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LETGS Calibration Overview and Status

LETGS Calibration Overview and Status


Calibration Data page: CALDB paths, filenames, dates, versions, and product files
ACIS Calibration
HRC Calibration

LETG Calibration Overview

There are two primary aspects to the LETGS calibration: the two-dimensional line response function (LRF), and the throughput of the instrument, or "effective area" (EA). The LETG LRF is complex, and the recorded positions of dispersed photons are affected by many interacting factors, including:

The LETGS EA comprises several elements corresponding to X-ray interaction with various components in the optical path, and additional factors related to data extraction. For mth order of a spectrum, these are:

There is no calibration product for LETG/detector EA per se. Instead, Grating Auxiliary Response Files (GARFs) are generated for each observation with a Spectral Response (SPECRESP) column that includes all but the last of the factors listed above. When fitting spectra to derive source fluxes, the model spectrum factors in the incident source spectrum, SPECRESP, and the Enclosed Energy Fraction (EEFRAC) of the spectral extraction region that was used. EEFRACs are interpolated from tabulated values in the LSFPARMs files and incorporated in the grating Response Matrix Files (gRMFs).

Theoretical grating efficiencies for all orders were based on a rhomboidal grating bar diffraction model calibrated pre-flight from laboratory near infrared characterization of each grating facet and on ground measurements (mostly of 1st and 0th orders) at XRCF and Panter. Higher order efficiencies were modified later based on LETG/ACIS-S flight data. Extraction efficiencies were originally estimated using MARX raytrace simulations. The 2010 recalibration of the LETG/ACIS EEFRACs used higher fidelity SAOSAC+MARX simulations with adjustments to match flight observations, and the 2015 work on the LETG/HRC EEFRACS used flight data.


The HRC UV-ion shield (UVIS) filter transmission was measured in the lab and at synchrotron facilities. Only the details of oxygen and carbon K-shell edge structure have been modified since launch. The HRC QE Uniformity (QEU) map is derived primarily from flat field lab data, from which an energy-dependent spatial model was built. The spatial model is constrained to be smooth over scales ∼20x line width. HRC-S QE was originally based on lab and XRCF measurements, but has been modified based on in-flight data as follows:


The LETG/ACIS-S effective area is based on the LETG transmission efficiencies for each diffraction order and on the ACIS-S QE model. The ACIS-S QE is based largely on laboratory and XRCF data obtained for the bare ACIS chips, the optical blocking filter (OBF), and the combination of both. An important additional component is the contamination layer on the OBF that has been calibrated using a combination of LETG/ACIS-S observations and bare ACIS observations of supernova remnants and clusters of galaxies.

Calibration Status

Product Last Update:
Date, CALDB              
effective area
QE: Sep 2015, 4.6.9
QEU: May 2017, 4.7.4
10-15% over whole range. As noted above, the LETGS EA comprises several elements, including detector QE and QE Uniformity. Time-dependent adjustments (via QEU) were first introduced in 2011 and are updated as needed.
effective area
QE: Dec 2006, 3.3.0
QEU: Dec 2012, 4.5.5
OBF: Dec 2016, 4.7.3
OSIP: Dec 2006, 3.3.0
10% over most of the effective range; OSIP errors may be larger on the S1 chip. The most frequently revised component of the net EA is the OBF contamination model.
LETG efficiency
Dec 1999 10% for 1st order. GREFF version 3.
LETG efficiency
Apr 2011, 4.4.3 5-10% relative to 1st order for mλ<80 Å. GREFF version 7. (The GREFF file contains data out to m=25.)
LETG/HRC spectral
extraction efficiency
Sep 2015, 4.6.9 LETG/HRC-S EEFRACs were recalibrated, with compensating changes in HRC QE in order to leave the LETG/HRC-S EA unchanged. New scripts were also developed to straighten spectra for use with an optional narrower extraction region that is optimized for long-exposure low-S/N spectra.
LETG/ACIS spectral
extraction efficiency
Dec 2010, 4.4.1 In combination with the Nov 2010 CALDB 4.4.0 correction for the LETG/ACIS tilt, the newly recalibrated Encircled Energy Fractions (EEFRACS) in the LSFPARM files are accurate to 1% for observations made with the standard setting of Yoffset=+1.5'. The default spectral region width was changed from |tg_d|<0.0020° to 0.0008° at the same time.
HRC-S Degap Map
and Dispersion Relation
Mar 2018, 4.7.8 Typical errors are 0.004 Å for λ<20 Å, a range with many strong lines that can be compared to HETG/ACIS spectra. At longer wavelengths, where wavelengths are less well known and lines are relatively weak, making calibration of dispersion non-linearities difficult, errors are around 0.015 Å with some localized deviations of 5-10 pixels (0.037-0.074 Å).
HRC-S gain map
(and adjustments)
Jul 2020 4.9.2
(Sep 2021 4.9.6)
Typical accuracy of ~1% with 1/3-tap grid (0.63 Å without dither). The gain map is time dependent, with two separate calibrations for the old and new HRC-S high voltage settings (changed in Mar 2012). There is an associated HRC-S/LETG background filter.
background filter
Jul 2020 4.9.2 This filter is used in conjunction with the time-dependent gain map and removes more than half of the L2 background beyond ~20 Å with a 1.0% loss of 1st order X-ray events. X-ray losses are also calibrated (and larger) for higher orders.
HRC-S Badpixel Map May 2017, 4.7.4 Additional area near the ends of the HRC-S plates is now removed by BADPIX filtering. These changes were made in concert with revisions to the HRC-S QEU to resolve discrepancies in LETG/HRC-S wavelength coverage near the plate ends.
Line Spread Function (gRMFs) March 2004 ACIS/LETG: Grating RMFs based on MARX raytraces match observed lines well.
HRC-S/LETG: Imaging non-linearities in the HRC-S distort and shift the line profiles from their expected wavelengths in some wavelength regions, so care is needed in applying gRMFs directly, e.g., through XSPEC or Sherpa model fitting.

Last modified: 09/24/21

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