WCSFixer Help
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Last Modified: Jan 7, 2006
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Introduction
WCSFixer is designed to correct the WCS of an image using the
specified reference catalog and the input WCS as a starting point. It is
implemented entirely as IRAF CL scripts, using a standard CGI-POST
method to invoke the script at the host level. Javascript embedded in the
output stream drives the user interface, providing a richer web application
and progressive display of results. These same scripts may be used from the
astronomer's desktop and would operate as a normal IRAF task. See below for
planned development.
The four Demo buttons will automatically fill out various form
elements and submit the demo image (via URL) for processing. Each demo
shows a different aspect of the task operation and are explained in the
detailed talking points (TBD).
Currently configured demos include:
- Demo 1:
- Large rotation and small shift image using
default parameters
- Demo 2:
- X-Ray image using the thresholding detection method.
Note the larger sample of reference stars needed for
the solution.
- Demo 3:
- Simple field, 20 arcmin correction using
user-supplied starting values for the field.
- Demo 4:
- Simple field, 10 arcmin linear correction.
As of 1/06/06:
- Coordinate readout not correct for rotated WCS
- Possible problems with SDSS/Nomad catalogs
- Image File
- The name of a local FITS file on your machine to submit for processing.
Filenames entered by hand must include the full path, the
Browse button can be used to navigate to a file. Files may
be compress via GZIP and should contain only a single image
per file (i.e. no MEFs at the moment). Filename extensions (e.g.
".fits" are not required).
- URL
- The URL to a file to be processed. The system will download the remote
file automatically, once corrected the file is available for download
using the "Download Fixed Image" button.
- Reference Catalog
- The reference catalog to use for the plate solution. USNO catalogs are
hosted locally at NOAO for speed and reliability, other catalogs are
offered but are not well tested at the moment and may be subject to
service interruptions.
- Help
- Display this page. This button will toggle to "Show Results" once the
help page is displayed
- Reset
- Reset the form to the default values.
- Submit
- Submit the given Image or URL for processing. Only one
or the other value must be supplied, specifying both will cause an
error box to appear. Use the Reset to reinitialize the form
if necessary.
- Clip catalog to image boundary?
- The reference catalog is queried for all stars in a search
radius covered by the given image. This circular pattern leads to
some number of stars which naturally fall outside the rectangular
footprint of the image. To maximize the possible overlap between
the image and reference catalog, this option allows the catalog to
be clipped to the image boundaries so that only stars in the field
(as determined from the initial WCS coordinates) are used for the
crossmatch. In cases where the shift/rotation may be large these
clipped stars may be required for the solution and a second attempt
will automatically be made that includes these stars if no solution
is found in the first pass.
- Ellipticity Filter?
- If enabled, detected objects with an ellipticity greater than the limit
set by the Cutoff parameter will be filtered from the image
catalog. This is done in an attempt to use only isolated stellar
objects when matching the catalog. In crowded fields, it may be
necessary to disable this parameter to get enough catalog objects to
successfully solve the image.
- Split detected objects?
- Segment blended objects in the image into individual components. This
option increases the proessing time required but generally leads to
more reliable catalogs.
- NStars
- Number of stars to use in the match. This basically refers to the
NStars brightest objects found in both the object and reference
catalogs (plus some overlap from the reference catalog). Larger
values increase the processing time required but since more image
objects will be detected increasing the value sometimes helps in
fields with a bright/nebulous background. Smaller values may be used
on fields with many well-resolved stars.
- Detection sigma
- The detection threshold for creating the image catalog. Objects with
flux this many sigma above the local background are used in making
the catalog.
If values are supplied, any image WCS keywords are deleted and a new
starting WCS is created using the values specified here. The RA/Dec/Scale
keywords are all required to use this option, the PA is optional.
- RA and Dec
- Initial guess for the plate center RA and Dec (J2000). RA must be
in hours, DEC in degrees, either may be in sexigesimal or
decimal notation.
- Scale
- Initial guess for the plate scale in arcsec/pixel.
- PA
- Initial guess for the position angle where the standard N-up/E-left
orientation is at zero degrees PA. This field may be left blank
since the task will solve for rotation.
Output Options
- Plot coord grid
- If enabled, a coordinate grid of the final WCS is overlayed on the image.
- Fill circles
- Reference objects are marked in red, matched catalog objects are marked
in cyan in the overlay. By default these objects are marked using a
filled circle of a specified size which may obscure the object
beneath it making it difficult to know whether there really is an
overlap. If enabled, this option will cause the circles to be hollow
in the center allowing you to see the image pixels beneath, but at the
cost of substantially more processing time when preparing the final
overlay image.
- Compass
- If enabled a small compass is plotted in the lower-left corner of the
overlay image in yellow. The arrow head points North, the orthogonal
line indicates East.
- Reverse video
- If enabled, the image overlay is displayed as black stars on a white
background and other overlay graphics are adjusted accordingly.
Image Overlay
The image overlay graphic shows the image overlayed by the reference
catalog objects (in cyan) and the objects used in computing the
solution (red). Each colored dot should overlay an image object
in a correct solution. (See the output
options for parameters controlling this graphic.
Solution Plot
The solution plot is a visualization of the image catalog objects (marked
in yellow circles), the reference catalog objects (green pluses
marked at their image position given the input image WCS), and the
matched objects (white circles) used in the plate solution. The
solid rectangle is the plate boundary, plot scale is in units of
image pixels. The fixed WCS reference point (CRPIX) is shown as a
yellow cross, and orientation compass is also displayed where North
is marked with the arrow and East as the perpendicular axis.
If the shift/rotation is large enough dashed white lines are used to
connect the reference catalog object with the matched image object.
Random lines indicate a wrong solution that should be refit, however
parallel lines or those indicating a rotation show the difference
between the input and solved coordinates intuitively.
Residual Histograms
The residual histograms are derived from the complete image catalog and
reflect the difference between the input and fixed WCS values for
each of the detected objects. Plot titles give the mean and standard
deviation of the residuals, units will be arcsec/arcmin as appropriate.
WCS Info
Simply, tables indicating the before and after WCS keywords for
comparison. Additionally, select keywords from the header are
displayed for reference. A complete display of the input image
header is available using the labelled button at the bottom.
This is the initial release of WCSFixer. Development is ongoing and
future versions will include (in no particular order):
- A client task that can run on the user's desktop to call the service
without a web browser,
- Additional return products such as the image and reference catalogs
for the field,
- Distribution of the software as an IRAF external package.
- Ability to handle MEF images and lists of files (e.g. the entire
VOTable returned by a SIAP service).
- User-supplied reference and image catalogs
- Expanded documentation providing hints on how to interpret the output,
improve the solution, and explain causes/workarounds for failed
solutions.
- Automatic invocation from other services. (A method for loading the
URL field automatically is already available, contact the author
for details.
- Additional parameters to control the task and results.
For comments, questions, to report bugs, or more information please email
fitz@noao.edu