The first version of the long-awaited RHESSI roll aspect data base is
now available on ATEST.
This database contains a mission long RHESSI roll solution tabulated
at 64s intervals. The new software on ATEST defaults to using this
database instead of calculating a custom PMTRAS roll solution for each
analysis. The main advantage of this is the (hopeful) elimination of
the star identification problem which occasionally gave incorrect roll
aspect. Secondary benefits are that the roll solution is faster, more
accurate, more consistent, and will support more complete flare
location lists, extensive self calibration of the PMTRAS system and
long integration intervals for non-solar applications.
The initial posted database covers only February through May 2002. A
larger version covering through 31 July 2003 has been generated and
will be posted in a few days if there are no reported problems with
the initial period. The database will be updated regularly thereafter
to incorporate more recent data. It currently requires about 1 Mbyte
per month, although this should be significantly reduced in future.
Although the roll database is over 90% complete, there are a large
number of individual gaps due to the absence of PMTRAS data, points
with lower quality solutions, and cases where data were acquired under
'unusual circumstances' (such as periods of offpointing, spin-up,
etc). These gaps will be gradually filled in by interpolation, custom
PMTRAS analyses and the use of CCD-RAS solutions.
The IDL log window will indicate whether the roll database is being
used for a given analysis and will issue a warning if there is are any
potentially serious gaps in the roll solution at the time of your
analysis. (For high-resolution imaging, gaps of more than a few
minutes may degrade the results.) If the roll database cannot be used
at all for your interval, the software will automatically revert to
doing a custom PMTRAS solution as before. In case of significant gaps
(or any other reason), you can override the use of the database by
specifying the control parameter, as_roll_solution='PMT'.
im_obj ->set, as_roll_solution='PMT' ;when using the command line or
hessi, as_roll_solution='PMT' ; when starting the GUI.
(Other valid as_roll_solution arguments are 'RAS' or the new default,
Three PMTRAS-related logs are being maintained on
roll_dbase_gaps.txt is a time ordered list of gaps in the current
database. roll_dbase_status.doc provides updates on the status of the
roll aspect database. pmtras_major_event_log.doc indicates 'major
events' (excluding gaps) affecting PMTRAS performance.
The database (and the new version of PMTRAS_ANALYSIS on ATEST) also
corrects a 1 millisecond timing error in all roll solutions obtained
to date. For all data acquired since 26 February 2002 the correction
of this error will rotate the absolute positions of all images by 1.5
milliradians clockwise about Sun center. This corresponds to a
position error on the Sun of less than 1.5 arcseconds (worst case)
except for the first 3 weeks after launch when the error was 3 times
larger. This error has had no effect on spectroscopy, image
morphology, image photometry, radial source locations or on relative
positions as a function of energy or time.
Please let me know if encounter any problems, have suspicions that
the roll database may be providing an incorrect solution for your time
interval, have a particular gap that you would like filled ASAP or if
you have any questions.
Thanks for your patience.