Output Options of the ESO Online DSS


Output images from the Online DSS at ESO/ST-ECF are delivered in two formats: FITS and GIF. Furthermore you can choose to display the images directly instead of first saving them to disk.

Image sizes: You cannot request images bigger than 14400 square arcmin. Also, the cutout generating software does not support requests for images of size smaller than 0.5 pixels in one direction (X or Y) and 1.5 pixels in the other one (Y or X); in such case the user's input is reset to 1.5 * 1.5 pixels. Given that the pixel scale for DSS1 is ~1.7 arcsec, and for DSS2 ~1 arcsec, the minimum X/Y sizes are:

DSS1 0.5 pixel = 0.0142 arcmin 1.5 pixel = 0.0425 arcmin
DSS2 0.5 pixel = 0.0084 arcmin 1.5 pixel = 0.025 arcmin

Image sizes for FITS, gzipped FITS and GIF are 260kB, 110kB and 70 kB respectively for a field of 10*10 arc minutes. There's a limit of around 4 MB for the largest image to be delivered. Images from the DSS2 are bigger, because the pixel size is smaller.

For image display, we use some MIME (Multi Purpose Internet Mail Extensions) types which can be used on your side to launch an appropriate application. You might have to configure the global or personal .mailcap file, or to configure your browser, to map the application/x-fits type to a FITS image viewer. FITS is the standard astronomical data interchange format. Possible viewers are SkyCat, SAOimage ds9, Aladin, FV, or FITSview.

The ESO SkyCat tool takes advantage of the world coordinates (WCS) keywords present in our DSS images and displays celestial coordinates at each cursor position. If you want to use this feature you need to make sure that SkyCat is installed in your system.

Then, to use SkyCAT as a viewer for FITS files please add the following line to your $HOME/.mailcap file: application/x-fits; skycat %s or similarly in your browser configuration.

Summary of DSS output options

Download as FITS file with WCS keywords
The image will be saved on your local disk.
MIME type: application/octet-stream

Download as gzip compressed FITS with WCS keywords
The image will be saved compressed on your local disk.
MIME type: application/octet-stream

Download as GIF file
A GIF will be saved on your local disk.
MIME type: application/octet-stream

Display as FITS file with WCS keywords
A FITS viewer will be started on your computer to display the image.
MIME type: application/x-fits

Display as GIF file
A GIF viewer will be started on your computer to display the image.
MIME type: image/gif


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Last update: Feb 21, 2014