Marx 5.0.0
Marx 5 Highlights
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Marx 5 is a major new release. This page is devoted to the new features.
Contents
Subpixel RandomizationNew Dither Model
Calibration Updates
Marx Parameter File Updates
Subpixel Randomization
The EDSER subpixel algorithm was incorporated into CIAO 4.3. When computing Sky coordinates,
acis_process_events first converts the
integer-valued chip coordinate to a floating point value. There are
several ways in which this may be done. One way is to simply cast the
integer to a float, e.g,, the integer 2 becomes 2.0. The problem with
this approach is that it can cause aliasing artifacts in the resulting
Sky image. For this reason, the default up until CIAO 4.3 was to
simply add a uniform random deviate from -0.5 to 0.5 to the integer
value so that 2 would become a random real number in the semi-open
interval [1.5, 2.5). The default was changed in CIAO 4.3 to use a
discrete probability distribution based upon flight grade and energy, known as
the EDSER method.
The marx 5 version of marx2fits introduces a --pixadj command
line parameter that allows the user to select one of several pixel
randomization methods. Simply running marx2fits without any command
line arguments will display its usage message:
marx2fits v5.0.0: Usage: marx2fits [options] marxdir outfile Options: --pileup Process a marxpileup simulation --pixadj=EDSER Use a subpixel algorithm (default) --pixadj=RANDOMIZE Randomize within a detector pixel --pixadj=NONE Do not randomize within a detector pixel --pixadj=EXACT Use exact chip coordinates
The effect of these randomization methods may be seen in the following plot, which shows that the EDSER distribution approaches the exact limit.
If using SAOTrace rays with marx, see the important caveat regarding the use of the EDSER method with such rays.
New Dither Model
The EDSER subpixel algorithm necessitated numerous changes to the marx aspect code. In particular, aspect reconstruction blur had to be cleanly separated into its independently contributing pieces, namely the telescope pointing uncertainty, the blur introduced by pixel quantization/truncation, and the blur associated with pixel randomization. Previous versions of marx incorporated these blurs under the guise of a single parameter,
DitherBlur, whose value
was the RSS sum of the contributing blurs. This was the source of a
lot of confusion among users resulting in numerous help desk inquiries
about how this value was obtained and why it was so large.
For these reasons, the DitherBlur parameter was removed from marx
and replaced by one called AspectBlur whose value represents
just the telescope pointing uncertainty,
0.07
arc-seconds. One consequence of this change is that marx 4.x
parameter files cannot be used with marx 5.0.
It was also necessary to remove the blur parameters from marxasp, which computes an aspect solution file for use in reprocessing a marx2fits generated event file. Hence any scripts that pass blur parameter values to marxasp will need to be modified.
Calibration Updates
As mentioned above, the DitherBlur parameter's value reflected
more that just that associated with the aspect uncertainty. It was
also tweaked to get the marx PSF to better match Chandra grating line
profiles. With this parameter gone, the marx HRMA blur parameters
had to be re-calibrated to get the widths of the marx simulated
grating line profiles to match those in the Chandra CALDB.
There was a long standing issue of a relative rotation between the
LETG and the ACIS detector. The root of this problem was tracked down
(with the help of marx) to a rotation offset between the aspect
coordinate system and the focal plane detector system. This offset
was masked by compensating rotations of the detectors from astrometric
analysis, and manifested itself as a small rotation of the LEG
dispersion arm on the ACIS detector. Changes were added to CIAO 4.3
that effectively adds an additional rotation to the LETG when used
with ACIS. The corresponding change in marx 5.0 is implemented via a
new parameter called LETG_ACIS_dTheta.
The marx calibration data have been brought up to date with the Chandra CALDB 4.4.7.
Marx Parameter File Updates
The parameter files for earlier versions of marx (e.g., the marx.par
file for version 4.5) cannot be used with marx 5.0. The recalibrations
that were necessary for subpixel support resulted in changes to all of
the HRMA blur parameters, as well as the introduction of the new
AspectBlur parameter discussed above.
The following marx.par parameter values have changed since marx 4.5:
P1Blur: 0.18129215 --> 0.303427 H1Blur: 0.13995037 --> 0.0051428 P3Blur: 0.11527828 --> 0.0951899 H3Blur: 0.16360829 --> 0.0713614 P4Blur: 0.1289134 --> 0.178899 H4Blur: 0.098093014 --> 0.0101367 P6Blur: 0.076202759 --> 0.151085 H6Blur: 0.079767401 --> 0.0239287 MEGRowlandDiameter: 8632.65 --> 8632.48 HEGRowlandDiameter: 8632.65 --> 8632.48 HETG_Shell1_Period: 0.400141 --> 0.400195 HETG_Shell3_Period: 0.400141 --> 0.400195 LETG_Shell1_Theta: -0.07 --> 0.07 LETG_Shell3_Theta: -0.07 --> 0.07 LETG_Shell4_Theta: -0.07 --> 0.07 LETG_Shell6_Theta: -0.07 --> 0.07 legCoarseNumOrders: 11 --> 121The following parameters have been removed:
DitherBlurThe following parameters have been added:
AspectBlur: 0.07 LETG_ACIS_dTheta: -0.0867 Use_This_Order: 0 DetExtendFlag: noThe latter two parameters were added for the purposes of calibration.
