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ds9
ds9, developed and maintained by the Smithsonian Astrophysical Observatory, is one of the most common image viewer for fits files. It can be installed on Linux/Unix, Mac, and Windows. ds9 has numerous menus to customize image visualization, and aid analysis steps.
The main ds9 window displays a part of an image, or several images. Two smaller windows to the top right show a zoom-on and a full-frame overview of the currently selected image. Information about the image, and the current cursor position (position, flux at that pixel) are displayed to the left of these windows. All features can be accessed through the top menu. Selected features can be accessed through the menu bars separating the large image window from the top.
Typical fits images are 16-bit or 32-bit; most monitors only support 8-bits per channel (R/G/B) - it is not possible to truthfully display a FITS image on a standard monitor. Various settings can be used to change the appearance of the image:
- min/max: the lowest count value is set to black, the highest to white
- zscale: sets black and white according to the IRAF zscale algorithm (This is one of the most useful ds9 buttons!)
- linear: linear color scale
- log: logarithmic color scale
Changes the colorbar used to display pixel values. Try them out - some are quite funky! Includes "Invert Colormap".
This menu controls what action clicking the mouse button performs. The default is "none". Very useful actions include:
- Region: place or select a region. Double-clicking a region opens an information window about it.
- Pan: move the window center to the mouse position
- Cat: when a catalog has been loaded, clicking on a highlighted object in the image selects it in the table
"Regions" are geometrical shapes that can be used to help certain image analyses. The shape can be selected under the "shape" sub-menu. For example:
- Circle: circular region (default). Double-clicking reveals its radius, among other things.
- Projection: lets you draw a line across the image, and opens a plot of pixel value vs. position along line. After you have placed the line, the middle point can be used to widen the line. This region is very useful for spectroscopy.
Found under "Analysis". Lets you load up an object catalog generated e.g. by Source Extractor. The catalog has to be in ascii format, and imported as "Tab-Separated Value". You have to specify the correct columns that contain (x,y) or (RA,Dec) coordinates.
Manages how to display multiple images. A particularly useful feature is "Lock->Frame", which you can use to match up two images according to their world coordinate system (WCS).