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Paper: Chapter 16: Web-based Tools—WCSFixer
Volume: 382, The National Virtual Observatory: Tools and Techniques for Astronomical Research
Page: 139
Authors: Fitzpatrick, M.J.
Abstract: While nearly all of the image data available in the VO will contain some form of World Coordinate System (WCS), not all image data will be the high-level data products that enjoy the rigorous reductions one might expect from survey data sources. Indeed, highly valuable images from e.g. HST and other major observatories have known errors in their astrometric solutions, and the ease by which any data publisher can make any image available through a SIAP service means that unless one is familiar with the reliability of a given resource, or is familiar with the shortcomings of that same resource, the conservative astronomer should not assume the WCS information in an image is correct.

This is especially important when doing the kinds of things VO astronomers want to do with access to images from many sources, nearly all of which rely on a correct coordinate system in each image being examined. The registration of images for stacking and mosaics, identification of objects in a crowded field given a reference catalog, crossmatching object detection catalogs with reference catalogs (see Chapter 14), and matching detections in an image of one bandpass with the same object in another, are just a few examples. The NVO Science Steering Committee recognized both the current state of coordinate systems in images available through VO services and the importance of this fundamental property of the data to enable good science when it rated the development of a service to correct an image WCS as a priority.

We describe the WCSFixer web-application ( as one means by which astronomers can improve (or derive) the WCS solution for an image, its implementation and the constraints imposed by that implementation.

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