Last modified: November 2020

AHELP for CIAO 4.16


Context: Tools::Aspect


Applies off-axis boresight correction to an L1 Aspect solution file (ASPSOL).


asp_offaxis_corr infile instrume [geompar] [verbose]


The aspect camera measures an offset versus time between the ACA (Aspect Camera Assembly) frame and the SIM (Science Instrument Module) using fiducial lights mounted on the instrument. It was initially thought that the ACA and HRMA (Chandra's High Resolution Mirror Assembly) were firmly attached and that the observed time variation in the offsets was between ACA/HRMA assembly and the SIM, but recent analysis has shown that the ACA and HRMA frames are moving with respect to each other but the HRMA and SIM frames are not. Observed drifts in CHIP position over time had been incorrectly interpreted as the HRMA optical axis also moving in CHIP position and a boresight offset correction was applied between the CHIP and DET (mirror) plane.

The new approach, implemented by this tool, is to move the boresight correction from being between the CHIP and DET plane to between the DET and SKY plane. This correction results in zeroing out the boresight offset columns (DY, DZ, DTHETA), and converts the pointing data (RA, DEC, ROLL) to be relative to the new optical axis. The pointing quaternions, Q_ATT, are also updated to match the new RA, DEC, and ROLL values.

Applying this correction to the aspect solution and reprocessing the event file with the resulting corrected solution will not make significant changes to photon celestial coordinates, but the detector coordinates (and the resulting off-axis angles) for each photon will change by up to of order 30 arcseconds.


This tool updates the input file in place. Care has been taken such that a problem during processing will not update the file, but leave it in the original state.

This correction can only be applied once. The tool adds a BOREVER=2 keyword indicating that the boresight correction has been applied.


Example 1

asp_offaxis_corr pcadf000000000N001_asol1.fits instrume="acis"

Apply boresight correction to the provided L1 Aspect Solution file using the ACIS pixel scale paramters.

Example 2

asp_offaxis_corr pcadf000000000N001_asol1.fits

Apply boresight correction to the provided L1 Aspect Solution file using the pixel scale paramters for the instrument identified in the L0.5 obspar file 'axaf1234_obs0a.par'.


name type ftype def min max reqd
infile file input       yes
instrume string input       yes
geompar string input geom     no
verbose integer   0 0 5 no

Detailed Parameter Descriptions

Parameter=infile (file required filetype=input)

Input L1 Aspect Solution file (ASPSOL)

This tool operates only on single files, stack syntax is supported, but the expectation is that there is only 1 entry.

Parameter=instrume (string required filetype=input)

Instrument name

Case insensitive instrument name ("hrc" or "acis"), indicates which set of pixel scale data to obtain.

Parameter=geompar (string not required filetype=input default=geom)

Parameter file for Pixlib Geometry files

Defaults to CXCDS default pixlib ARD parameter file, "geom.par"

Parameter=verbose (integer not required default=0 min=0 max=5)

Verbose output level (0-5)

Changes in CIAO 4.16

See Also

coords, level, pileup, times
autoname, ciao, ciao-install, history, parameter, stack, subspace
dm, dmascii, dmbinning, dmfiltering, dmmasks, dmopt, dmregions
asphist, dither_region
dmcoords, reproject_aspect
aconvolve, acrosscorr, arestore, dmfilth, dmregrid
arfcorr, make_psf_asymmetry_region, psf_project_ray, simulate_psf, src_psffrac