Aspect Solution Files
The aspect solution describes the pointing of the telescope as determined by the Pointing Control and Determination (PCAD) system. The PCAD uses three instruments to determine the alignment of the detectors relative to the sky. They are:
- two 2-axis gyroscopes
- a 4.25" star-tracking optical CCD camera
- an internal set of fiducial lights (LEDs mounted on the science instrument focal planes)
The accurate determination of pointing is necessary to take full advantage of the high spatial resolution of the HRMA. During a normal science observation, Chandra is continually "dithering" in a repetitive 8" (ACIS) or 20" (HRC) Lissajous pattern to minimize the affect of bad pixels and lessen the effect of radiation on the detectors. The aspect solution therefore allows the ground processing pipeline to remove this dither pattern (as well as any other random disturbances) and assign an RA and Declination to each detected photon.
The string of numbers in the file name (e.g. "pcadf111767021N001_asol1.fits") refers to the start time of the period for which the aspect solution is valid. The time is measured in seconds, starting at 0h UT, Jan 1, 1998.
In many instances more than one asol file will be included in the package of data products. The files should be provided to the CIAO tools as a comma-separated list or as a stack. They may also be merged with the tool dmmerge to create a single asol file for the observation.
Creator pipeline: ASP1
There are no creator tools; the aspect solution files are generated internally.
- Chandra Aspect page - the prime source for the latest information on aspect determination.
- Positional Accuracy Monitor - the accuracy of the aspect solutions.
|ra||degrees||RA of MNC frame (x-axis). The Mirror Nodal Coordinates (MNC) have their origin at the optical axis. See the coordinates manual for more info.|
|dec||degrees||Dec of MNC frams (x-axis). See 'ra' description.|
|roll||degrees||Roll of MNC frame. See 'ra' description.|
|ra_err||degrees||uncertainty in RA|
|dec_err||degrees||uncertainty in DEC|
|roll_err||degrees||uncertainty in Roll|
|dy||mm||Shift of STF frame realtive to FC frame. The SIM (Science Instrument Module) Translation Frame (STF) takes into account the position of the optical bench and the instrument's position on the bench. The Focal Coordinate (FC) system is centered on the telescopes nominal focus. The difference between STF and FC is usually very small, and due to shifts in the telescope structure. For more info see the coordinates manual.|
|dz||mm||Shift of STF frame relative to FC frame. See 'dy' description.|
|dtheta||degrees||Rotation of STF frame relative to FC frame. See 'dy' description.|
|dy_err||mm||Uncertainty in dy|
|dz_err||mm||Uncertainty in dz|
|dtheta_err||degrees||Uncertainty in dtheta|
|roll_bias||degrees/second||Change in the Telescope roll, as determined by gyroscope.|
|pitch_bias||degrees/second||Change in the Telescope pitch, as determined by gyroscope.|
|yaw_bias||degrees/second||Change in the Telescope yaw, as determined by gyroscope.|
|roll_bias_err||degrees/second||Uncertainty in roll_bias|
|pitch_bias_err||degrees/second||Uncertainty in pitch_bias|
|yaw_bias_err||degrees/second||Uncertainty in yaw_bias|