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
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
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
You can also download the COSMOS
developement purposes. The SkyWalker is distributed within the GNU General Public License
More SkyWalker applications have been created for the following projects:
library for layer handling. This code can be adapted to display any other
dataset, but not by plug-and-play. Required steps:
- 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
Note: The SkyWalker will work with any Mozilla and Internet Explorer,
but might not with other browsers.
- 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
- Create an overview image (shrunken version of original).
- Compute scale factors between overview image and original size image in
- 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!
- 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)