Last modified: 7 November 2022


Aspect Solution Files

Filename Description Content ICD SDP Pipeline
pcad_asol1.fits aspect solution ASPSOL v2.4 ASP1


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:

  1. two 2-axis gyroscopes
  2. a 4.25" star-tracking optical CCD camera
  3. 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.

ASOL-specific columns:

Column Name Units Description
time seconds -
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
q_att[4] - -
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