Aspect Solution File
| Level | Instrum | ICD | Data product | Content | Pipeline | Filename template |
|---|---|---|---|---|---|---|
| L1 | PCAD | v2.4 | Aspect Solution File | ASPSOL | ASP1 | pcad...asol1.fits |
Aspect Solution File
| Level | Instrum | ICD | Data product | Content | Pipeline | Filename template |
|---|---|---|---|---|---|---|
| L1 | PCAD | v2.4 | Aspect Solution File | ASPSOL | ASP1 | pcad...asol1.fits |
Description:
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.
In some instances, more than one asol file will be included in the package of data products. The files may be merged with the tool dmmerge, to create a complete asol file for the observation. Alternatively, many tools which require the asol file(s), such as asphist, will accept a list of asol files, as a stack.
file name template:
pcad*_asol1.fits - the string of numbers in the asol 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.
creator pipeline:
ASP1
creator tool:
no tool (asol files generated internally).
Useful links:
- Chandra Aspect page - the prime source for the latest information on aspect determination.
- Positional Accuracy Monitor - Up to date info on the accuracy of the aspect solutions.
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 |
