Calibration of Chandra Grating Spectrometers

linked off of

Grating-related people planning to attend the Nov.13-15 meeting are:
HETG/ASC: Dan Dewey, Kathy Flanagan, Herman Marshall, Mike Juda, Jeremy Drake
LETG: Bert Brinkman, Peter Predehl, Theo Gunsing, Jelle Kaastra, Heinrich Brauminger
Other calibration review pages:

Return to HETG at XRCF page.

This is a DYNAMIC page so please direct comments and questions to

Introduction to Chandra Grating Spectrometers

    Grating Spectrometer Operation

      General Diagram
      Rowland Geometry
      Example Image from Rehearsal Data
      Spectrum from above with linear scale

Calibration Product(s) for Chandra Grating Spectrometers "... the creation of a model or set of models that describes the performance characteristics of the instrument to within a certain accuracy..." Effective area vs. E The effective area calibration data must allow converting an observed counts histogram (counts/sec bin) into an incident spectral flux (e.g., photons/cm^2 sec). For the grating spectrometers this generally refers to the combined (plus and minus) first-order effective area. Line-response function (LRF) vs E With the presence of continuum and adjacent lines it is important to have a known or constrained LRF to do accurate model fitting in regions with many lines. Response Matrix The Effective area and LRF can be combined into a single entity or response matrix.
Science drivers and required calibration accuracies The absolute effective area of the grating spectrometers should be known to +/- 10 % over the relevant energy ranges. Plasma diagnostic methods, models, and their science lead to a desired relative effective area calibration of +/- 3 % . The instrumental shape of an infinitely narrow X-ray line, the line response function LRF, should have its energy resolution, e.g., FWHM, known to 1 part in 30 to support line-width measurements and upper limits. The extended LRF, or wings, must be known or constrained to within 1% of the peak of the response in energy bins that are the instrumental width for all energies (this includes higher orders implicitly). To support doppler shift and line-energy determination, the absolute energy scale of the grating spectrometers must be known to the FWHM/30, e.g., 0.03 eV at 1 keV for HEG, or 0.002 A for the LETG at 60 A.
Definition of Effective Area and related terms for Grating Spectrometers Effective Area (E,m,offaxis) is ratio: detection rate (counts/sec) ------------------------------ flux at HRMA aperture (photons/cm^2sec) where source is mono-chromatic (superposition assumed to work) and "detection" includes PSF effects (cross-dispersion) and detector effects (e.g., grading) Effective efficiency (E,m,offaxis) is the Effective Area of the HETGS/LETGS divided by the corresponding Effective Area for the non-grating configuration. Shell N Efficiency (E,m,offaxis) is the ratio of photons diffracted to order m by grating facets covering shell N to the photons leaving HRMA shell N. Membrane Efficiency (E,m) is the diffraction efficiency of the grating membrane, e.g. for LEGs includes support structure vignetting, for MEGs/HEGs includes platingbase/polyimide support. Does not include facet size vignetting nor unfaceted regions between gratings. Grating-bar Efficiency (E,m) Diffraction efficiency for the gold grating bars alone.
LRF error budget and terms The LRF is dominated primarily by the HRMA PSF (especially at XRCF) and aspect system - except at highest dispersions. The gratings are expected to introduce only a core blurring to the LRF based on subassembly diffraction tests. XRCF will be an important place to look for other unexpected components of the response, e.g. scattering.
Cross-dispersion Range for Calibration The Effective Area / LRF calibrations apply to a cross-dispersion width sufficiently large that variations in this width will not produce large (few percent) changes in detected counts. For the HEG and MEG a cross-dispersion width of 200um (d = 0.100 mm in Dave H's plots below) encloses more than 99% of the flux. For the LEG sprectra, 200 um is used for dipsersions below 60 mm, and 500 um for dispersions greater than 60 mm. Further away from the dispersion axis are the support structure orders. The first of these has ~1/100 of the main peak intensity, and each subsequent is ~0.8 times the previous. The first two "blips" from both sides would then have ~ 3.6 % of the main counts.
Grating Calibration Products Component pieces: HRMA area (E,shell,offaxis) LETG/HETG Effic (E,m,shell) Detector Effic (E,other) Drake reflectivity(E,shell) HRMA PSF (E,offaxis) HEG,MEG,LEG LRF (E,m) Drake PSF (E, shell) Detector PSF Aspect PSF Effects Top level: Response matrices for: HETGS(HEG,ACIS-S) HETGS(MEG,ACIS-S) HETGS(HEG,HRC-I) HETGS(MEG,HRC-I) LETGS(HRC-S) LETGS(Drake) LETGS(HRC-I) LETGS(ACIS-S)

LETG Calibration Considerations

LETG Simulations and Analysis

LETG Subassembly Tests

LETG Subassembly IR/Diffractometer data Panter, Zeta, BESSY X-ray measurements Likely errors from IR or X-ray tests Range of LEG parameters Subassembly prediction, modeling it Significant edges/features LETG Membrane efficiencies derrived from IR Data
Assumes a factor of 0.809 for the support-structure vignetting.

ASCII tables of LETG Effective Efficiency predictions

Effective efficiency of combined shells (mirror weighted) for direct comparison with XRCF measurements. For LETG using an extra vignetting factor of 0.75 in addition to the "membrane efficiencies".

Gratingm=0m=1 m=2m=3m=4m=5 m=6m=7m=8m=9
LEG leg 0 leg 1 leg 2 leg 3 leg 4 leg 5 leg 6 leg 7 leg 8 leg 9

Plots of LETG Effective Efficiency predictions

Gratingm=0m=1 m=2m=3m=4m=5 m=6m=7m=8m=9
LETG letg 0 letg 1 letg 2 letg 3 letg 4 letg 5 letg 6 letg 7 letg 8 letg 9

HETG Subassembly Tests

Basis for model is scalar diffraction theory for trapazoidal or many vertex modeled bar shape. Synchrotron and X-ray facility testing. HETG HETG Sub-assembly tests Synchrotron results, compare with model and X-GEF Likely error on X-GEF measurements Range of grating parameters for shells 1,3,4,6 Subassembly prediction, modeling it Significant edges/features Efficiency Model composed of many gratings (and for HETG different mirror shells) Can be fit with a several-grating model? Adjusting model to match data HETG Membrane Efficiencies derrived from X-GEF data

ASCII tables of HETG Effective Efficiency predictions

Effective efficiency of combined shells (mirror weighted) for direct comparison with XRCF measurements. For HETG using an extra vignetting factor of 0.93 in addition to the "membrane efficiencies".

Gratingm=0m=1 m=2m=3m=4m=5 m=6m=7m=8m=9
HEG heg 0 heg 1 heg 2 heg 3 heg 4 heg 5 heg 6 heg 7 heg 8 heg 9
MEG meg 0 meg 1 meg 2 meg 3 meg 4 meg 5 meg 6 meg 7 meg 8 meg 9

Plots of HETG Effective Efficiency predictions

Gratingm=0m=1 m=2m=3m=4m=5 m=6m=7m=8m=9
HETG hetg 0 hetg 1 hetg 2 hetg 3 hetg 4 hetg 5 hetg 6 hetg 7 hetg 8 hetg 9

"Drake Flat"

- Jeremys Presentation (130Kb .ps) - Model Details for Drake Flat(360kb .ps.gz)

Pre-XRCF Performance Prediction

- Physical models of key sub components HRMA (P1H1 P3H3 P4H4 P6H6) Reflectivity off of Ir coated surface, Ir optical constants Uncertainty for all HRMA shells of order 10%, more at edges. Created and tested by MST and TS. Significant edges/features: Ir N and M edge regions, high energy fall off Expect it will be well constrained after XRCF. ACIS-S, ACIS-I CCDs UV/Optical-S, UV/Optical-I Significant edges/features: O edge, Al edge, Si edge HRC-S, UV/ion HRC-I, UV/ion MCPs UV/ion Significant edges/features: For each physical model above summarize: Basis for model Model Creation and Testing Significant edges/features Uncertainties (pre XRCF) Unconstrained (even) after XRCF - Pre-XRCF Calibration Status Plots of "products" and their errors if only subassembly data are used HETG and LETG effective efficiency predictions Resolving Power plots and data files based on "worst-case" error budget

XRCF Grating Performance

Off-Rowland Operation

The gratings are built for operation at a Rowland diameter (grating to focal plane distance) of 8635 mm. At XRCF they are located at a distance of 8772.8 mm (TBR) from the focal plane driven by the requirement that the gratings intercept the HRMA rays. This Rowland diameter "error" produces insignificant (1 micron) additional blur terms.

Effective Area: Vignetting

XRCF-Flight differences

As outlined in a detailed memo the spacing of 8772.8 mm (TBR) is not optimum even from a vignetting point of view and some additional LETG vignetting (about 5% for shell 1) will occur. Somewhat different regions of the gratings will be illuminated at XRCF as compared to flight. It is expected that the size of these effects and the uniformity of the gratings are such that these diferences will not compromise the XRCF to flight calibration transfer.

XRCF Point Spread Function

Because of a variety of effects the HRMA PSF at XRCF will differ from the flight PSF. In addition, because the XRCF shutters allow selection of mirror shells/quadrants it is useful to consider the predicted performance of each shell/quadrant when planning XRCF tests. The images and plots below provide just such detailed looks for the Al 1.487 keV line.

Images and 1D projection plots for each shell/quadrant. The projection along the dispersion axis (horizontal, XRCF-Y axis) is shown solid and the cross-dispersion axis (vertical, XRCF-Z) projection is dashed.

XRCF Calibration Priorities

  Summary of identified calibration priorities

    Measure grating dispersion directions and (roll) angles in facility
    Measure XRCF vigneting by shell
    Measure axial repeatability of grating insertion mechanism at XRCF

    Use a "Shutter Focus" iterative test to focus detector and define X=0
    Make "Focus Check" measurements - images in each quadrant - to
     determine focus of dispersed orders.
   LRF Inner Core
    Verify grating dp/p values
    Measure LRF near core for a few energies
    Measure cross-dispersion response

   LRF Outer Core
    Search LRF for unexpected features (in wings)

   LRF Modeling
    Verify defocus and offaxis effects

   Effective area related:
    Efficiency/Effective area vs E to high accuracy for 1st order
    Measurement of LETG high-order efficiency at several energies
    Scans over known edges/features
    Search for unexpected edges/features
    Verify/measure detector effects on effective area 
      (e.g., grading, mode, gain, rate)

   Wavelength Scale
    Set energy scale relative to edges/features
    Generally parasitic on other measurements

   Molecular Contamination
    Use "featureless" continuum to search for unexpected edges and
     features in the response

XRCF Calibration Measurements

Measurement priorities for Grating Calibration Measurements Test list - CMDB [Test lists in /proj/asc/cal/Calibration/{LETG|HETG}/{phase1|phase2}] Subphases for Phase 1: A - find any major surprises in HRMA/Gratings B - minimum ground calibration of HRMA/Gratings C - proper calibration of HRMA/Gratings, model over determination, etc. .exp file written to clearly indicate purpose of measurements Phase 1 HXDS CMDB Summary Table Phase 1A CMDB Summary Table Phase 1X Phase 2 HRC-I, HRC-S, ACIS-2C Summary Material Model constraints that these measurements will provide Assummed calibrations (XSS,HXDS,HRMA,FPSIs,...) - have HSI map identifying bad areas, gain variations - HSI effic at 130A calibrated post test - SSD-500 and BND-500 cross-calibrated - FPC efficiency @ 108 eV > 1% - Effic(HSI_for_real) > Effic(HSI_rehearsal)/20 - comparable no-grating measurements are made

Simulation and Data Analysis

Simulation of XRCF Measurements

Both SAOSAC and MARX can be used to simulate XRCF measurements. The SDS/ASC-MIT developed MARX ("csim") is being used with an IDL XRCF spectrum generator, is having its parameter files generated from a cmdb.trw database file, and is being modified to include the HXDS instruments. Details of csim/MARX were recently presented at the '96 ADASS meeting, manuscript in preparation.

Pipeline Processing for XRCF Grating Tests

PURPOSE: The purpose of pipeline processing is to automatically process as much data as possible with minimal human interaction in order to prepare data products for inspection, modeling, and analysis by calibration scientists. Pipelines have been defined for each type of test, with the detector/grating combination primarily specifying variant parameters and slightly alternative processing (e.g., if HSI or HRC, then skip PHA-based order sorting). Processing specifications were derived from input from Calibration scientists.

CONTENT: For Phase 1, there is strong reliance on MST processing of the focal-plane detector data. Those results are then subjected to grating-specific processing (for example, scan-positions are converted to diffraction coordinates before profiles are extracted and measured).

The tables below diagram the types of tests and the detectors used, based on a recent Calibration Measurement Data Base (CMDB). Numeric designations correspond to individual pipelines. Sub-designations (1a, 1b, etc) mean that the processing is identical at Level 2 (ASC level 2 - individual test measurements). (priorities are not indicated)

After Level 2 pipelines, data for each test and type are collected into tables for detailed analysis (e.g., collect all effective areas vs energy).

HETG XRCF tests:
		   |       HXDA     |          SI         |
	Test       |  HSI  SSD  FPC |  2C  AS  HS  AI  HI |
1a   Shutter focus |  x             |  x   x   x   x   x  |
1b   Focus check   |  x             |  x   x   x   x   x  |
2    Alignment     |  x             |                     |
3    Mol. Contam   |  x             |  x   x   x          |
4a   PSF/1D        |  x         x   |                     |
4b   PSF/2D        |                |                     |
4c   PSF/Inner     |                |  x   x   x   x   x  |
4d   PSF/Outer     |                |                     |
4e   dFocus        |                |                     |
4f   Dispersion    |  x             |  x   x   x          |
5a   EffArea/EE    |  x    x    x   |                     |
5b   EffArea/QE    |                |  x   x   x   x   x  |
6    Scattering    |  x             |  x                  |

LETG XRCF tests:
     Test          |  HSI  SSD  FPC |  2C   AS  HS  AI  HI |
1a   Shutter Focus |  x             |  x    x   x       x  |
1b   Focus check   |  x             |  x    x   x   x   x  |
2    Alignment     |  x             |                      |
3    Mol. Contam   |  x             |                      |
4a   PSF/1D        |            x   |                      |
4b   PSF/2D        |            x   |                      |
4c   PSF/Inner     |  x         x   |  x    x   x       x  |
4d   PSF/Outer     |                |  x    x              |
4e   dFocus        |  x             |                      |
4f   Dispersion    |  x             |  x    x   x          |
5a   EffArea/EE    |            x   |                      |
5b   EffArea/QE    |  x             |  x    x   x       x  |
6    Scattering    |                |                      |

HSI = High Speed Imager		    SSD = Solid State Detector
FPC = Flow Proportional Counter


Released pipelines can be found on the ASCDS web pages. They can be executed under the ASSIST environment, or via the UNIX command-line. The current XRCF release of grating pipes includes:

Level-1 (formatting, "linearization") / Rehearsal:
       ACIS/HRC event processing
       ACIS/HRC SAO-simulation (SAOSAC) connector
       ACIS/HRC MIT-simulation (csim/MARX) connector
       ACIS-2C quicklook PSF
       ACIS-2C quicklook spectra
       HSI quicklook PSF

Level-1.5 (diffraction coordinates)


Level-2 (test-specific measurements):

	ACIS/HSI Molecular contamination
	ACIS/HSI PSF (inner, outer, defocus, dispersion)
	ACIS/HSI Shutter focus, focus check

PIPELINES under development:

        HSI Alignment: 50% done. Borrows from HRC PSF
	    (radial/azimuthal profiles). Completion requires profile
	    ratios and statistical test of constancy with azimuth.

	SSD/FPC effective area and PSF: These borrow heavily from MST
             processing for L1 products, after which they in turn
             borrow from existing grating pipes - e.g., for L1.5
             diffraction coordinate transformation and then profile
             extraction and measurement.

	 HSI effective area: borrows heavily from ACIS and HRC
	     effective area (which are released pipelines).

	 ACIS/HRC effective area: will be similar to HSI.

	 HRC L1.5: release depends upon coordinate transformation
	     library updates which include HRC-S and grating routines.
	     Otherwise, this is very much like ACIS-S, with the
	     exception of PHA-based order sorting.

	 HRC PSF: follows easily after L1.5 - similar to ACIS, HSI.


After pipeline processing, a large variety of software is available on the 3rd floor for detailed, interactive analysis. Many ASC-developed tools have been released in the data-system. IDL is available, as well as IRAF/PROS, XSPEC, and FTOOLS.

ASC-DS released tools can be found on the ASCDS web pages. (http://???). They include:

       - image manipulation
       - data conversion 
       - event manipulation and filtering
       - table manipulation
       - source detection
       - image processing (radial profiles)
       - curve fitting
       - graphics display tools

   Some examples of post-pipeline processing are:

      - Fitting effective area vs energy
      - determination of dispersion constants and clocking angles from
        PSF-pipe output line-centroids vs energy.
      - spectral resolution vs energy.
      - modeling shutter focus results to determine whether centroid
        variance is due to astigmatic focus, or exceeds it due to

Summary and Conclusions

   XRCF Data Analysis plan (Tasks, who, dates: prelim, final)


MSFC-RQMT-2229 "Scientific Requirements for the Calibration of AXAF"
   o HETG Homepage
   o LETG Homepage
   o HETG SOP01 Operations Handbooks
   o LETG SOP01 Operations Handbooks
   o ASC SIN
   o Various SPIE papers