Reprojecting Coordinates of a Solar System Object
CIAO 4.15 Science Threads
Overview
Synopsis:
When an object is observed moving with respect to the observer, e.g. a planet or comet, the resulting event file looks "blurred" when viewed in (x,y) coordinates. The sso_freeze tool reprojects the coordinates to the frame of the object, eliminating this blurring.
This thread contains general information on running sso_freeze. The Object-centered Aspect Solution and Exposure Map thread explains how to use the object-centered aspect solution file.
chandra_repro will now automatically run sso_freeze if it locates a solar-system ephemeris file in the primary directory. To check if sso_freeze has been run, look for the Object Centered coordinate columns, ocx,ocy, in the output file
unix% dmlist 1463/repro/acisf01463_repro_evt2.fits cols | grep ocx,ocy 17 oc(ocx,ocy) pixel Real8 0.50: 8192.50 - Object centered coordinates
Users must not (re-)run sso_freeze on an event file with existing Object Centered coordinate columns. This triggers a bug in sso_freeze which will create new columns with the same name; this causes problems with other tools.
Purpose:
To reproject coordinates from the frame fixed to the celestial sphere to a reference frame which is fixed to the moving solar system object (SSO).
Related Links:
Last Update: 05 Dec 2022 - Updated for CIAO 4.15. chandra_repro will now automatically run sso_freeze for solar system objects. Users must not re-run it again to avoid a bug that creates duplicate columns in the output.
Contents
- Choosing the Ephemeris Files
- Run sso_freeze
- Display the Results
- Parameter files:
- History
- Images
Choosing the Ephemeris Files
Download the sample data: 1463 (ACIS-S, Jupiter)
unix% download_chandra_obsid 1463 evt2,asol,eph1
There are two ephemeris input files needed for the tool to run: the spacecraft ephemeris file (scephemfile) and the SSO ephemeris file (ssoephemfile). Both of these files are included in the primary directory.
unix% ls *eph1.fits jupiterf059875200N001_eph1.fits jupiterf059875200N002_eph1.fits orbitf059443264N002_eph1.fits
The ephemeris files are selected based on the start time (TSTART) of the observation:
unix% dmkeypar acisf01463N006_evt2.fits TSTART echo+ 59968797.485273
Make sure that the timestamp in the filenames is less than the start time of the data. Also, if there is more than one version of a file - e.g. N001 and N002 - choose the newest (higher number) version available. The two ephemeris files for this observation are orbitf059443264N002_eph1.fits and jupiterf059875200N002_eph1.fits.
Spacecraft and SSO ephemeris files are also available from the CDA FTP site (cda.harvard.edu) if you have trouble retrieving them via WebChaser. Login as "anonymous", using your email address as the password; the files are in the /pub/arcftp/bary/ephem directory.
Run sso_freeze
Once you have the necessary input files, you are ready to run sso_freeze.
In many cases, there will be more than one aspect solution file (pcad_asol1.fits) for an observation. All the files must be input to the asolfile parameter, either as a comma-separated list or as a stack. Here we use:
unix% cat pcad_asol1.lis pcadf059968984N004_asol1.fits unix% punlearn sso_freeze unix% pset sso_freeze infile=acisf01463N006_evt2.fits unix% pset sso_freeze scephemfile=orbitf059443264N002_eph1.fits unix% pset sso_freeze ssoephemfile=jupiterf059875200N002_eph1.fits unix% pset sso_freeze asolfile=@pcad_asol1.lis unix% pset sso_freeze ocsolfile=1463_oc_asol1.fits unix% pset sso_freeze outfile=frozenjupiter.fits unix% sso_freeze Input event file or stack (acisf01463N004_evt2.fits): Input spacecraft ephemeris file (orbitf059443264N002_eph1.fits): Input solar system object file (jupiterf059875200N002_eph1.fits): Output event file name (frozenjupiter.fits):
The contents of the parameter file may be checked using plist sso_freeze.
Display the Results
The new object-centered coordinates are stored in the ocx and ocy columns of the file. When displaying the "frozen" image, we must specify to bin on those columns (note that the binning syntax for ds9 is not the same as for the DM):
unix% ds9 "frozenjupiter.fits[bin=ocx,ocy]" &
Alternatively, the image can be loaded in the usual way into ds9:
unix% ds9 frozenjupiter.fits &
Then the binning may be changed with the "Bin → Binning Parameters..." dialog box. Choose "ocx" and "ocy" with the buttons in the "Bin Columns" section.
Figure 1 shows the original and "frozen" images side-by-side.
[Version: full-size]
Figure 1: Comparison of original and "frozen" images
The Object-centered Aspect Solution and Exposure Map thread explains how to use the object-centered aspect solution file.
Parameters for /home/username/cxcds_param/sso_freeze.par #---------------------------------------------------------------------- # # sso_freeze.par- Parameter file for planetary transformations routine # #---------------------------------------------------------------------- infile = acisf01463N006_evt2.fits Input event file or stack scephemfile = orbitf059443264N002_eph1.fits Input spacecraft ephemeris file ssoephemfile = jupiterf059875200N002_eph1.fits Input solar system object file outfile = frozenjupiter.fits Output event file name (asolfile = @pcad_asol1.lis) Input aspect solution file (ocsolfile = 1463_oc_asol1.fits) Output asol in OC coordinates (logfile = STDOUT) debug log file ( STDOUT | stdout | <filename>) (scale = 0) Image pixel scale (km/pixel, 0=angular coords) (lookuptab = ${ASCDS_CALIB}/dmmerge_header_lookup.txt -> /soft/ciao/data/dmmerge_header_lookup.txt) lookup table (clobber = no) Overwrite existing output dataset with same name? (verbose = 0) debug level (0-5) (mode = ql)
History
03 Jan 2005 | reviewed for CIAO 3.2: no changes |
16 Dec 2005 | updated for CIAO 3.3: bug in ChaSeR means that orbit ephemeris files must be retrieved from the CDA FTP site |
01 Dec 2006 | reviewed for CIAO 3.4: no changes |
18 Jan 2008 | updated for CIAO 4.0: sso_freeze can create object-centered aspect solution (Object-centered Aspect Solution thread); include aspect solution files in sso_freeze run; parameter file updated with new asolfile and ocsolfile |
04 Feb 2009 | updated for CIAO 4.1: image is inline |
15 Jan 2010 | reviewed for CIAO 4.2: no changes |
13 Jan 2011 | reviewed for CIAO 4.3: no changes |
10 Jan 2012 | reviewed for CIAO 4.4: no changes |
03 Dec 2012 | Review for CIAO 4.5; no changes |
23 Dec 2014 | Reviewed for CIAO4.7. Updated image; minor edits. |
14 Feb 2022 | Reviewed for CIAO 4.14. No changes. |
05 Dec 2022 | Updated for CIAO 4.15. chandra_repro will now automatically run sso_freeze for solar system objects. Users must not re-run it again to avoid a bug that creates duplicate columns in the output. |