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Last modified: 10 Mar 2015

URL: http://cxc.harvard.edu/ciao/threads/reproject/

Overview: Reprojecting Files

CIAO 4.9 Science Threads


Overview

Synopsis:

This thread describes the tools available in CIAO for reprojecting event, image, and aspect files, and provides links to the threads that use them.

Last Update: 10 Mar 2015 - Added reproject_obs and merge_obs


Contents


reproject_obs: reproject and merge event files for multiple observations.

reproject_obs is a script that uses reproject_events to reproject the event files for a stack of observations to a common tangent point and then merges them together with dmmerge. The user may input the location of the new tangent point or the script will compute a mean value from the input files.

The script also provides diagnostic information based on the observation configurations since combining observations with different aimpoints or different CCDs may limit the usefulness of merged event file. Finally, the script also manages the auxiliary files for each observation (bad-pixel files, aspect solutions, etc) by providing either copies or symbolic links to the files CIAO uses.

Threads:

This tool does not apply any astrometric correction to the data. Users needing to align their data should follow the Correcting Absolute Astrometry thread before combining observations.


merge_obs : combine observations and produce exposure corrected images.

merge_obs run the reproject_obs script followed by the fluximage script to create exposure corrected images for a combine dataset in the user specified energy bands.

Threads:

This tool does not apply any astrometric correction to the data. Users needing to align their data should follow the Correcting Absolute Astrometry thread before combining observations.


dmregrid: rebin and combine a stack of images

dmregrid regrids each input image based on the binning specification, offsets, rotation angle, and center of rotation. The stack of regridded images is summed into a single output file.

This tool was previously used to combine the exposure maps in the Multiple Chip ACIS Exposure Map thread. The thread now uses reproject_image instead.


reproject_aspect: minimize position differences between two files

reproject_aspect applies corrections for translation, scale, and rotation to the WCS of a file. It compares two sets of source lists from the same sky region. If three or more sources are found to be a close match in position, the tool calculates the transformation that relates the two files and updates the WCS mapping from SKY(X,Y) to (RA,Dec) either by modifying the aspect solution or by revising the WCS keywords, depending on how the parameters are set.

reproject_aspect is actually a script which runs two tools: wcs_match and wcs_update. These tools may be run individually for slightly more flexibility; see the help files for details.

This tool is the most accurate method of reprojecting a file, but does depend on source lists for each observation being available. Note that the source list from the archive (src2.fits file) is not accurate enough for use with this tool. Users should create a new source list by running one of the detect tools; wavdetect is recommended because of its ability to separate closely-spaced point sources. The Running wavdetect thread illustrates how to use the tool.

Threads:


reproject_events: regrid an event file to a common tangent point

reproject_events calculates new sky coordinates for the input event file using the WCS from the match file. There are two modes of operation:

  1. the new sky coordinates are evaluated assuming the old aspect solution was correct
  2. the user supplies a new aspect solution and the sky coordinates are calculated from scratch, independent of the original aspect solution.

Refer to the help file for details on each approach.

Threads:

reproject_events supports large mosaics that exceed the standard image sizes (ACIS 8k x 8k, HRC-I 32k x 32k). The data range (FITS TLMIN/TLMAX keywords) are updated based on the reprojected data. This allows CIAO and applications like ds9 to make images that cover the entire range of the data.


reproject_image: match one image to another

reproject_image maps an image in one WCS reference frame to the WCS of a match image. If a match image is not available, use reproject_image_grid instead.

reproject_image has a parameter (method) which controls whether the input files are summed or averaged together; "sum" is typically used to reproject the counts image and "average" is used to reproject the exposure map when making fluxed images.

Threads:


reproject_image_grid: project an image to a WCS grid

reproject_image_grid maps an image in one WCS reference frame to a specified WCS grid. This tool works similarly to reproject_image, but does not require a match image.

reproject_image_grid has a parameter (method) which controls whether the input files are summed or averaged together; "sum" is typically used to reproject the counts image and "average" is used to reproject the exposure map when making fluxed images.

Threads:


sso_freeze: reproject a solar system object

sso_freeze reprojects the coordinates in an event file from the frame fixed to the celestial sphere to the frame of the object. This eliminates the blur that occurs when an object (e.g. a planet or comet) is observed moving with respect to the observer. The tool can also create an object-centered aspect solution file, which may then be used to generate an exposure map or response file (ARF) for the data.

Thread:


Note: using stacks of input files

All of the reproject tools can take a stack of files in the input parameter, but they do not all handle the stack in the same way. For instance, reproject_events creates a separate output file for each item in an input stack, while reproject_image creates a single merged image from the stack. Refer to the individual tool help files for details on how they handle a stack of input files.


History

04 Apr 2006 new for CIAO 3.3: original version
24 Apr 2006 The Correcting Absolute Astrometry with reproject_aspect thread is now available; link added to this thread
06 Jun 2006 added "Merging Data from Multiple Imaging Observations" thread to reproject_aspect section
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 (added Object-centered Aspect Solution thread link)
07 Jan 2009 reviewed for CIAO 4.1: no changes
08 Feb 2010 reviewed for CIAO 4.2: no changes
13 Jan 2011 reviewed for CIAO 4.3: no changes
13 Jan 2011 added dmregrid as a reprojection option
08 Jul 2011 added a link to fluximage in the reproject_image section
10 Jan 2012 reviewed for CIAO 4.4: no changes
03 Dec 2012 Review for CIAO 4.5; new note about reproject_events updating the range of the image to match the limits of the data to make large mosaics.
01 Jul 2014 Minor updates to reflect the behavior of the fluximage, merge_obs, and reproject_obs scripts.
22 Dec 2014 Reviewed for CIAO 4.7; no changes.
10 Mar 2015 Added reproject_obs and merge_obs


Last modified: 10 Mar 2015
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