Figure vsg.1: Time line of Flight and Vacuum Storage Gratings Activities
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The grating properties fall into Period properties and Efficiency properties. These have now been monitored over a two year baseline.
All grating periods and period variations appear stable (periods within 20 ppm and dp/p within 20%); the only unexplained exception is grating HA2003 which varied by -110 ppm but has returned to its original period.
The HEG efficiencies appear stable or reasons have been found for the several anomalies seen.
The MEG efficiencies continue to show a drift with time. At 4 keV at least the three of six gratings show a trend towards lower efficiency with time at a rate of roughly -3% per year. At 1 keV, the MEGs show an increase in efficiency with time at a rate of about +4% per year.
Investigating the "MEG drift with time" has lead to clear evidence that the X-GEF system detectors are being very slowly coated with a contamination layer, changing their detection efficiency properties. The detector "contamination rate" is ~ 1 micron of polyimide-equivalent per year for the SSD, ~ 0.4 microns/year for the PSPC and negligible accumulation on the SMD. Analysis and re-analysis of data-in-hand is on-going to assess the effects on the VSG X-GEF measurements. Future testing will monitor and decontaminate the SSD and PSPC windows.
Finally, while the details and effects of the detector changes are under study, we consider the possibility that the observed MEG efficiency drift is a real sample change. Assuming as a worst case that all of the drift seen is due to real sample changes, the MEG 4 keV efficiency will remain above the Level 1 requirement for over 10 years and the 1 keV efficiency is actually improving with time!
Appendix A. X-GEF Contamination and Effects
Version 1.0, June 1998 is available at http://space.mit.edu/HETG/vsg9806/vsg.html
Selected gratings were put into vacuum storage and their properties are being periodically measured. This vacuum storage grating (VSG) program will allow us to detect or set upper limits on changes in the grating properties with time. The properties being measured for each sample are the period and period variation ( "LR Measurements") and X-ray diffraction efficiency ( "X-GEF Measurements"). Familiarity with these quantities and test setups is assumed in the following.
The figure below shows a time line and events relevant to the HETG flight and VSG gratings. The flight gratings were X-GEF tested in the laboratory as they were produced between mid-1995 and September 1996 (date codes 950615 to 960914). Due to LR system upgrades and the relative ease of the test, all but a handful of the flight gratings were LR (re-)tested in the April to September 1996 time frame. These detailed, facet-by-facet laboratory data sets provide the basis of the instrument model and calibration.
The HETG was next tested as a full system at XRCF from December 1996 through April 1997. These system-level measurements represent an independent set of data to complement the laboratory measurements. The result of analyzing and synthesizing these two data sets will yield the final pre-flight calibration of the HETG and HETGS (HRMA-HETG-ACIS-S).
As the time line indicates, flight data will not be taken until more than two years after the XRCF calibration measurements were made! Thus, it is important to estimate the degree to which the HETG properties might change with time - especially during the period from XRCF to flight first-light. In order to accomplish this, a total of five vacuum storage retest cycles are planned to cover the XRCF to flight first-light time period, as indicated. We plan to continue the tests at six (TBR) month intervals throughout flight operations.
Figure vsg.1: Time line of Flight and Vacuum Storage Gratings Activities
Currently four retests have been carried out, indicated by the vertical lines in the diagram above with date codes 9611, 9707, 9801, and 9810. Ideally plots of the measured facet properties with time ("trend plots") would show unchanging horizontal lines; variations are, however, visible. The following sections summarize the data and list significant variations that are seen. In most cases possible explanations for the variations have been identified and traced to a "measurement problem" rather than a true sample variation. In these cases a "problem code" (e.g., LRmp1) is given along with relevant remarks; details and explanations of the measurement problem codes are given in the LR Measurement Notes and X-GEF Measurement Notes sections below.
Anomalies seen in these plots are listed and described here:
The individual facet LR data can be combined to produce average period and the dp/p values for MEG and HEG grating sets as is done for the flight HETG, using the "active region" of the facets to approximate the flight-illuminated region on the facets. The four gratings with non-flight-like periods (HD2334, HA2045, MC1212, and ME1408) have not been included.
Vacuum Storage LR Results: MEG Ensemble
Test Date : 9611xx 9707xx 9801xx 9810xx
P_ave : 4000.36 [-68ppm] [-11ppm] [-30ppm]
dp/p : 192.07 [-22%] [-18%] [+19%]
Vacuum Storage LR Results: HEG Ensemble
Test Date : 9611xx 9707xx 9801xx 9810xx
P_ave : 2001.95 [-70ppm] [-37ppm] [-8ppm]
dp/p : 229.48 [-14%] [+5%] [-2%]
In tracking down the sources for these drifting efficiencies, it was discovered that the SSD and PSPC windows are very slowly coated with a thin, nonuniform layer of hydrocarbon from the X-GEF system, see X-GEFmp3 in the "X-GEF Measurement Notes" section below and Appendix A.
The common trend among samples also suggests the possibility of "reference variations" - that the X-GEF MEG reference grating's properties are changing with time, possibly due to the same contamination that is seen on the detector windows, X-GEFrv1. These possibilities are under investigation, see the "X-GEF Measurement Notes" section below and Appendix A.
Finally, while the details and effects of the above are under study, we must consider the possibility that the observed drift is a real sample change. Assuming as a worst case that all of the drift seen is due to real sample changes, the MEG 4 keV efficiency will remain above the Level 1 requirement for over 10 years and the 1 keV efficiency is actually improving with time!
General information on the 14 grating facets which make up the Vacuum Storage Gratings are given in the tables below. As Table vsg.4 indicates, the VSGs come from a subset of flight batches that together yielded 180 of the full 336 flight gratings. The Period and Diffraction properties of the flight gratings and VSGs are compared in Figures vsg.5 and vsg.6 - showing that the VSGs cover a wide range of flight grating types.
Specific information and notes regarding each grating especially as it effects test results are collected here, organized by the facet name.
9511 9611 9707 9801
'p4 0 % 20 % 33 % 42 % <-- blemish region
'p0 0 % 2.8% 0.0% 3.6%
'm4 0 % 4.3% -0.2% 0.5%
When a sample is re-measured there are several general categories of effects that can be responsible for a change in the measured quantity - these are discussed in general as well as with respect to the LR and X-GEF setups themselves.
A variety of measurement problems can be caught by reviewing the
LR data sheets:
IDL> gratview, 'MA1019.981005125313.dat' IDL> plot, lrd_data.id, PSYM=4 ; diffracted beam IDL> plot, lrd_data.ir, PSYM=4 ; reflected beam
LR reference variation, that is a change in the period of the LR reference samples built into the LR system , is unlikely. However illumination of different regions of the samples may create apparent changes - this could be investigated further. The general stability of the LR system can be monitored with the Reference Stability data and plots - however, changes in these plots (not shown) do not immediately translate into sample period changes.
LR sample variations may show up as clear changes in the dp/p pattern or clear changes in the frame flatness (LR surface plot.)
Other ideas and actions for improved LR testing are noted here:
Identified sources of X-GEF measurement changes are listed here:
The absolute zero-order efficiency of these gratings can be directly measured by looking at the ratio of the zero-order beam with the gratings in place to the direct X-ray beam. This is most accurately carried out with the SSD because of its superior energy resolution. Plots of these SSD-measured zero-order efficiencies as a function of time are shown in Figures vsg.7 and vsg.8; note that any effects of X-GEFmp3 have not yet been included in these analyses.
The following is a summary of the tasks that need to be carried out in order to perform a VSG retest. The retest period generally refers to the period during which X-GEF measurements are made - for the 14 VSGs this is about two weeks. Note, however, that there are preliminary and follow up activities as well.
The analysis and presentation of the VSG measurement results is carried out by the IDL routine vsg_info.pro (in ~dd/idl/meta/). This routine has multiple sections starting with "if 0 EQ 1 then begin" which may be changed to "if 1 EQ 1 then begin" to produce that section's product. In this way a large variety of VSG-related analyses are assembled in one location.
This web page is: http://space.mit.edu/HETG/vsg/vsg.html.
Please send any comments and updates to: dd@space.mit.edu.