sep 8, 20082008 nvo summer school1 wcsfixer a web-enabled iraf plate solver mike fitzpatrick, noao t...
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Sep 8, 2008 2008 NVO Summer School 1
WCSFixerA Web-Enabled IRAF Plate Solver
Mike Fitzpatrick, NOAO
THE US NATIONAL VIRTUAL OBSERVATORY
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Sep 8, 2008 2008 NVO Summer School 2
Motivation and Requirements
• A correct image WCS is fundamental to a lot of VO processing (e.g. Catalog generation and X-match, data quality). So,a service is needed that – Given an image with a poor WCS, will improve it;– Given an image with no WCS, will find one;– Given an image with a non-standard WCS, will
standardize it (e.g. DSS plate solutions)• Must run from a web-browser and return the
corrected image• Provides good feedback to the user about what
was done and the quality of the solution
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Sep 8, 2008 2008 NVO Summer School 3
Design Goals
NGC3104
• Interactive interface
• Single-page display
• Exploit IRAF capabilities
• More than a simple web-based form
• Should be fun to write
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Sep 8, 2008 2008 NVO Summer School 4
DEMO
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Sep 8, 2008 2008 NVO Summer School 5
Technology Used
• SOA – AJAX
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Sep 8, 2008 2008 NVO Summer School 6
Technology Used
• SOA – AJAX (Simple, Only Apparently AJAX)
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Sep 8, 2008 2008 NVO Summer School 7
Technology Used
• SOA – AJAX (Simple, Only Apparently AJAX)
• IRAF/CL scripts for plate solution/graphics (~15 separate tasks)
• HTTP/POST service• C-shell CGI script for form processing• HTML iframes for layout• CSS and Javascript for style/dynamic content• Embedded Javascript in return stream for
driving the user interface
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Sep 8, 2008 2008 NVO Summer School 8
Algorithm
• Verify image is valid (2-D, not an MEF, etc)• Set initial conditions from user input or image header• Generate reference catalog of what should be in the
field• Generate a catalog of detected objects in the image• Cross-match the two catalogs (triangle algorithm, N
brightest objects used)• If (no solution found) {
iterate by searching param space (e.g. No clipping) } else {
compute plate solutionupdate image, data products }
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Sep 8, 2008 2008 NVO Summer School 9
Why Doesn't it Always Work?
• Too many points• Not enough points• Scale problems lead to bogus solution• Selection Effect• Image Geometry
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Sep 8, 2008 2008 NVO Summer School 10
Other Astrometry Engines
• Astrometry.net– Downloadable software– Image formats supported– No starting info required
• SCAMP - WCS solutions from SExtractor catalogshttp://terapix.iap.fr
• Automatch - John Thorstensen’s toolshttp://mdm.kpno.noao.edu/Manuals/automatch.html
• Pitt WCS Fixerhttp://nvogre.phyast.pitt.edu/wcs/
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Sep 8, 2008 2008 NVO Summer School 11
Future Work (Maybe)
• Web-Service interface• Return reference/image catalogs
(VOTable)• Process multiple images (SIAP query
table input instead of single image)• Desktop application via VO Client• Automatically explore parameter space
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Sep 8, 2008 2008 NVO Summer School 12