LETGS In-flight Performance Issues

last updated: 6 June 2000

Low Energy Transmission Grating

The LETG itself is functioning extremely well and as best we can tell at this stage, in accordance with pre-flight predictions.

High Resolution Camera Spectroscopic array (HRC-S)

There are two main performance issues for the HRC-S which are different to pre-flight predictions.

1/ Due to a problem in the detector electronics, the anti-coincidence shield is not vetoing particle events and the net detector background is significantly higher than pre-flight estimates. Position dependant PHA filtering can reduce background counts by ~ 50% with < 1% loss of X-ray photons. The particle radiation environment of Chandra in-flight is also considerably more active than expected, and the observed background count rate under times of "flaring" can rise to several times the quiescent rate.

Comparison of the approximate observed pre- and post-filtered background during times of quiescence with numbers published in the AO-1 Proposers' Guide is as follows:

AO-1 Prop. Guide Typical Observed Filtered Units
1.5e-6 7.0e-5 3.0e-5 count/s/arcsec^2

The higher in-flight background is likely not a problem for stronger sources at shorter wavelengths where the cross-dispersion profile is narrow (< 50AA or so), but will be increasingly important towards longer wavelengths as the cross-dispersion profile broadens out and covers more detector (~6 arcsec wide at 160 AA). In order to help you determine whether or not the background might have an impact on your observation, we have placed a sample observed background spectrum on the CXC LETG web page.

2/ The HRC-S QE was essentially uncalibrated at launch for wavelengths above 44 AA. The effective area for the LETG+HRC-S combination is therefore still uncertain at low energies. On-orbit calibration observations of Sirius B and HZ 43 have enabled us to refine the low energy HRC-S QE. The pre-launch data assumed for AO-1 overestimated this by a factor that is typically ~1.6, but which varies with wavelength. We have placed information concerning our revised estimates of the HRC-S QE on the LETG webpage. Over the long term, knowledge of the effective area is expected to improve as analysis proceeds and more data are acquired.

3/ Issues directly related to higher background:

- To avoid telemetry saturation (183 count/s for the HRC), the HRC-S is currently operated by default in a "windowed down" configuration, whereby only 6 of the 12 coarse taps in the center of the detector in the cross-dispersion direction are telemetered. This corresponds to an active detector area slightly less than 10mm, or 3.2 arcmin, across in cross-dispersion. This window easily accomodates the dithered spectra of point sources, but could be problematic for observations of extended sources.

- The current total quiescent count rate for the windowed-down HRC-S is about 85 count/s, down from 120 count/s at the beginning of the mission. However, this value rises to > 183 count/s (the telemetry saturation limit for the HRC) for significant fractions of some observations, especially at times close to the radiation belt passages. The telemetry saturation effectively means that both source and background events are "missed" (equivalent to a systematic lowering of the effective area). This can be corrected for using the deadtime coorection factor computed in level 1 processing. However, this deadtime correction is currently only believed to be accurate to a level of about 10%. It is not certain that this can be easily improved in the short term. Consequently, accurate relative and absolute photometry could be affected.


If you are observing an object for which you are expecting little or no flux at longer wavelengths and you are especially concerned with telemetry saturation issues, you might want to consider narrowing the default spectroscopic window to reduce the detector area that is telemetered in the dispersion direction. Please contact me if you think this might be appropriate for your observation.

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 LETG 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 LETGS 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 ameliorate the effects of CTI, LETG+ACIS-S observations of POINT SOURCES are therefore currently aimed at a detector location 8mm closer to the ACIS-S readout nodes using a SIM Z offset.

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 LETG+ACIS-S because the event pulse height filtering that can be applied for order sorting greatly reduces any background signal. However, higher than expected background might be relevant for observations performed in Timed Exposure/Very Faint (TE/VF) mode. Observers whose LETG+ACIS-S specify this mode might consider changing to TE/Faint mode.

3/ FEP0 problem. You do not need to consider this issue unless you have specific interest in data from the half of chip S0 that lies furthest from the framestore. Unless you specify otherwise, point sources will be observed at the opposite side of the chip using the SIM Z offset described in (1) and are not affected. You are therefore only likely to need to consider the FEP0 issue if you are observing an extended source. A brief explanation of the issue is provided below. If you are still in any doubt about this issue, please contact me with any questions you might have.

During the early phase of the mission, an intermittent problem was discovered with one of the Front End Processors (FEPs) which processes pixel data from the ACIS CCD's. Specifically, FEP0 would suddenly experience a corruption in the bias map from rows 512 to 1024 at an indeterminate point in the observation. This would lead to many spurious events being reported from the top half of that CCD for the remainder of the observation, flooding the telemetry stream. A patch has been developed and installed which detects when such an error has occurred and prevents data from the top half of the affected CCD from being telemetered from that point in the observation. By default this CCD is S0, but can be changed if there is a compelling scientific reason to do so.

4/ If any part of your observation will be affected by pileup, this 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 4% of events will pile for a flux of 0.1 event per readout frame. Pileup can affect both the shape of the PSF and the PH spectral energy distribution of your source. 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.

Please send questions or comments to:Jeremy Drake