Figure A1. Creation of the I-K column in TOPCAT.
We did a limited set of tests with TOPCAT, exploring the X-ray selected EROs data set as was done with VOPlot and Mirage.
To calculate the I-K values we defined an extra column with
the "Define Synthetic Column" function (figure A1). This allows creation
of new columns using a very powerful expression syntax.
The TOPCAT "Help" explains that these are actually expressions in the
Java language that are compiled into Java code before evaluation.
The examples list the standard arithmetic and logical operators
plus the ability to use conditional statements. They explain that
the expression syntax extends to all the methods of the Java Math
and String classes, which appears to enable very powerful possibilites
for manipulation of tabular data. Our limited tests here use only
the basic arithmetic function to calculate the I-K values.
Figure A1. Creation of the I-K column in TOPCAT.
Subsetting the I-K values to those less than 3.75 is possible
using the "New Subset Expression" function in the "Subsets"
menu. Figure A2 shows how this was done.
Figure A2. Defining subsets of data in TOPCAT.
We plotted our data and newly created columns using the
TOPCAT "Table Plotter". The I-K > 3.75 subset was selected
separately in the "Row subsets" box, which allows plotting of
subsets in another colour or point style as shown in figure A3.
Figure A3.
EROs selected from an (I-K) vs I colour-magnitude plot in TOPCAT.
TOPCAT provides a way for the original data, newly created columns and the selected EROs to be written to output files in a range of useful formats; text, VOTable (e.g. eros.vot), FITS, and even the Mirage ascii format (e.g. eros.mirage). We note that output in the VOTable format does not appear to allow specification of UCDs, although it is not very clear how this should be done in any case. Also, out attempt to read the output VOTable into VOPlot did not succeed.
TOPCAT appears to handle the small number and column addition problems
that were problematic in our VOPlot and mirage tests. Figure A4 shows
the Hardness ratio HR, plotted against the total (soft plus hard) X-ray
flux. The ERO subset is shown in red.
Figure A4. Hardness Ratio vs Total X-ray flux.
Overall TOPCAT goes quite a long way towards our desire for a VOTable editor and viewer. It serves a current need to easily view and manipulate tabular data, with plotting capabilities along the same conceptual lines as VOPlot. Perhaps it's most exciting feature is the powerful data manilpulation ability, using Java classes, without having to invest too heavily in learning Java itself.