Footprint Services for Everyone

Astronomical Data Analysis Software and Systems XVI ASP Conference Series, Vol. 376, 2007 R. A. Shaw, F. Hill and D. J. Bell, eds.

Footprint Services for Everyone Tam´ as Budav´ ari1 , L´ aszl´ o Dobos2 , Alex S. Szalay1 , Gretchen Greene3 , 4 Jim Gray , Arnold H. Rots5 Abstract. We present high-level user and web services for dealing with astronomical survey geometry of arbitrary size and complexity. Based on our highperformance spherical library, we built an on-line public repository of footprints pre-loaded with the coverage of some of the most widely used datasets today including the Sloan Digital Sky Survey, the Galaxy Evolution Explorer and the Hubble Space Telescope exposures. We publish a web-based toolkit that enables advanced spatial searches for regions of interest on the sky, Boolean operations on selected footprints (union, intersection), on-the-fly visualization, and exact area computation. We also provide an easy-to-use interactive footprint editor, as well as a simple upload facility. The results of the searches and region manipulations can be saved and published on the footprint server or downloaded in various formats, including ASCII and the VO-compliant Space-Time Coordinate (STC) region representation.

1.

Introduction

There is much more to astronomical observations than just source catalogs. For example, when working with multiple observations at different wavelengths, knowing their precise coverage is very important, e.g. to look for dropouts, and can often be very difficult when the most significant observations also have the most complex geometry. This functionality has been missing almost entirely from the toolbox of many astronomers until today. The sphere is arguably the most beautiful object and certainly the simplest one. Yet its surface has a strange topology: if one starts going in a direction, one eventually ends up at the same place where one started. As trivial and banal as it sounds, this has been a source of frustration of many scientists, including astronomers who observe objects on the celestial sphere. Spherical geometry is difficult and there are no easy solutions or shortcuts, especially if one is after an efficient algorithm.

1

Dept. of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218

2

Dept. of Complex Systems, E¨ otv¨ os University, Budapest, Hungary

3

Space Telescope Science Institute, Baltimore, MD, USA

4

The eScience Group of Microsoft Research, San Francisco, CA, USA

5

Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA

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Budav´ ari et al. The Math of Painting on the Sky

There are many ways to represent shapes on the surface of the unit sphere. Most, however, are cumbersome in practice, e.g. when determining whether a certain point lies within the specified shape. For example, most geospatial libraries ignore the poles, and so are mostly useless for astronomers who need libraries that treat the whole sphere in a uniform way. Going from a two-dimensional (2D) surface representation to a three-dimensional (3D) representation provides such a uniform framework. A half-space, a directed plane that splits the 3D space in two, represents a circle (or cap) when intersected with the 3D sphere. Many halfspace constraints build up a convex that describes a 3D convex hull and its 2D intersection with the unit sphere. Many 3D convexes do not define simple convexes on the surface of the sphere. For example, the corners of a cube centered on a sphere may define 8 triangular patches. But with suitable restrictions, connected 2D spherical hulls are defined by 3D half-spaces hulls. More general spherical regions are defined as the union of convexes. Figure 1 illustrates the regions of the Sloan Digital Sky Survey’s and the Galaxy Evolution Explorer’s sky coverage in a stereographic projection that enhances the details of nearby convexes and demonstrates the problem’s complexity. Within this framework, the boolean algebra of regions maps very well on set operations of half-spaces and convexes. The intersection of two or more convexes is a convex that includes all their half-spaces and the union of two or more convexes is a region that includes all the convexes. It is also straightforward to define the differences of convexes, etc. The real challenge is to derive the irreducible representation of the results of these operations, discarding empty regions and redundant constraints. This is a compute-intensive task. Once the simplification is done, one can calculate the area of the region by summing up the areas of its convexes. 3.

Footprint Services

We have developed a lightweight spherical library to deal with regions of arbitrary size and complexity as an OS-independent, managed .NET assembly implemented in C#. The library links easily to desktop and web applications, e.g. the Hierarchical Triangular Mesh (HTM) library (whose latest version builds on the spherical library), and also integrates with astronomical databases such as the SkyServer of SDSS. The library is deployed in the database, so that specialized stored procedures and other SQL routines can leverage its functionality. Our Footprint Service6 is a web portal built on top of a specialized SQL Server 2005 database engine that uses the Spherical Library to perform spatial searches and region operations. The Footprint Service database is pre-loaded with the sky coverage of some of the most widely used datasets today, including the Sloan Digital Sky Survey (SDSS, DR4 and DR5), the Galaxy Evolution Explorer (GALEX, GR2) and the Hubble Space Telescope (HST ACS) exposures. The site serves as a public, on-line repository, where registered users can publish their footprints. Since our oral presentation at the ADASS XVI conference, 6

Visit our website at http://www.voservices.net/footprint

Footprint Services for Everyone

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Figure 1. Stereographic projection of the footprints of the Sloan Digital Sky Survey’s 5th Data Release and the Galaxy Evolution Explorer’s 2nd Public Release as visualized by the Footprint Service.

several astronomers have already contributed significant footprints of surveys that, as of this writing, include UKIDSS, 2dF and CFHTLS. The web-based toolkit enables both advanced spatial searches for regions of interest on the sky, whose results are listed along with all-sky thumbnail views, e.g. in Figure 2, and also Boolean operations on selected footprints (union, intersection), on-the-fly visualization in various projections, and exact area computation. The site features an interactive footprint editor, as well as a simple upload facility that accepts formal region descriptions in different formats. The results of the searches and region manipulations can be saved and published on the footprint server or downloaded in various formats including ASCII and the VO-compliant Space-Time Coordinate (STC) region representation. 4.

Summary

The Footprint Service and its novel underlying infrastructure fills in a major void in astronomical data analysis systems. The modular architecture provides a friendly interface to all scientists from the programmer, through the database developer, to the web user. Its functionality coupled with the recent International Virtual Observatory Alliance recommendation of Space-Time Coordinate

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Figure 2. The Footprint Service provides various search options, e.g. by the object’s name, and the results are presented along with all-sky thumbnail views for quick inspection.

region description serve as one of the most basic building blocks for the next generation astronomical tools. The novel technology enables more efficient dynamic federation of astronomy archives, i.e. SkyQuery and the kind, and provides the missing components for proper on-the-fly catalog cross-matches that also find non-detections called dropouts. We are committed to supporting the Footprint Service in the future by improving the tools, including more surveys and helping others to publish their own data.