High Energy Transmission Grating Spectrometer Calibration
This page provides an overview of the HETGS in-flight calibration. It is
linked from the
HETGS Users web page and from the
Chandra Science Center
In most areas, the HETGS is performing according to pre-flight expectations.
For recent developments regarding HETGS calibration, see the
progress section of this web page. References that may be of interest include:
HETGS in-flight performance Summary
The relative accuracy of wavelengths assigned to events on the basis
of the distance from 0th order has been reduced significantly by computing
ACIS-S chip rotations and positions to better than 0.05 pixel (see a
in PS form by Bish Ishibashi). Thus,
wavelengths should now be accurate to less than 0.0055A
(0.0028A) for MEG (HEG) spectra.
The absolute accuracy of wavelengths was
reduced by eliminating
relative errors (above) and taking out a systematic
fractional error of 1.34e-4 (or
40 km/s) in the MEG spectra by adjusting the MEG period from 4001.41 A to
Dan Dewey's HETGS TechNote from October 2004).
This adjustment is similar in magnitude to that determined
by Marshall, Dewey, and Ishibashi (SPIE 20003).
Line response function -- See the HETGS
Technotes, where Dan Dewey (MIT/HETGS) has verified that the 0th
order images are consistent with models using MARX. The cores of
the lines are used to determine the preliminary E/dE plots of the MEG
results are available from his HAK analysis of an observation of Capella.
See also the study
of the line response function by David Huenemoerder and the
of profiles from the Line Spread Function library with lines from
Capella by Bish Ishibashi.
The wings of the lines are under analysis but do not appear to deviate
significantly from pre-launch expectations. Some of these results are
shown by Marshall, Dewey, and Ishibashi
Cross dispersion profiles -- The cross dispersion profiles were
derived from the HETGS observation of the Crab pulsar for the MEG
and the HEG spectra.
The cross dispersion enclosed energy fraction
(PS form) was computed in rectangular apertures
of various sizes as a function of wavelength using an observation of
Mk 421 (observation ID 1714). Also plotted in the image is the 1D profile
of a point source with minimal pileup. Note that a point source is always
narrower than the HETGS cross dispersion profile.
See also a
May 2004 memo (PS form)
by Bish Ishibashi for encircled energy fraction verification using
Dispersion angles -- The HEG and MEG dispersion angles were measured
relatively early in the calibration phase by Dan Dewey and verified by
HLM (MIT/CXC). Dan's analysis is available.
Focus is stable
(Updated 26 October 2004). The average FWHM of the zeroth order is 38.1 +/- 0.19 microns,
giving an angular size of 0.780 +/- 0.004 arcsec. Note that the HETG does not affect
the PSF of images so this size should be the same as that obtained without the
HETG in place.
System Effective Area --
Prior to implementation of new HEG and MEG efficiencies,
the estimated HETGS relative uncertainties were
based on the simple power law fits to the HETGS spectra of
pulsar and Mk 421. The following
estimates of the systematic uncertainties of HETGS spectral fluxes
were suggested, after
correcting for the system effective area using the calibration
data products as of September 2005:
The quoted uncertainties are meant to give the uncertainty
on fluxes determined in one band relative to those in another band.
Absolute flux uncertainties have not yet been verified but are expected
to be good to a better than 5-10%. The uncertainties include systematic
errors in many instrument components, including the HETGS efficiencies
and ACIS filter and QE models, and are meant to reflect the system
10% for 1.5 < E < 6 keV (both MEG and HEG),
20% for 6 < E < 8 keV (HEG only),
20% in the Si-K edge region (1.83-1.84 keV) (both MEG and HEG),
20% for 0.8 < E < 1.5 keV (both MEG and HEG),
30% for 0.5 < E < 0.8 keV (MEG only),
50% in the O-K edge region (0.525-0.57 keV) (MEG only).
The ACIS team has provided QE models that eliminate most of the
discrepancies between the BI and FI chips, as documented in a
preliminary report (PS form) on the LETG/ACIS calibration and by
Marshall, Dewey, and Ishibashi
There is also a significant
contamination buildup which is also observed in direct ACIS
spectra. This contamination was
first detected in LETG/ACIS data but
clearly affects all ACIS observations. See also the paper by
Marshall et al. (SPIE 2003b),
which provides details about the composition of the contaminant.
A web page
documents some work on comparing the
MEG and HEG spectra as of April 2005.
A more refined
on the HETGS effective area, dated 10/14/2005, gives an
update of the HEG and HEG efficiencies and a fit to
the HRMA Ir-M edge.
Using these updated HEG and MEG efficiencies as well
as the suggested correction to the Si-K and Ir-M edges,
fractional residuals should now be less than 5% from
0.5 to 8 keV.
Work in progress (Updated 10/14/05)
The effects of pileup are examined in an analysis
by Dan Dewey and are apparent in this plot
comparing the MEG and HEG spectra of the Crab pulsar. (The dashed
line is the MEG spectrum, which shows a deviation from the HEG spectrum
near 2.1 keV, which is associated with the EA increase due to the mirror
Ir M edges.) The HETGS calibration observation of Cyg X-2 is
very heavily affected by pileup, to the extent that the continuum is very
difficult to measure. From the Crab and Cyg X-2 observations, we
surmise that pileup has a greater effect on HETGS spectra than would be
predicted from naive models and that the spectral region that is most often
affected is where the MEG EA peaks near 2 keV.
Pileup correction may be considered to be a calibration task.
At present, there
for correcting for pileup within
ISIS and the
function in sherpa (see the
model thread). A
correction method seems to be sufficient for calibration tasks,
which does not involve iteration or forward folding.
We will be assessing the accuracy of pileup
correction methods with calibration data.
We will be testing the models of the high order efficiencies of the HETG
using data from 3C 273 and Cyg X-2, among other sources.
This work is on hold while we complete analysis of the first
order efficiency, which has higher priority.
There are a few stages in grating data analysis that involve
data filtering or corrections which have not yet been examined
as part of the calibration program. Issues that may have an
effect on the reliability of derived spectra include:
order sorting --
using a very wide order sorting selection
window will yield the best reliability at the expense of statistical
(see Dave Huenemoerder's
at the 2002 Chandra Calibration Workshop for a discussion
of order sorting in ciao);
destreaking -- the streaks in the S4 chip are flagged and
then removable using
the destreak tool (see the
ACIS data" web page) but for very bright sources some real events
may be removed so the threshold could be raised to reduce this loss;
removing "afterglow" events -- the afterglow events are
isolated in time and chip location and can be interpreted incorrectly
as sources but are generally not a problem for grating spectra,
so one should ordinarily eliminate this step of the ACIS processing
or reset the afterglow status bit for grating data analysis using
continuous clocking mode -- this mode has been used often to
observe bright sources but order sorting and background selection are
sufficiently different that the calibration of the resultant spectra is
not readily comparable to that of spectra
taken in standard timed exposure (TE) mode.
Contact Herman L.
Marshall (hermanm at space.mit.edu) for further information about HETGS
Last modified: 1/30/07