HETG Vacuum Storage Gratings

HETG Vacuum Storage Gratings, Version 1.0, June 1998

http://space.mit.edu/HETG/vsg9806/vsg.html go to more HETG Calibration
last modified 6/11/98 dd

Busy-person Summary

A sample of HETG gratings has been aged in a high vacuum environment and retested periodically since late 1996 to determine if there are any changes in the performance parameters. The evaluations in early 1998 indicate that there are no trends that would change the calibration of the HETG instrument. Several changes in performance parameters have been detected but most seem to be caused by measurement or sample anomolies. Aging and evaluations of these gratings will continue through this year and updates will be made as testing is completed.



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 is a 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 almost two years after the XRCF calibration measurements were made! Thus, it is particularly important to estimate the degree to which the HETG properties might be changing with time - especially during the period from XRCF to flight first-light.

Figure vsg.1 : Time line of Flight and Vacuum Storage Gratings Activities

Summary Results to Date

As the diagram above indicates, a total of five (possibly six) vacuum storage retest cycles are planned to cover the XRCF to flight first-light time period. Currently three retests have been carried out, indicated by the vertical lines at 9611, 9707, and 9801 above.

A gross summary of Period changes at these times can be made by presenting the changes in the average period and the dp/p value for MEG and HEG grating ensembles:

         Vacuum Storage LR Results: MEG Ensemble
       Test Date :      9611xx     9707xx     9801xx 
           P_ave :     4000.36   [-68ppm]   [-11ppm]
           dp/p  :      192.07     [-22%]     [-18%]
         Vacuum Storage LR Results: HEG Ensemble
       Test Date :      9611xx     9707xx     9801xx 
           P_ave :     2001.95   [-70ppm]   [-37ppm]
           dp/p  :      229.48     [-14%]      [+5%]
These results are from analysis using the "active region" of the facets to approximate the flight-illuminated region on the facets. The four gratings with non-flight-like periods (see caption to Figure vsg.5) have not been included.

A similar summary for the diffraction efficiency is not tabulated but may be inferred from Figure vsg.4 below.

The measurement results to date for the individual facets are presented in the following plots and tables:

Ideally these data would show unchanging horizontal lines; variations are, however, visible. The following items summarize significant variations seen in these data and their likely explanations. More details and an explanation of the measurement problem codes (e.g., LRmp1) are available in the sections that follow.

Period Plot Anomalies

Period Variation (dp/p) Plot Anomalies

First-order Efficiency Plot Anomalies

The Vacuum Storage Gratings

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 given here, organized by the facet name.


Why Do Measurements Change?

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.

measurement noise (mn)
sample variation (sv)
reference variation (rv)
measurement problem (mp)

LR Measurement Notes

A variety of measurement problems can be caught by reviewing the LR data sheets:

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 recorded here:

X-GEF Measurement Notes

Identified X-GEF measurement problems are listed here:

X-GEF reference variation is more likely an effect than LR reference variation. The two reference gratings mounted in the X-GEF, MX078 and HX220, have been there continously since before 950615. They are maintained in vacuum, subject to any low-level contaminants in the system, and exposed to atmosphere whenever samples are being changed. The absolute 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 available:

Other ideas and actions for improved X-GEF testing are recorded here:

VSG Retest Procedure

The following is a high-level list of 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.

VSG Software

The analysis and presentation of the VSG measurement results is carried out by the IDL routine vsg_info.pro. 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/vsg9806/vsg.html.
Please send any comments and updates to: dd@space.mit.edu.