Calibration Data page:
CALDB paths, filenames, dates, versions, and product files
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:
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.
|QE: Sep 2015,
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.|
|QE: Dec 2006,
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.|
|Dec 1999||10% for 1st order. GREFF version 3.|
|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.)|
|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.|
|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
||Typical errors are
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
errors are around 0.015 Å with some localized deviations
of 5-10 pixels (0.037-0.074 Å).
||HRC-S gain map
(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.
||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
||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
LETG/HRC-S wavelength coverage near the plate ends.
||Line Spread Function (gRMFs)
||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
The Chandra X-Ray
Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory.
60 Garden Street, Cambridge, MA 02138 USA. Email: firstname.lastname@example.org
Smithsonian Institution, Copyright © 1998-2004. All rights reserved.