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Last modified: September 2016

AHELP for CIAO 4.9


Context: tools


Correct a Chandra aspect solution for temporal drift calculated from a fit of the drift in the sky positions of the events in a supplied circle region.


correct_periscope_drift  infile evtfile outfile [corr_plot_root] [x]
[y] [radius] [src_min_counts] [corr_poly_degree] [verbose] [clobber]


Thermal cycling on the spacecraft can result in an apparent temporal drift of the sky position of an X-ray source during an observation. This appears as a drift of up to about 0.5 arcsec in X-ray event sky X, Y coordinates over time. Because of the thermal variation time scales, this effect is usually most prominent in long observations (more than about 50 ksec).

As of DS 8.4, a drift correction is applied to the aspect solution using the periscope gradients telemetry. However, temporal drifts have continued to increase with thermal variation of the spacecraft. Therefore the Aspect team suggests that to accomplish science related to sub-arcsec source structure, users should follow this thread to correct residual drift induced by the periscope. This requires a relatively bright, on-axis source (within a few arcmin off-axis angle) to perform a "self-calibration" of the aspect solution.

This tool takes as input a source aspect solution and event files and returns a new/corrected aspect solution that may be applied to the events to correct residual temporal drift.


% correct_periscope_drift infile=pcadf537654279N001_asol1.fits.gz
evtfile=acisf16659N001_evt2.fits.gz x=4133.76 y=4078.74 radius=6
outfile=driftcorr_asol1.fits verbose=2
Running: correct_periscope_drift
  version = 0.1
with parameters:
  and ASCDS_INSTALL is /soft/ciao-4.8
Fitting a line to the data to get reduced stat errors
Fitting a polynomial of degree 2 to the data
Fitting a line to the data to get reduced stat errors
Fitting a polynomial of degree 2 to the data
Fit results
Events show drift range of 0.24 arcsec in yag axis
Max absolute correction of 0.14 arcsec for yag axis
Events show drift range of 0.20 arcsec in zag axis
Max absolute correction of 0.11 arcsec for zag axis
Writing out corrected aspect solution file to driftcorr_asol1.fits
To review fit see correction plots in:


name type ftype def min max reqd stacks
infile file input       yes no
evtfile file input       yes no
outfile file output       yes no
corr_plot_root string output       no no
x float     1      
y float     1      
radius float     0      
src_min_counts float   250 0      
corr_poly_degree integer   2 1 8    
verbose integer   1 0 5    
clobber boolean   no        

Detailed Parameter Descriptions

Parameter=infile (file required filetype=input stacks=no)

Input Chandra aspect solution.

Parameter=evtfile (file required filetype=input stacks=no)

Input Chandra event file (evt1 or evt2).

The event file should have been made with/processed with the aspect solution supplied with infile (evtfile ASOLFILE header key value should match infile)

Parameter=outfile (file required filetype=output stacks=no)

Output filename for corrected aspect solution file

Parameter=corr_plot_root (string not required filetype=output stacks=no)

Output prefix for fit and data plots.

The output prefix for fit and data plots. The output names will look like ${corr_plot_root}_fit_${axis}.png and ${corr_plot_root}_data_${axis}.png where ${axis} will be 'yag' or 'zag'.

Parameter=x (float min=1)

Sky X position of center of circle region used to extract events to fit correction.

Parameter=y (float min=1)

Sky Y position of center of circle region used to extract events to fit correction.

Parameter=radius (float min=0)

Radius in pixels of circle region used to extract events to fit correction.

Parameter=src_min_counts (float default=250 min=0)

Minimum number of counts required in extracted source region (below which the tool will complain and not fit).

Parameter=corr_poly_degree (integer default=2 min=1 max=8)

Degree of polynomial used in the fit model to fit and correct drift in the two Aspect Camera axes.

Parameter=verbose (integer default=1 min=0 max=5)

Amount of tool chatter level.

Parameter=clobber (boolean default=no)

Overwrite output files if they already exist?


Bad fits

This tool is provides output plots of the fits of the events. The user must evaluate those plots to determine if a correction is warranted and if the output fit is reasonable. This tool should not be blindly applied to observations in aggregate.

Cannot be run headless or without ChIPS

The output plots are not optional.

Counts needed for good fit

The tool is not intended for short observations (less than 50ks) and will not fit observations well without a bright point-like source.

Multiple aspect solutions files

This tool is only intended for observations with single aspect solution files.

About Contributed Software

This script is not an official part of the CIAO release but is made available as "contributed" software via the CIAO scripts page. Please see the installation instructions page for help on installing the package.

See Also

asphist, dither_region, reproject_aspect, reproject_image, reproject_image_grid, sky2tdet, skyfov

Last modified: September 2016
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