Example 1: Blocked Zeroth Order Image
The S-Lang script tg_findzo is quite useful when the zeroth order image is completely blocked out and cannot use any conventional means to find the reference position for zero wavelength. In the case of ObsID 660 (target: GRS1915+105), we have:
unix$ tg_findzo acisf00660_002N003_evt2.fits m Running TG_FINDZO version 0.9.1 Assuming that the RA_TARG and DEC_TARG values are correct for this particular target. Derived zeroth order position: X0 = 4093.65 Y0 = 4063.48
which derives the centroid position of [X0, Y0] = [4093.65, 4063.48] (see Figure 1).
Since this particular dataset is quite large, the user may wish to
use only the first 100,000 frames to derive the centroid coordinate,
instead. A simple
unix$ tg_findzo "acisf00660_002N003_evt2.fits[EXPNO=1:100000]" m Running TG_FINDZO version 0.9.1 The extension block name is always set to be [EVENTS] by default. Assuming that the RA_TARG and DEC_TARG values are correct for this particular target. Derived zeroth order position: X0 = 4093.65 Y0 = 4063.48
Figure 1: Example #1. Blocked Zeroth Order Image (ObsID 660). The red line traces the data streak; the green line traces the grating arm (MEG). The intersection of these two lines coincides with the zeroth order position for this datum.
Example 2: Severely Piled Zeroth Order Image
When the source is extremely bright (or even moderately bright, e.g., Capella),
the pile-up becomes so severe that the central core of the PSF is often
burned out. The existing CIAO detection tool
In the case of ObsID 3814 (target: Cyg X-1), the core of the PSF is "burned out" so that the source appears as a crater. The script tg_findzo, however, has no problem finding its centroid position since the algorithm does not utilize the zeroth order image itself (see Figure 2). Suppose the user knows roughly where the zeroth order position should be and refine the position by using tg_findzo, it can be achieved as:
unix$ tg_findzo acisf03814N001_evt2.fits h 4 4099.5, 4025.5 Running TG_FINDZO version 0.9.1 User specified values will be used Derived zeroth order position: X0 = 4099.53 Y0 = 4025.7
Figure 2: Example #2. Severely Piled Zeroth Order Image (ObsID 3814). The color code is the same as in Figure 1 (except for the use of HEG in this case).
CaveatTwo independent testings (with MARX and Capella datasets) show that the accuracy of the algorithm in tg_findzo is good to about 0.1 ACIS CCD pixels.
Figure 3 (derived from Capella datasets) illustrates that, by using tg_findzo, the errors (scatter) in the offset values in line wavelength between plus and minus 1st orders of HEG and MEG are as small as and , respectively. The scale of the errors in wavelength corresponds to 0.10 and 0.0.9 ACIS CCD pixels. For more details, see the in-depth description of tg_findzo.
Figure 3: The mean differences in the line centroid position of selected bright
emission lines between plus and minus order spectra as a function of
This page was last updated Sep 20, 2011 by David P. Huenemoerder. To comment on it or the material presented here, send email to firstname.lastname@example.org.