This Legacy journal article was published in Volume 4, February 1994, and has not been updated since publication. Please use the search facility above to find regularly-updated information about this topic elsewhere on the HEASARC site.

SkyView : An All-Sky-Survey

Data Service

Thomas A. McGlynn (NASA GSFC), Keith Scollick (NASA GSFC),
Nicholas E. White (NASA GSFC) & Conrad A. Sturch (STScI)


SkyView is a new network service that allows users to get digital images of selected regions from all-sky and large area surveys. In recent years, digital information from such surveys has become much more widely distributed within the astronomical community, but it remains difficult to use. Data come in different formats, projections, coordinate systems, equinoxes and scales. This diversity makes it tedious for users to get at the data. SkyView addresses these essentially geometric issues - providing the data to users in a convenient form, and allowing astronomers to concentrate on research rather than on geometry. SkyView is also a single source for many of the most useful astronomical surveys. This paper describes the capabilities and contents of the current SkyView system and our planned enhancements to the system.


SkyView is an on-line service that allows users to pick a region of the sky and then obtain images of it at a number of wavelengths. The user chooses the projection, coordinate system, scale, orientation, and size of the desired image. The user selects these parameters to match exactly existing data from ground- or space-based observations, or to meet their research needs. The data is transformed from the available all-sky and wide-area surveys according to the users specifications and the user is given the resulting FITS file or PostScript image of the region.

SkyView can be used for many different applications. Users can generate finding charts, look for optical counterparts to sources seen in other wavelengths, or check for time variability of sources. SkyView images can be used to look at large-scale features and see if there are correspondences among different wavelengths, or to look at very small regions where the user has some of his or her own data. SkyView can be used anywhere that a user needs an image of the sky in the wavelengths of the surveys that SkyView covers. While it is not intended to be the facility where one pushes data to its extreme limits, SkyView provides a convenient, single access point to all useful publicly-available large-scale surveys.

SkyView can provide coordinate and contour overlays and locations of objects from a number of catalogs. Future versions of SkyView will provide capabilities to contour one image over another and to generate multicolor images from several inputs.

SkyView is now available within the HEASARC xray account on legacy. To use SkyView, type skyview once you have logged into the xray account. Users with special needs can get personal accounts on the SkyView system.

Using SkyView

When a user logs into the SkyView systems (either directly or through the XRAY account), he or she is presented with a simple form interface to request images of the sky. The user makes the request of the system to generate images. The request can specify the following tasks:

  • Selection of data from one or more surveys centered on any point in the sky

  • Data sampling in any of gnomonic (tangent), rectangular, and Aitoff projections (Other projections will be available soon)

  • Data sampling in any of Equatorial, Galactic or Ecliptic coordinates

  • Coordinate system overlays

  • Overlays of objects from SkyView catalogs

  • Arbitrary equinox for equatorial or ecliptic coordinates

SkyView takes care of automatically mosaicking data when the survey data is split into multiple regions, resampling of data in the desired coordinate system and scale, and other geometric issues. The data from the surveys are resampled into the users' desired frames so that users need not worry about how the data is stored, but only about how it can be managed most conveniently for their own needs.

The user may specify that the image is to be displayed immediately or can save it as a FITS or PostScript file. Files are retrievable to the users' home system using anonymous ftp.

SkyView has extensive on-line help, both on how to run the system and on the included catalogs and surveys.

The SkyView GUI

The SkyView graphical user interface uses IDL widgets to generate a simple forms interface. The current appearance of the GUI is illustrated in Figure 1. The menu may be thought of having five areas from top to bottom. Since the SkyView system is still undergoing changes you may see some small changes when you start up the system.

The top area contains pull-down menus. The FILE menu allows the user to start a request, to clear user inputs and to exit SkyView. The TOOLS menu allows the user to start tools that manipulate the image, e.g., change the color table, smooth the image, and the like. The entries in this menu are generally IDL tasks that perform simple image manipulation or display tasks. The comments button displays a window that tells users how to contact the SkyView managers. The OPTIONS box, which is grayed out until a user has made an image request, allows the user to add overlays to an image, e.g., catalog positions or a coordinate overlay. Finally the HELP button starts the internal SkyView help.

The second area from the top is the image specification area. Here the user specifies the attributes of the image that is to be created. The user may select the Coordinate System box to switch among the available coordinate systems. The user then enters the coordinates for the center of the image in the boxes below whose labels will change according to the selected coordinate system. Coordinates can be specified in both hh mm sss (or ddd mm ss) and ddd.fff notation where the assumption is that if the string the user specifies contains a space, then it is in the former format, otherwise the user has entered decimal degrees. RA is assumed to be in hours when a space is included, but in degrees if not.

The Map Projections button allows the user to change the projection used while the user enters the Scale in arcseconds per pixel. The scale is given at the special point in the projection, e.g., at the point of tangency for the gnomic projection and at the 0,0 point in the Aitoff projection. Finally the user specifies the number of pixels in the image using the X and Y boxes.

Shortly, the system will be enhanced to allow arbitrary rotations of the coordinates. The user also will be allowed to specify the center point of the data independently of the special point of the projection. For example, in a tangent (gnomonic) projection, SkyView currently places the tangent point at the center of the image. This enhancement will allow the tangent point to be anywhere in the image (or even beyond its borders). For the Aitoff projection, the projection is made around the coordinate origin, e.g., (RA,Dec) = (0,0).

The third section of the interface consists of a number of selection boxes. Users may check from which IMAGE SURVEYS they wish to extract images. An image will be generated for each survey. At least one survey must be checked. Zero or more CATALOGS may be checked. Catalog sources that lie within the image will be marked on the image, and the a table of these catalog objects displayed (and included in any FITS output). The user can select one or more output from in the OUTPUTS box. The OUTPUT OPTIONS allows the user a few more capabilities. The user may ask for contours of the image to be plotted, or for a coordinate grid. The user also may ask that the data be resampled more carefully. Users also may ask to be queried for the names of any files as they are created.

When SkyView needs to resample data, the user has a choice of how the resampling is to be done. By default SkyView uses the nearest-neighbor method for resampling. Here the program finds the closest pixel in the archived data to the one desired and uses that value. This works well when the user-defined scale is close to the data being retrieved, or when the archived data is reasonably smooth. In practice, nearest-neighbor works remarkably well for most of the SkyView surveys. However, when the user wishes to retrieve data with much larger pixels than in the original data, nearest-neighbor resampling may miss significant features in the data.

In this case, the user may choose to rebin the data before it is sampled by the nearest-neighbor technique. The data is rebinned to generate pixels of approximately the same size as the users' desired pixels. The new pixels then are sampled using the nearest-neighbor technique. With the rebinning, the nearest neighbor technique will conserve flux reasonably well even in data with large fluctuations.

The development version of SkyView also has the capability of integrating the flux over the pixel boundaries of the new image. This is the most accurate resampling technique but is currently quite slow. We anticipate this technique will be sped up significantly and offered as an option in the standard version. If a user has a need for this capability immediately, he or she should get in contact with the SkyView managers using the addresses below.

The final output option requires a little more explanation. Users may wish to compare SkyView images with their own data where the data may be some pixel array with NX and NY even, and the image center given not for the center of the image, but for the center of one of the pixels nearest the center. The 'center at pixel center' option makes SkyView put the user specified coordinates at the center of a pixel rather than at the center of an image. For example, suppose we have a 500 by 500 image with a center at RA=r, and DEC=d. The center of the image is the corner formed by pixels (250,250),(250,251),(251,250) and (251,251). If the option is not checked, the this corner corresponds to the (r,d) position. If the option is checked, then (r,d) will correspond to the center of pixel (250,250).

The fourth area of the GUI is a set of accelerator buttons that include features available from the pull down menus at the top.

The final area of the GUI is a status display line at the bottom. Any errors encountered are displayed there. During the retrieval of data, the status display will show the various stages of image retrieval.

Planned enhancements to SkyView

Several major enhancements are planned to the SkyView system. A command line interface will be provided. This interface will allow users who do not have X-compatible terminals to use SkyView. Perhaps more important, SkyView users will be able to generate and run command scripts.

SkyView will soon have more generic capabilities for specifying the users desired coordinate system. Users will be able to specify arbitrary rotations in the coordinate system and also will be able to generate maps where the special point in the coordinate system is not necessarily at the center of the image.

SkyView also will soon have multi-image capabilities which will allow the user to compare two or more images. Users will be able to generate contours of one or more images on an image display of another. SkyView also will support multi-color overlays, where one image is used to supply each of the primary colors.

Another major feature will be a capability for the user to download their own images (in FITS) to be used in the image comparisons.

We hope to begin integration of optical survey data into SkyView by the end of the year, and many more surveys and catalogs will be made available.

SkyView's current holdings

Surveys currently available or scheduled for ingest in SkyView are shown in the table below. We welcome any suggestions for additional surveys, and in the next few months, we anticipate adding many small surveys not shown.

Name		 Regime     Resolution   Available	     Notes

Bonn 21cm Radio .5 deg Now Dec >-19 deg IRAS Infrared 90" Now 100, 60, 25, 12 mu m HEAO A-2 X-Ray 1 deg Now Contains glitches Preliminary EUVE UV 1 deg Now 555, 405, 171, 83 A Palomar SS Opt 1.4" Starting 1994 Compressed data Southern SS Opt 1.4" Starting 1994 Compressed data COS-B gamma-ray 1.5 deg 1994 Galactic plane COMPTEL gamma-ray 2 deg 1994 EGRET gamma-ray 1 deg Early 1994 Several energies ROSAT WFC UV 1995 Final EUVE UV When available

The current catalogs, shown in the second table, are intended to be representative of the kinds of catalogs that will be available. Adding catalogs to SkyView is very easy and with SkyView's integration into the ADS and HEASARC, the vast catalog holdings of those systems also will be available.

Catalogs currently available within SkyView

Catalog Regime

4C Radio Veron-Cetty Radio Dark Nebulae(Lynds) Optical-nebulae Bright Nebulae(Lynds) Optical-nebulae HII (Sharpless) Optical-nebulae FK5 Optical-stars Bright Star Optical-stars POSS Plate centers Optical-observations Seyfert (Weedman) Optical-galaxies RNGC Optical-galaxies Markarian Optical-galaxies Uppsala Optical-galaxies Abell Optical-clusters 4th Uhuru X-ray

How do I get started?

To start SkyView use please enter skyview in the HEASARC xray account. Documentation for SkyView is available in the /skyview/doc directory of the legacy anonymous ftp area. This documentation includes a preliminary users guide and design document. If you have questions or problems please get in touch with:

Keith Scollick, (301)-286-8143 or
Thomas McGlynn, (301)-286-7743.

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