Sep 8, 2008 2008 NVO Summer School 1

WCSFixerA Web-Enabled IRAF Plate Solver

Mike Fitzpatrick, NOAO

THE US NATIONAL VIRTUAL OBSERVATORY

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

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

Sep 8, 2008 2008 NVO Summer School 4

DEMO

Sep 8, 2008 2008 NVO Summer School 5

Technology Used

• SOA – AJAX

Sep 8, 2008 2008 NVO Summer School 6

Technology Used

• SOA – AJAX (Simple, Only Apparently AJAX)

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

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 }

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

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/

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

Sep 8, 2008 2008 NVO Summer School 12