LETGS Calibration Overview and Status
LETGS Calibration Overview and Status
Links
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:
- Point Spread Function (PSF) and focus position of each of the 4 HRMA shells
- Photon energy
- Diffraction by the main grating bars, and by the fine and coarse support
structures
- Alignment of the 540 grating facets
- Accuracy of the HRC-S degap map
- Increasing astigmatism in the cross-dispersion direction at longer
wavelengths as the Rowland circle deviates from the imaging focal plane
- Deviations of the HRC-S plate surfaces from the Rowland circle
- Time- and/or temperature-dependent flexing of all the relevant optical
elements and support structures.
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:
- HRMA effective area
- LETGS diffraction efficiency for order m (GRating EFFiciency, GREFF)
- Detector QE (including filter transmittance and QE uniformity map)
- Order Sorting Integrated Probability (OSIP; ACIS only)
- Exposure corrections (FRACEXPO) for dither across detector gaps and
bad pixels
- Extraction efficiency of the spectral region
(Enclosed Energy FRACtion, EEFRAC)
- Efficiency of the PI vs. wavelength filter (if applied).
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.
LETG/HRC-S
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:
- Fine tuning the edge structures at C-K, O-K, and Cs-M.
- Correcting the low-energy QE (at wavelengths > 50 Å, where
ground calibration is very difficult) based on
observations of the hot white dwarfs Sirius B and HZ 43. Current efforts are
underway to further modify the low-E QE
using updated white dwarf emission models.
- Correcting the QE from 6 to ∼40 Å by comparing the predicted
spectrum of PKS 2155-304 based on contemporaneous observations
with HETG/ACIS and LETG/ACIS.
Additional small scale corrections have been
made using observations of the bright blazar Mkn 421.
LETG/ACIS-S
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 |
Accuracy |
LETG/HRC-S
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.
|
LETG/ACIS-S
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
(m=0,1)
| Dec 1999
| 10% for 1st order. GREFF version 3.
|
LETG efficiency
(m=2-10)
| 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.
|
HRC-S/LETG
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
