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This document is retained for historical interest

This documents discusses the schedule and implications of the major SkyView hardware and software upgrade which is currently underway.

Schedule and Hardware Configurations

At noon, May 29, 2007, the address was pointed to a pair of load-balanced machines with new hardware and software. For a transition period the name continued to point to the older SkyView hardware and software. The address was moved at noon (EDT), Tuesday, June 12. In this configuration the skys and skyview addresses are interchangeable. They do not point to different machines but to the same load-balanced cluster.

The new machines will continue to support the old IDL software (though not in the old locations) through September 21, 2007. The old interfaces will be removed after that time. In addition to the SkyView service itself, the SkyView hardware has hosted a few other services. These should continue to be supported unchanged under the new hardware.

Transition to the new Java-based version of SkyView

The new machines use a new Java-based version of SkyView. This new version has many new features and capabilities and supports almost all of the capabilities of the old system. Subseqent sections discuss some of the changes and outline areas where users may see changes in the system. We hope that for most users the transition is completely seamless other than the new (and we hope improved) user interfaces. Please let us know if have problems or concerns.

Single interface

Only a single Web interface is provided in the new version rather than the three Non-astronomer, Baick and Advanced interfaces in the old SkyView. This interface uses JavaScript to enable users to view the more complex and advanced options easily, but only when needed. The Non-Astronomer interface has been replaced by a discussion of how lay users might use the standard features of the system. If you find this interface confusing or would like additional capabilities please send us a note.

Significant problems with JavaScript processing in new user interface were discovered for Internet Explorer users after our initial transition. We believe that these issues have largely been rectified. To support users who wish to forego JavaScript in the longer term, a version of the interface which does not use JavaScript was provided.

Pixel data

We have compared image results pixel by pixel for most of the surveys. We have found the results are identical or have differences due to differing roundoff errors. Note that when resampling using the nearest neighbor algorithm, roundoff errors will occasionally cause a different pixel to be sampled. For nearest neighbor resampling the pixel values will normally be identical but may be a few pixels where an adjoining pixel was sampled. For other resampling methods one typically gets small differences in the values of all pixels.

By default on the Web users will get 4-byte real data as before. Unchecking a checkbox allows the users to get 8-byte reals which are what is used in all calculations.

Quicklook images

There are substantial differences in the quicklook data. Quicklook images are now provided as JPEGs rather than GIFs. These are more generally supported and often smaller. The underlying software supports both JPEG and GIF images and we can provide GIF as an option if there is a significant demand. GIF images are supported in Batch mode requests (as well as TIFF, BMP and PNG formats).

In appearance the most significant change is that by default images are now black and white. Users can select any of the old and many new color tables if desired but we feel that the black and white images most accurately reflect the intensity scaling. Some novice users were confused about the meaning of the colors in the old versions. The old system used the Stern Special color table.

Users can invert any color table. Only gray-scale images could be inverted in the old system.

The algorithms for logarithmic and histogram equalization scaling are provided by ImageJ and are somewhat different than what is used in the old SkyView system. A SQRT scaling is now provided as well.

The overlays may look slightly different. There are many more options for contours. Smoothing is available as a separate options for the contour maps and RGB overlays.

The color for plot overlays can be specified by the user.

Mosaicking Algorithm

When mosaicking component images of a survey together, a more robust algorithm is used. This should eliminate small regions that ran past the edge of the source images that were sometimes seen in previous versions. The new algorithm should generally find the 'best' image for each pixel in the image.

Images used

The old interface had a link which showed the user which images were used in constructing an output image. This information is now included in the FITS header of the output file.

New Resampling Algorithms

A new set of 'high-order' resampling algorithms is available. These allow for use of Lanczos resamplers with kernels of a user specified size, and for second to fifth order spline resamplers.

An efficient exact-area resampler has been developed to replace the very slow and buggy triangular decomposition resampler used in the IDL version. This is useful when exact conservation of flux is required or where the original data is poorly sampled or incomplete.

Currently supported samplers include:
Nearest Neighbor (NN)
Bi-linear interpolation (LI)
Splines from 2-5 order (Spline or Spline3, Spline2, Spline4, Spline5)
Lanczos interpolation with a specified kernel size (Lanczos, or LanczosN where N >= 2, the default is 3)
exact-area resampling using Sutherland-Hodges clipping (Clip).

The default is Nearest Neighbor but the Clip sampler is relatively fast and users should seriously consider using it.

Faster Image Generation

The Java-based system uses less memory than the IDL version, so that larger images can be generated and large images are generated more quickly. For most surveys images of up to at least 5000x5000 pixels can be generated while the previous version was limited to about 2000x2000 pixels. Please only ask for large images if you need them.

New Edge Adjustment Algorithm

A new edge adjustment algorithm is available whose invocation can be controlled by the user. Edge-adjustment attempts to hide the visible boundaries between source images caused by differing background levels in the source image. The older version used a median test where the medians of images were matched. The new version matches medians along the edges associated with pairs of images and can be turned on or off (but is turned on by default for the DSS and 2MASS).


Support for catalogs has been substantially upgraded. Users can request data from HEASARC and Vizier catalogs as well as any URL that supports the Virtual Observatory Cone Search Protocol. HEASARC and Vizier catalog names or the Cone Search URLs can be entered in a text box, and HEASARC catalogs are also available from a menu.

Users can now add criteria to their selection, e.g., 'vmag < 15' to select only the brightest objects.

Catalog query results are now stored in a separate page which can be viewed or downloaded and is linked to the image display. The catalog query results are no longer appended as a FITS extension to the image file. Nor are there links to a similar (though not identical) HEASARC query that was available from the results page. Please let us know if these are useful capabilities to restore.

Deleted and added surveys

A few surveys have been deleted as obsolete: the very low resolution UV surveys and the PSPC (old) survey. It is much easier to add surveys and the new code has the latest (DR5) release of the SDSS, the IRIS improved IRAS data, and the Very Low-frequency Sky Survey (VLSS) were not available in the old version.

Survey are now displayed in 5 regime areas. Currently within each regime surveys are displayed in alphabetical order. Previously some attempt had been made to display surveys in frequency order within regimes.

Other changes

The internal directory structure used for the creation of FITS and quicklook data has changed. Users of the undocumented RETURN=filename option may need to modify their code.

The Image center at pixel center option is no longer available. That shifted the center of the image by half a pixel when the number of pixels was even. The underlying code supports the offset setting which can be used to emulate this option if needed.

The survey documentation now repeats survey information for each band rather than combining related surveys. We may address this in the future. E.g., the IRAS survey is described in full for each of the 4 bands rather than having the 4 bands refer to the same document.

The SkyView Java applet has been ported but will no longer be actively supported. We suggest using Aladin or OASIS.

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SkyView has been developed with generous support from the NASA AISR and ADP programs (P.I. Thomas A. McGlynn) under the auspices of the High Energy Astrophysics Science Archive Research Center (HEASARC) at the NASA/ GSFC Astrophysics Science Division.

We gratefully acknowledge the support of NASA and contributors of SkyView surveys.

A service of the Astrophysics Science Division at NASA/ GSFC
and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory (SAO)

HEASARC Director: Dr. Alan P. Smale
HEASARC Associate Director: Dr. Roger Brissenden
Responsible NASA Official: Phil Newman
Web Curator: Laura McDonald

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