Issues relevant to planning HETGS Observations

Most observation planning issues relate to setting ACIS-S parameters in order to make the best of HETGS observations. An example of a commonly used set of parameters can be examined by checking out the obscat entry for observation ID 1802. The source is point like, has flux down to 0.8 keV in the HEG, so all ACIS-S chips are on, the SIM has been shifted to reduce CTI, and a custom subarray is used to reduce the frame exposure time to 2.5 s. For more about using a subarray with the HETGS, see HETGS Subarray web page. See the HETGS Calibration web page for calibration updates and current HETGS calibration issues.

For some details on observation scheduling, pre-observation planning, and target parameter explanations, see the observation planning message that is sent to HETGS observers. Consult the contact scientist to make any changes. See the AO7 contact scientist list to find out if your contact scientist is Norbert Schulz (nss@space.mit.edu, 617-258-5767) or Herman Marshall (hermanm@space.mit.edu, 617-253-8573).

If the CC mode is used when observing a point source with the HETGS, then there is no need to allow room near the readout for a spatial background region because columnar spatial information is not recoverable. Thus, one may reduce the effects of ACIS-S CTI by offsetting the zeroth order closer to the readout. Allowing 16 rows for dithering and 20 pixels for up to 10" of acquisition error, a shift of -4.7 mm is feasible and neither the MEG nor the HEG spectra will end up off of the array.

Advanced CCD Imaging Spectrograph Spectroscopic array (ACIS-S)

1) Radiation damage in-flight and attendent charge transfer inefficiency (CTI) has significantly degraded the energy resolution of the ACIS front-illuminated CCD's. The energy resolution of ACIS-S is important for HETG observations because it allows discrimination between spectral orders that are spatially overlapping on the detector. In order to be able to sort spectral orders a fairly low resolution is adequate and the detector CTI is not thought to be a serious problem from the standpoint of HETGS observations. However, it does render data more difficult to analyse because orders are not so sharply defined, and it also prevents removal of some background that could otherwise be excised through a tighter pulse height filter. CTI decreases strongly toward the CCD readout node; in order to reduce the effects of CTI, we suggest that HETGS observations of POINT SOURCES be aimed at a detector location 3mm closer to the ACIS-S readout nodes using a SIM Z offset of -3 mm.  One may shift the spectrum closer to the readout than -3 mm at the expense of placing the low energy portions of the MEG -1 order and HEG +1 order closer to the readout and possibly off the detector altogether.  Larger shifts may well be appropriate for sources with large ISM absorption, for which few counts are expected at low energies.

2) The BI chips (S1 and S3) may be affected by particle background flares with count rates many times the quiescent rate. These flares were not anticipated prior to launch. In early data, the background rate in the BI devices (after the standard event screening) was >2 times the quiescent rate during roughly 30% of the time; this fraction is considerably lower for the FI chips. The quiescent background rate, after the standard event screening, is larger by a factor of 1.5-3 for the BI devices compared to the FI devices. Background is not generally a problem for the HETGS because the event pulse height filtering that can be applied for order sorting greatly reduces any background signal. Furthermore, background flares have not generally been observed during HETGS observations, perhaps because the polyimide grating bases block the low energy particles responsible for the increased background.

3) If any part of your observation will be affected by pileup, it can be mitigated for observations that do not require the whole detector area in the cross-dispersion direction by using a subarray. Pileup is most likely to affect 0th order, but for bright sources can also affect dispersed photons. As an example, roughly 50% of events will pile for a flux of 0.1 event per readout frame per dispersion element (one ACIS pixel). Pileup can affect both the shape of the PSF and the PH spectral energy distribution of your source and, due to grade migration, some events will be lost, mimicing a QE loss. A subarray reduces pileup because a smaller region of the detector is read out and frame times are comensurately shorter. If you are not observing a fairly extended source and have not yet chosen to use a subarray you might want to consider this option. For more about using a subarray with the HETGS, see HETGS Subarray web page.

4) If the 0th order spectrum is valuable, then there are two steps one may take to make the data somewhat easier to analyze. Because the current aimpoint is very close to the boundary of nodes 0 and 1 of S3, the user may offset the telescope pointing direction by about 20" (0.33 arcmin) along the Y axis. If the source is a point source and the > 7 keV region may be important, then an offset of +0.33 arcmin is recommended. If the source has extended emission that may fall into the S2-S3 gap, then the user may wish to offset by -0.33 arcmin. Another step the user may take is to use a smaller subarray, especially if the source is point-like and the -3mm SIM shift is to be applied. In this case, there are at least 6 mm of ACIS-S rows that may not be needed (unless the user desires serendipitous sources). The reduced array would be 1024 - 250 = 774 rows and start at row 1. The readout time is reduced to about 2.5 s from 3.2 s. For more about using a subarray with the HETGS, see HETGS Subarray web page. See other options in the Proposers' Observatory Guide, section 6.16.3 .


Contact Herman L. Marshall (hermanm@space.mit.edu) for further information. 

Last modified: 11/22/05