|HETG on Orbit||
This page contains occasional HETG Status summaries.
|"..., Happy Birthday dear HETGS, Happy Birthday to you!"|
On August 28 the HETG was retracted and completed its first year of observations.
The HETG ended 1999 having successfully
completed observations of ~25 different astronomical targets.
Performance has remained as expected and no significant
issues are outstanding. A summary of instrument status
was presented to the Chandra SWG via telecon on Jan.7 2000.
Status Notes, presented to SWG Telecon, Jan.7 2000
The High Energy Transmission Grating Spectrometer, HETGS, is the integrated combination of the Chandra HRMA, HETG, ACIS-S, aspect system, and ground processing; the HETGS performance thus depends on all of these components. To date there are no indications that any changes are needed to the characterization (calibration data) of the HETG per se. Performance of the HETGS is summarized in the two sections below.
Resolution related The starting point for the HETGS performance from a spectral resolution point of view is determined primarilly by the imagining performance of the non-HETG components and can be monitored by the quality of the HETGS zeroth-order images. For point sources, as-measured FWHM of the 1D projection of the zeroth-order along ~TDETX axis shows consistent values of 1.3 to 1.8 ACIS pixels. The core and wings of the zeroth-order 1D projection are very close to expectations, e.g., MARX simulations.
The spectral resolving power achieved, "E/dE", falls in the expected range of pre-launch expectations and appears to be constant with time.
The ACIS detector is providing geometrically accurate event locations as expected. The size of the gaps between the ACIS chips has been adjusted post-launch based on flight data and currently are accurate to ~0.3 pixels. Absolute wavelength accuracy is currently better than 0.02 (0.01) A for MEG (HEG) and will be studied in more detail.
Effective Area related Some ACIS effects which can influence the HETGS effective area have come to light, otherwise there are no clearly required changes to the HRMA and HETG area/efficiency calibration products to date.
CTI effects in the ACIS FI devices still allow them to perform spectral order sorting. Small QE effects due to CTI-induced grade migration (< ~10%) may have to be included in the HETGS effective area and will be investigated. CTI effect is reduced soomewhat by observing with the ACIS-S offset in SIM Z axis by a small amount to move the "X" pattern closer to the readout nodes.
- 991202 SIM-Z Default Values to minimize CTI effect
- Recommended SIM-Z offset values for HETG observations.
Pileup in the dispersed spectra has been seen and can be a large effect for bright lines or sources. The pileup level in an observation should be evaluated to see if effects are important.
- 991230 Pileup in HETGS Dispersed Spectra
- Qualitative pileup levels seen in HETGS calibration observations.
The "FEP 0" problem showed up around Oct. 10 and in two HETG GTO targets caused the data from S3 to be scientifically useless. Steps were taken to associate FEP 0 with S0 and only run S1-S5 in the S-array. A patch to provide '5-and-a-half-plus' mode ACIS operation is process of being tested and will restore the 6-CCD ACIS-S operation and catch and contain a FEP 0 problem if it occurs.
The ACIS QE calibration data currently used is spaced in 10 eV steps in the Si-K region (1.8 keV) - this is too coarse for some HETGS analyses, e.g., the Si He-like triplet lines. The ACIS team has synchrotron data which will allow much finer detail of the instrumental Si-K edge region and will get in into the calibration products in future.
No "background flares" are seen in the ACIS-S BI CCDs when the HETG is in place (whereas they are seen with LETG in place.)
HETG performance has been "nominal" with no hints of performance differing from ground calibration expectations. The HRMA-HETG-ACIS-S system is achieving expected resolving power, e.g., E/dE values for as-measured lines from Capella are over 700 for the MEG at 0.653 keV, and over 1100 for the HEG at 0.825 keV.
HETG has recently observed its 3rd GTO target, E0102-72 (September 28-30), and has several more GTO observations scheduled for the weeks ahead. Calibration observations of Capella, HR1099, the Crab, and Cygnus X-2 have been carried out; only two of the initial calibration observations remain: Mrk 421, to be observed soon, and 3C 273, planned for Jan.'00.
The "degraded" ACIS frontside CCDs have energy resolution adequate for use in order sorting.
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HETG Web page:
HETG sucessfully observed SS 433 (GTO target), Cygnus X-2, and Capella on September 23-25. (Sept.27, 1999)
HETG is observing HR1099 for 200 ks as part of the Emission Line Project. HETG Observations currently planned for the next week or so include HETG GTO observation of SS 433, Calibration observation of Cygus X-2, and a further 70 ks of Capella exposure for Emission Line Project. (Sept.15, 1999)
Initial HETG activation and checkout observations were completed on 8/29. The data from Capella and the Crab Nebula are being processed and assessed. HETG first-light spectra of Capella look great! In a week or two HETG will observe Capella again, Cyg X-2, and HR1099. (Aug.30, 1999)
The HETG was VERY sucessfully inserted at 1:35 am, August 28, 1999 !
The Capella focus set is in process (8/28 - 8/29). Stay tuned for data.
This web page is: http://space.mit.edu/HETG/flight/status.html.
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