HETG Efficiency

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HETG Ground
Spectroscopy Group

HETG Efficiency

November 7, 2000

Version "N0004" HETG (and LETG) efficiency products were created in April 2000 as briefly summarized in this note:


Version "N0003" HETG (and LETG) efficiency products have been produced based on subassembly measurements, improved optical constant values, and corrections to the effective subassembly reference gratings efficiencies. The general steps are described in Subassembly Efficiency Analysis ; and some specifics are highlighted here.

Based on XRCF measurements made in Phase 1 (non-flight detectors) and Phase 2 (with ACIS-S), see below, we have confidence that the tabulated first and zeroth order efficiencies are accurate at the 10% level in an absolute sense. The relative effective area tests (a.k.a. "continuum" or "molecular contamination" tests) confirm the detailed structure in the HETGS effective area at the few percent(TBR) level. Note that MARX version 2.22 uses 'N0002 versions of both HETG and LETG gratings. See the n0003_effic_corr.pro procedure below for changes from 'N0002 to 'N0003 for HETG; for LETG the first order efficiency changes were less than 4% over the energy range.

HETGS XRCF Effective Areas with 'N0003 Efficiencies

Analysis of the HRMA-HETG-ACIS-S XRCF data sets produced measurements of the HETGS effective area. The the report "Absolute Effective Areas of the HETGS" ( 11.5 Mb .ps , 2.2 Mb .pdf ) presents the measurements and compares them to the model based on the 'N0004 HRMA XRCF areas, and 'N0002 HETG efficiencies. The plots below show the same data plotted with the new 'N0003 efficiencies. Note that these are NOT the flight predicted effective areas!

Plots of the measured HETGS effective areas compared
with the HRMA('N0004)-HETG('N0003)-ACIS-S model predictions.

Grating/Order: HETG0 MEG-1 MEG+1 MEG-3 MEG+3 HEG-1 HEG+1 HEG-2 HEG+2
Plot: .gif .gif .gif .gif .gif .gif .gif .gif .gif
Plot: .ps .ps .ps .ps .ps .ps .ps .ps .ps
N0003 Efficiency Notes

There are two changes which have lead to the 'N0003 efficiencies: i) the effective efficiency of the reference gratings in the HETG X-GEF setup have been revised and ii) new optical constants for gold, optical-constants.990510.tbl, have been produced.

For the LETG, new efficiencies were directly calculated with the new optical constants.

For the HETG, it is not possible to simply recalculate the efficiencies with new optical constants because the bar-shape models fitting the X-GEF data are based on the 96116 optical constants. A proper recalculation requires that the X-GEF measured efficiencies be fit with the 990510 optical constants to generate the grating model parameters; this will take some time to carry out.

However, the effects and validity of these two changes on HETG efficiency values have been assessed by reprocessing some X-GEF measurements with the new values. The general changes seen in the 1st and 0th order efficiencies were encoded in an IDL routine, n0003_effic_corr.pro, and applied as corrections to the N0002 "membrane" efficiencies. The HETG N0003 efficiencies thus have the same edge structures as the N0002.

Error values were created as part of the 'N0003 products by the procedures cip_weights_and_errors.pro and, for LETG, letg_cip_etc.pro. For the HETG the errors are:

In addition, "pruned" efficiency files were created by the procedures cip_hetg_prune.pro and, for LETG, cip_letg_prune.pro; the actual pruning is accomplished with the pruning routines interp_pruned_x.pro and interp_prune_err.pro.

See also the terse record in the hetg/docs/cip_config.txt and letg/docs/cip_config.txt files.

N0002 Efficiency Notes
For the HETG, these efficiencies have improved modeling of the polyimide edges (C, O, N, etc.) using optical-constants.981023.tbl values. For the LETG, the only change is the removal of the extra factor of 0.81: the efficiency is for all photons in the focal plane, that is it includes photons diffracted by the fine and coarse support structures.

These efficiency values are included in MARX version 2.22.

N0001 Efficiency Notes
These efficiencies were used for AO-1 and are based on the optical-constants.961116.tbl. The gold edge is improved over Henke values but the polyimide edges are simple. The LETG N0001 efficiencies have an extra factor of 0.81 included.

MARX 2.04 used a rectangular grating efficiency model for the gratings which approximated the N0001 efficiencies.