Overview: Reprojecting Files

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:

• Correcting Absolute Astrometry with reproject_aspect thread

• The Correcting Aspect Prior to Merging section of the Merging Data from Multiple Imaging Observations has an example of using wcs_update as a standalone tool.

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, and
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 is called by the merge_all script, which is used in many of the CIAO Imaging threads to merge observations and create exposure maps.

• Merging Data from Multiple Imaging Observations

• Using the ACIS "Blank-Sky" Background Files

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:

• Reprojecting Images: Making an Exposure-corrected Mosaic

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:

• Reprojecting Images: Making an Exposure-corrected Mosaic

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.

Thread:

• Reprojecting Coordinates of a Solar System Object

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.