EARLY DATA ANALYSIS WITH RHESSI
Gordon Hurford, 20 March 2002
After a month in orbit, RHESSI is operating well.
Dozens of events have been well-observed to date, including several
M-class x-ray events.
The purpose of this report is to briefly outline the
hardware and software issues that users will face during early analyses of
RHESSI data. Note that software
improvements discussed below will be fully applicable to all data acquired
since the beginning of the mission.
POINTING AND ASPECT
The spacecraft is rotating at 15 rpm with its spin axis
pointed to Sun center within programmed
limits of 3 to 6 arcminutes. The solar
panels have been adjusted so that the imaging axis is aligned to the spin axis
to about 4 arcminutes. An apparent
orbital variation in this alignment has been noted, but this has no significant
effect on imaging.
Off-line analysis of Solar Aspect System (SAS) data
indicated that it easily meets the (0.4 arcseconds rms) requirement for 2
arcsecond imaging. Using a simpler algorithm,
the current SAS analysis software is constrained to a 1 or 2 arcseconds rms
solution, which is adequate for imaging with grids 3 through 9. The on-line version of the higher
resolution algorithm is currently under test.
Users are also cautioned that for now, there may be intermittent periods
of higher aspect uncertainty shortly after sunrise.
In the first few weeks of the mission, aspect diagnostics
required rebooting the aspect computer twice near the middle of each daylight
orbit. Such reboots introduced ~20
second data gaps during which imaging is not possible. Until software filters are installed, users
can use hsi_aspect_solution() to verify that their imaging does not extend into
or across these gaps.
The primary roll aspect system (RAS) is operating, but its
performance and analysis are
complicated by a higher-than-expected number of false triggers and by issues
related to earth albedo. We are hopeful
that these matters will be resolved and that RAS software with suitable software
filters will be available within the next few weeks.
Meanwhile, the backup phototube-based roll aspect system
(PMTRAS) is operating nominally and is providing robust and accurate data. The spin period has been found to exhibit a
significant (for imaging) orbital variation
(2 ms peak-to-peak). As
described in an earlier memo, the pmtras_analysis routine can be used to
determine the roll period that is then input into the imaging routines. Absolute roll aspect can also be determined
at present, but at an unacceptable level of inconvenience. Integration of the PMTRAS output into the
aspect solution is very close to completion, at which time a complete,
user-transparent, absolute aspect solution will be available.
Overall, the spectrometer is operating well and has yielded
splendid high-resolution continuum spectra of flares. Background lines are also readily seen and so nuclear
spectroscopy awaits only solar cooperation.
The cooler is operating well at about 50 watts with low vibration
levels (~10 mG), keeping the detectors in the temperature range 72 - 75 K.
Seven of the nine germanium detectors are operating
perfectly with the nominal 1-keV resolution and 3-keV threshold levels.
Detector 7 is operating as it did during integration with 3
keV resolution and a threshold of 7 to 8 keV.
After the first week or so, detector 2 has been operated as
a single large detector (viz, no segmentation) that provides a 25 keV threshold
and 10 keV resolution. An on-board
software configuration issue inadvertently suppressed events from detector 2
from Feb 26 through March 8.
Detector 8 operates normally, except for demonstrating a
susceptibility to RFI from the rear transmitter. The transmitter is on for about 50% of the time during ground
contacts. This represents an average of
about 6 5-minute sunlit periods per day.
Suitable software filters will be implemented, but in the meantime,
users should disregard data from detector 8 during these intervals.
Accurate energy calibrations of all detectors have been
achieved using known lines in the background spectrum. Compared to prelaunch predictions,
background levels range from expected levels at equatorial latitudes to ~x2
higher at high geomagnetic latitudes.
When background levels are an issue, a significant improvement in
background reduction for front segment events can be obtained by considering
only events in anticoincidence with rear segment events.
The front segments of all detectors are showing substantial
periods of dead time (data gaps) that last from a few milliseconds to several
tenths of a second. This is believed
to be due to large amounts of charge deposited by hi-Z cosmic rays. The most important immediate consequences
are that uncorrected, high-time-resolution, single-detector light curves
exhibit obvious data gaps; and that image quality is degraded, particularly for
integration times less than ~30 seconds.
The good news is that a reliable RESET-based signature of such
occurrences has been identified and the live-time algorithm is being adapted
accordingly. When these corrections
are implemented, the effects of the data gaps on imaging and composite light
curves should be largely eliminated, except for an increase in statistical
At present, while the software and hardware supports
science-quality continuum spectroscopy, early spectroscopy will require
considerable care. In the coming
weeks, users can anticipate improvements in background subtraction algorithms
and convenience. During their initial
analyses, users should be aware that the instrument response is still uncertain
at energies below 12 keV.
With some important caveats, 7-arcsecond imaging with
subcollimators 3 through 9 is now possible.
Image quality is currently constrained by the data gap issue discussed
above. Until the software incorporates
this into the dead time, image quality will be limited, although useful results
have been obtained with CLEAN and temporary versions of MEM-SATO and
PIXONS. Phase calibration for grids 3
through 9 is preliminary but usable and will be improved over the next few
weeks. Phase calibration for grids 1
and 2 will be done when suitable compact flares have been identified, and the
high-resolution aspect solution becomes available.
Both the thick and thin attenuators have been exercised
successfully. Data has been acquired in
attenuator states 0 and 1 ('both-out' and 'thin-in' respectively). With both attenuators open, significant steady-state solar count rates
have been observed, along with flares as weak as the GOES B2 level. The on-board algorithm for automated
attenuator operation is currently under test.
For the most part, operations are running smoothly. Memory management strategies (e.g. event
decimation models) continue to be evaluated.
Data are being converted to the level-0 format using automated
algorithms with a latency of a day or two.
Catalog information will be available shortly, at which time it will be
added to previously processed data. In
particular, flare locations await the integration of the absolute roll aspect
Direct comparisons with complementary observations - GOES,
SOHO (EIT and MDI), TRACE, radio (Owens Valley, Nancay, and Nobeyama), and
H-alpha (Big Bear) - are readily accomplished thanks to the mirroring of these
data sets and their accessibility through the RHESSI software. One identified weak point in the
complementary observations is the paucity of high-time resolution (~5 s) imaging
data in optical, EUV or X-ray wavelengths.
This list of issues indicates that analyzing RHESSI data will
be more challenging (but of course more rewarding) than working with the
simulated data. At first, it will not
be easy. But as we move up the learning
curve and as the software and calibrations are adapted to meet both our needs
and the demands of RHESSI's real-world performance, it will get
easier. At the same time, we can expect the quality of our images,
spectra, etc to improve significantly.
Because of this, users doing early analyses may want to 'do the easy
stuff first' and avoid the temptation to immediately push the RHESSI data
to its limits in terms of image quality, energy range, statistics, etc.
Documentation concerning data access and software can be
Please send bug reports to
Good luck !