The Aspect Solution & pcad_asol1.fits Files
Introduction
The aspect solution describes the orientation of the telescope as a function of time. The detected position of an event and the corresponding telescope aspect are combined for an accurate determination of the celestial position of that event. The most current aspect-related information, including accuracy reports, is located on the Chandra Aspect page.
Collecting & Storing Aspect Information
Aspect data is collected through three instruments of the Pointing Control and Attitude Determination (PCAD) system. Gyroscopes are used to measure the short-term changes in the aspect, while a 4.25 inch optical telescope images guide stars for long-term tracking on the celestial coordinate system. Finally, a set of fiducial lights from the Science Instrument Module (SIM; where the ACIS and HRC detectors are located) is projected onto the optical imager to track the internal movement and flexure (focusing mechanism) of the telescope. Star positions from Hipparcos and other astrometric surveys are used to put the aspect solution into a standard reference frame.
The aspect information is stored in the aspect solution files, which are of the form pcad<TSTART>_asol1.fits, where <TSTART> is taken from the header keyword of the same name (e.g. the aspect solution file for ObsId 1838 is pcadf084244404N003_asol1.fits). The asol1 file(s) are located in the primary directory of the standard data distribution; if you don't see the files there, check the secondary/aspect directory. For more details about this file, see the ASOL section of the Data Products Guide.
For some observations there may be more than one aspect solution file. The aspect solution is only computed when the solution is stable, also know as being in "Kalman lock." That is the Kalman filter used to combine the star, FID, and gyro data into the final aspect solution has small residuals errors. Any number of things can cause a break in Kalman lock ranging from satelite operations, to external influences, to software rate limiting parameters. These breaks in Kalman lock often introduce short gaps (few seconds) in the aspect solution which are reflected in the Level 2 Good Time Intervals (and the Level 1 Mission Time Line).
Note: The time range covered by the asol files is not expected to be the same as the time ranges in the event files. It is computed with little knowledge about which detectors are in the focal plane and their status (eg powered on, collecting data, etc).
IMPORTANT: There is a set of files located in the secondary directory with the file extension osol1.fits. These may not be used in place of the aspect solution files describe above. The asol1.fits files are the result of ground-based aspect analysis using all PCAD data, while the osol1.fits files are based on aspect determined by the on-board computer. Ground processing gives a better product because more information is available. The osol1.fits files are NEVER needed in Chandra data analysis.
Using the pcad_asol1.fits Files
Since an aspect solution file is created for each stable aspect interval, there is often more than one asol1.fits file for an observation.
If there is a single aspect solution file, it can be provided directly to the tool:
unix% pset asphist infile=pcadf245733620N002_asol1.fits
In some cases, there will be more than one aspect solution file for an observation. Then all the files must be provided in chronological order. (The time is in the filename, so "ls" lists them in chronological order.) This may be done as a comma-separated list:
unix% pset asphist infile="pcadf063874624N002_asol1.fits,pcadf063875522N002_asol1.fits,pcadf063902942N002_asol1.fits"
The list of aspect solution files may also be given as a stack. Any number of files can be specified in the stack, one:
unix% cat pcad_asol1.lis pcadf245733620N002_asol1.fits unix% pset asphist infile=@pcad_asol1.lis
or more than one:
unix% cat pcad_asol1.lis pcadf063874624N002_asol1.fits pcadf063875522N002_asol1.fits pcadf063902942N002_asol1.fits unix% pset asphist infile=@pcad_asol1.lis
Note that an at symbol ("@") is used before the filename to indicate that it is a stack file.
They may also be combined into a single aspect solution file by running dmmerge:
unix% dmmerge \
"pcadf063874624N002_asol1.fits,pcadf063875522N002_asol1.fits,pcadf063902942N002_asol1.fits" \
full_asol.fits
However, one must be sure that all the meta-data was cleanly combined. In particular, the SIM related keywords must be idential for this dmmerged file to be valid.