Welcome to the SkyWalker page

This tool was written to view large astronomical images, that are too large to be transported through a network in one file, with the use of only HTML and JavaScript. The large image is sliced into many small (256pixel squared) images that are mosaicked to give the impression of a single large, contiguous image. Only the images that are presently viewed are downloaded.
This allows the transport of arbitrarily large areas, through the slowest network connections, without installation of extra software but a single web-page.
The functionality of different SkyWalker applications evolved with time and dataset. The most advanced one is the COSMOS SkyWalker, providing access to the 2 square degree image of the COSMOS project, obtained with the Hubble Space Telescope and its Advanced Camera for Surveys (ACS). The full image is 40.000x40.000 pixels in size in the highest resolution we use.

Follow this link to access to the COSMOS SkyWalker

You can also download the COSMOS SkyWalker HTML/JavaScript page including the DynAPI cross-browser library for developement purposes. The SkyWalker is distributed within the GNU General Public License

More SkyWalker applications have been created for the following projects:

  • Overview image (top left) with movable navigation marker
  • Wievport (right), showing the region marked in the overview window
  • Viewport image can be moved with mouse (click-drag)
  • Switch between three "zoom levels" (aka three sets of images of different resolution)
  • Calibrated world coordinate system (with on/off switch), computed in JavaScript using gnomonic projection and reference values
All of this is written in JavaScript, using the DynAPI JavaScript cross-browser library for layer handling. This code can be adapted to display any other dataset, but not by plug-and-play. Required steps:
  • Chop up your image into subimages 256 pixel on a side (or similar value). Best to choose a value that does not leave a border or make your input image a size that is a multiple of 256 pixel. Dimensions can differ in X and Y. Get the naming scheme right (incremental from top to bottom, left to right).
  • Make it a JPEG or GIF or PNG image. Or first do conversion then cut the image. If you start with FITS images, we recommend stiff for conversion to TIFF, and the fitscopy routine from CFITSIO for image cutting. Both work on (almost?) arbitrarily large images!
  • Create an overview image (shrunken version of original).
  • Compute scale factors between overview image and original size image in viewport.
  • Compute different offset factors for image placement in the Web-window
  • If you want world coordinates displayed, input your own computation for any other projection but gnomonic. Use reference point coordinates to make pixel and world coordinate system match.
  • Modify all the links, naming scheme and design.
  • Include credit line in the code and send us an email where we can find your new application. If you have new tricks or a more cleaned-up version, tell us, too. If you use the SkyWalker in a publication, please reference this paper: Jahnke, Sánchez, Koekemoer, 2006, PASP, 118, 1186 (ADS | astro-ph).
  • You're done!
Note: The SkyWalker will work with any Mozilla and Internet Explorer, but might not with other browsers.


  • K. Jahnke, S. F. Sánchez, B. Häußler, 2004, astro-ph/0403520, "Skywalking GEMS and UDF"
  • K. Jahnke, S. F. Sánchez, A. Koekemoer, 2006, PASP, 118, 1186, "Seeing the sky through Hubble's eye: The COSMOS SkyWalker" (ADS | astro-ph)
Design & Programming: Knud Jahnke (MPIA).
Image data: Anton Koekemoer (STScI)
The COSMOS Skywalker is based on the GEMS Skywalker by K. Jahnke and S. F. Sánchez.

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Last update: 25.07.2006, Knud Jahnke, kostenlose
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