The script TG_FINDZO is written in S-Lang and can be used from a command line in conjunction with CIAO software. Its syntax, despite the complexity of the algorithm, is quite simple:

        tg_findzo infile="input_event_file" grttype=grating_type [, flype=option_mode [, x0, y0]]

where input_event_file is the input event L1 (or L2) file, grating_type is ["h", "m", "l"] (either HEG, MEG or LEG, respectively). The derived coordinate value [X,Y] is both prompted on screen and stored in the CIAO parameter file tg_findzo.par by the script. For fltype, users have four choices:

  1. 1. (default) Use RA_NOM, DEC_NOM for the initial guess of ZO.
  2. 2. Set the initial guess of ZO as [median(X), median(Y)].
  3. 3. Use the dummy coodinate [4096.5, 4096.5] for ACIS-S.
  4. 4. Use a user-specified coordinate [x0, y0].

By default, the fltype is set to 1. For example, the script tg_findzo can be executed as simple as:

	tg_findzo acisf0001N0001_evt1.fits m

to locate the zeroth order position of the point source near RA_NOM and DEC_NOM for MEG. For HEG, users should replace ``m'' with ``h''. If users wish to specify the initial guess more explicitly, then execute the script as:

	tg_findzo acisf0001N0001_evt1.fits m 4 4090.2, 4061.7

In this case, the algorithm will look for the source near the SKY coordinate [X0, Y0] = [4090.2, 4061.72]. This option may be useful when the specified nominal coordinate position differs significantly from the actual location of the point source. At the end of each run, the result of fitting is stored in a postscript file named in the working directory. It is possible to control the plotting behavior through tg_findzo.par. See the ahelp file for tg_findzo for more.

The use of simple CFITSIO extended file name syntax (filtering) is also allowed by fitsio S-lang module. For example, if the user wishes to filter events with the energy range of 1000 to 6000 eV:

	tg_findzo infile="acisf0001N0001_evt1.fits[energy=1000:6000]" m

Note that the extension block name ([EVENTS]) is set by default and there is no need to type in the extension name.

The derived zeroth order coordinate position [X, Y] is then used to create a region mask using the CIAO tool tg_create_mask. For example, to create a region mask for a user-specified coordinate position [X,Y] = [4090.23,4061.72], users need to set its parameters as follows:

	pset tg_create_mask use_user_pars=yes last_source_toread=A
	pset tg_create_mask sA_id=1 sA_zero_x=4090.23 sA_zero_y=4061.72
	pset tg_create_mask sA_zero_rad=20 sA_width_heg=15 sA_width_meg=20

The width and radius parameters are specified here arbitrarily. Users should consult the CIAO/tg_create_mask documentation for details.

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This page was last updated Sep 20, 2011 by David P. Huenemoerder. To comment on it or the material presented here, send email to
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