Topcat - anjavdl/PHY517_AST443 GitHub Wiki

Topcat is "an interactive graphical viewer and editor for tabular data". It is specifically designed to work well with astronomical data tables, including working with sky coordinates, and data formats used in astronomy, namely fits files and catalogs in the Virtual Observatory format (VOTables). The full manual can be found here. One feature to point out is that topcat can interact with ds9, i.e. you can make selections on a catalog, and then identify the selected objects in an image (see below for details on how to do this).

Column calculations

  1. Select "Views" -> "Column Info" from the main Topcat window. This will bring up a second window, showing a list of columns in the table. (Note that you can use this list to determine which columns are shown.)
  2. In the new window, select "Columns" -> "New Synthetic Column". This causes a third window to come up.
  3. In the third window, specify your new column. Consult the function help menu, if necessary.

Plotting

Various plotting tools can be accessed from the main window, under "Graphics". Various options (setting the axis ranges, using logarithmic axes, inverse axes, etc.) are available.

Catalog subsets

Subsets can be defined in several ways:

  1. With a mathematical expression: from the main window, select "Views" -> "Row Subsets" -> "New subset" .
  2. Using a plot window: "Subsets" -> "New subset from visible" will select all objects currently visible in the plot.
  3. Using a plot window: "Subsets" -> "Draw subset region" will let you draw on the plot to define the subsample.

Topcat and ds9

Topcat can interact with ds9. Given a catalog of objects with sky coordinates (or pixel positions x,y), you can "send" your table (or a subset that you selected) to be overlaid onto ds9.

  1. Open your image in ds9.
  2. Start up topcat from the same terminal, and open your catalog.
  3. In ds9, click on File -> SAMP -> Connect .
  4. In topcat, click on File -> Send table to -> SAOImage ds9 .
  5. This will bring the ds9 catalog tool. If necessary, specify the correct coordinate columns and system (wcs vs. image).