUME keyword:

  unix% dmkeypar acisf01010N001_evt2.fits INSTRUME echo+
  ACIS
  

  The instrument is ACIS, as indicated by the filename as well (acisf01010N001_evt2.fits).

Now that we have all the information, we can run tg_scale_reg; we let the script prompt for the necessary parameters:

unix% punlearn tg_scale_reg
unix% pset tg_scale_reg scale=kev
unix% tg_scale_reg 
Name of FITS file with obs info (or blank) (): 
Name of output file (): meg_en_1010.reg
x source pos in sky coords (for infile='') (): 4101
y source pos in sky coords (for infile='') (): 4089
Roll angle of obs (for infile='') (): 262.65257
Grating arm (for infile='') (leg|getg|heg|meg) (leg): meg
Instrument used (for infile='') (acis|hrc) (acis): 

        Output region file "meg_en_1010.reg" has been created

Recall that setting scale=kev forces the ruler scale to energy instead of wavelength.

We can use the ds9 command-line syntax to load the region file on top of the image:

unix% ds9 acisf01010N001_evt2.fits -region meg_en_1010.reg &

Figure 3  shows the scale overlaid on the image.