Introduction to TOPCAT

TOPCAT is a tool designed for astronomers to analyze and manipulate large sets of data. It features a very intuitive graphical interface, can perform calculations using data, and quickly produces readable diagrams. For more details on the program’s development and purpose, or to install it to your computer, look at the official TOPCAT site.

This documentation will walk through the procedures specific to the Star Clusters expedition. The content discussed here can be extrapolated fairly easily to many other applications, but if you want to do more than what is covered on this page, refer once again to the TOPCAT site for more generalized documentation.

For a narrated version of the content below, follow along with this video.

Preparing Your Data

If you collected your data using an SQL query through the SDSS form, then you need to first open the .CSV file in a spreadsheet editor or text editor. Excel, Notepad, and TextEdit should all work. The first line will say “#Table”; you need to delete just that very first line in order for TOPCAT to read the column names. This is a quirk of the SDSS form, so if you collected your data through CasJobs then you can ignore this step.


Every time you open the program, you’ll be confronted with a blank slate, so the first step is to open a data table using the button in the upper-left:

The “Format” drop-down menu is very important, since it tells TOPCAT what kind of file your data table is. This will affect how it needs to be imported, so if you select the wrong format the program won’t know how to access any of your data. In this window, you also need to click on “System Browser” and locate the file on your hard drive.

Now that your data has been imported into TOPCAT, you can select it in the “Table List” to view its properties, rename it, and start your analysis. To create a new column calculated from data in the table, click on the “Display column metadata” button.

A window will open up showing the name and data type (ie string or integer) of each column in the table. You can uncheck individual columns if you don’t want them to show up as options while you’re graphing later on, but that feature is mostly to reduce clutter when working with very large data sets. Click on the green plus to create a new column.

You can name it whatever you like, but try to keep it concise and descriptive. For example, in this case, I’m calculating the color index using the g and r magnitudes. The “Expression” is the important part, since that’s what TOPCAT uses to calculate the values of the new column. You need to type in the exact names of the columns you use as inputs, so that it knows which ones to reference, and you also need to use operations that it can recognize such as subtraction and multiplication. When you’re done, click “OK,” and then you can close the “Table Columns” window as well.

Now you can start plotting your data by clicking on the empty Cartesian axes to access the “plane plotting window”:

This is the most complex interface you’ll use in TOPCAT, but also the most powerful. To start with an overview of the buttons you’re most likely to use:

  • “Rescale plot” will scale the axes so that all data points are visible.
  • “Add plot” allows you to layer different tables onto the same diagram for direct comparison.
  • “Delete layer” removes the selected plot. You can also check and uncheck them in the lower-left to do this temporarily.
  • “Axes” allows you to plot your data on decreasing or logarithmic axes, and you can use the “Range” tab to manually input the range of values to plot.

While your table is selected in the lower-left, you can access drop-down menus labelled “X” and “Y”; these are the columns which will be used as your x- and y-coordinate inputs. Each point corresponds to one row, or object; darker spots on the diagram mean that multiple objects are overlapping with each other there.

To save a modified data table or your TOPCAT session to return to later, use the file menu to select “Save Table(s)/Session” and locate the directory you want to store the files to. You can use the default “FITS-plus” file type, and then open that file in TOPCAT the next time you want to access the table or session.