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Subsections

13. Digital Sky Survey images

Digital Sky Survey images are scanned from plates taken from Palomar and other observatories. These images cover all sky and are very convenient for checking our images for novae, supernovae and other transient phenomena. Fitsblink can access the Digital Sky Survey images in three ways, provided that your computer is connected to the internet. With the downloaded images you can perform all operations that fitsblink allows you to do with other images.

13.1 Loading from command line

This feature is entirely due to the CFITSIO library which is capable of loading images not only from the local file system but also from ftp and http servers. All what is needed is a correct URL specification. An example of such a command line call is given here:


fitsblink "http://archive.eso.org/dss/dss/image?-c.ra=17:17:39.00&
-c.dec=+43:03:00.0&-c.bm=5.000000/5.000000&-source=DSS1&
-mime=image/x-fits(m92.fts)"

The line is broken into three lines for better readability. The quotation marks are necessary because otherwise fitsblink takes characters after - as option names. Parameters following the URL are

-c.ra right ascension in hh:mm:ss format
-c.dec declination in +dd:mm:ss format
-c.bm width and height of the image in arcminutes
  format is width/height
-source can be DSS1 or DSS2
-mime-type must be x-fits

Last item it the command line is the name under which the image will be stored on the local disk.

13.2 Using currently displayed image as a template

The most interesting option regarding DSS is the ability to use the currently displayed image as a template for the download of DSS image. WCS values in the header of currently displayed image must be correct. We can also detect stars and match them to the catalog for the same effect. How to do this, is described in the previous sections.

Although we can use the whole image as a template, this is sometimes not desired, especially if we have large image. In such case it is necessary to select the region of interest with the middle mouse button. After that, just press the DSS button and the image will be downloaded from the server. After that fitsblink will rescale and rotate DSS image so that it matches the image which is currently displayed on the screen. Than just press Blink button and you cam immediately compare the images.

A word of caution is necessary here. The resampling process is rather basic and the signal of objects, such as stars, is generally not preserved. This can lead to degradation of astrometry and photometry. It is therefore recommended to use these images for visual inspection only. If you want to do astrometry of some objects, use the original file which is saved on the disk by fitsblink. File name is the same as the of the template, but with -dss suffix before the .fts.

The file that is actually shown after loading DSS images is not preserved when you exit fitsblink unless you manually save it before.

13.3 Interactive input of parameters

To download an arbitrary sky portion from the DSS, just press the DSS input button, fill in the input fields and press Get image. The image will downloaded to the current working directory of your disk and then loaded by fitsblink. No rescaling or any other modification of the file is performed.


next up previous contents
Next: 14. Telescope and CCD Up: Fitsblink 3.195 user manual Previous: 12. Astrometry   Contents
Jure Skvarc
2000-06-11