XRCF HRMA On-axis Effective Area
XRCF HRMA On-axis Effective Area
Ping Zhao
Smithsonian Astrophysical Observatory
zhao@cfa.harvard.edu, 617-496-7582
http://hea-www.harvard.edu/MST/mirror/www/xrcf/hrma_ea.html
NASA's Chandra X-ray Observatory (CXO) is currently expected to be
launched soon by the space shuttle Columbia. CXO has unprecedented
capabilities of high resolution imaging and spectroscopy over the
X-ray energy band of 0.1 keV - 10 keV. The effective area of its
X-ray mirror - High-Resolution Mirror Assembly (HRMA) - was measured
using the X-Ray Calibration Facility (XRCF) at the Marshall Space
Flight Center (MSFC) in Huntsville, AL from late 1996 to early 1997.
At first, there were substantial discrepancies between the
experimental results and the predicted effective area based on the
raytrace simulation according to the HRMA model at that time. The
calibration data show that, for energies higher than 2 keV, the
measured effective area is less than the predicted effective area
beyond the experimental errors.
During the period from late 1998 to early 1999, we made substantial
improvement in our raytrace model and hence reduced those
discrepancies in the effective area. The measured effective area is
still a few percent less than the raytrace simulations for energies
higher than 2 keV, except for Shell 1, which still has a larger
disagreement. And there is still an unknown systematic gap between
the SSD continuum data and the FPC line data. However, the agreement
between the data and the model are much better than before. While we
are still continuing to work on the remaining discrepancies of the
HRMA effective area, by popular demand, we are releasing our current
best predictions of the HRMA on-axis effective area for both
on-orbit and at the XRCF.
The raytrace simulations were generated based on the HRMA model to the
best of our knowledge. Its trace-shell configuration file can be found
here.
The key raytrace configurations are:
- Optical Constant:
- E < 0.940 keV: Gullikson '95 optical constant table.
- E > 0.940 keV: BNL synchrotron measurements of the AXAF mirror witness GO flat 065 made by Dale Graessle et al.
- Reflectivity:
- Fresnel equation of multilayer reflection with Iridium, Chromium,
and semi-infinite Zerodur.
- Interface reflection reduction with the Modified Debye-Waller factor.
- Mirror Surface Roughness Scattering:
- Based on PSD produced from the HDOS metrology measurements and
calculated by Leon Van Speybroeck's program "foldw1", which is based
on the scattering theory by Beckmann and Spizzichino.
- Scattering table was generated on 6/23/98.
- Mirror rigid body specs.
- Mirror surface deformations.
- Mirror cap, pre- and post-collimators, p6 ghost baffle.
- XRCF source distance.
There are three kinds of calibration measurement of the HRMA on-axis effective area:
X-ray spectral line measurements with the Flow Proportional Counter (FPC).
X-ray spectral line measurements with the Solid State Detector (SSD).
X-ray carbon continuum measurements with the Solid State Detector
(SSD).
Here are the comparisons of the XRCF HRMA on-axis effective area between the
calibration results and the raytrace simulations with a 2mm aperture,
which was the largest aperture used with the SSD measurements:
where the top panels show the raytrace simulations (generated by Diab
Jerius), spectral line FPC and SSD data (analyzed by Richard Edgar)
and C-continuum SSD data (analyzed by Ping Zhao). Because of a little
discontinuity in the transition between Gullikson95 and synchrotron
optical constant, there is a small jump at 0.94 keV in the raytrace
simulation curve. The bottom panels show the ratio of data/raytrace.
A polynomial was fit to each of the data/raytrace ratio curve.
As we can see in the above figures, the data currently still show a
few percent less than the raytrace simulations for the effective area.
Based on the principle that theory should yield to the experiment, we
use the XRCF calibration data to scale down the raytrace for both
on-orbit and at XRCF HRMA effective area predictions.
The XRCF data scaling factors are:
- E < 2 keV: Use the average ratio between the raytrace and FPC rerun
data.
Errors include: FPC data errors; standard deviation of the FPC data;
beam nonuniformity errors; aperture size errors; raytrace statistical
error (this is very small).
- E > 2 keV: Use a fourth order polynomial fit of the ratio between
the SSD C-continuum data and the raytrace. (The SSD data analysis has
included pileup, pulser deadtime, relative QE and beam uniformity
corrections.)
Errors include: standard deviation of SSD continuum data within each
energy bin; errors from the polynomial fit; beam nonuniformity errors;
aperture size errors; raytrace statistical error (this is very small).
Here are the results of XRCF HRMA on-axis effective area predictions
(The small jump at 0.94 keV is an artifact of the transition between
Gullikson95 and synchrotron optical constant.):
- XRCF HRMA on-axis effective area within 2 mm diameter aperture:
rdb table and
figure.
- XRCF HRMA on-axis effective area within 35 mm diameter aperture:
rdb table and
figure.
- XRCF HRMA on-axis effective area within 2 pi steradian:
rdb table and
figure.
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