GNU Astronomy Utilities


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5.3 Table

Tables are the products of processing astronomical images and spectra. For example in Gnuastro, MakeCatalog will process the defined pixels over an object and produce a catalog (see MakeCatalog). For each identified object, MakeCatalog can print its position on the image or sky, its total brightness and many other information that is deducible from the given image. Each one of these properties is a column in its output catalog (or table) and for each input object, we have a row.

When there are only a small number of objects (rows) and not too many properties (columns), then a simple plain text file is mainly enough to store, transfer, or even use the produced data. However, to be more efficient in all these aspects, astronomers have defined the FITS binary table standard to store data in a binary (0 and 1) format, not plain text. This can offer major advantages in all those aspects: the file size will be greatly reduced and the reading and writing will be faster (because the RAM and CPU also work in binary).

The FITS standard also defines a standard for ASCII tables, where the data are stored in the human readable ASCII format, but within the FITS file structure. These are mainly useful for keeping ASCII data along with images and possibly binary data as multiple (conceptually related) extensions within a FITS file. The acceptable table formats are fully described in Tables.

Binary tables are not easily readable by human eyes. There is no fixed/unified standard on how the zero and ones should be interpreted. The Unix-like operating systems have flourished because of a simple fact: communication between the various tools is based on human readable characters55. So while the FITS table standards are very beneficial for the tools that recognize them, they are hard to use in the vast majority of available software. This creates limitations for their generic use.

‘Table’ is Gnuastro’s solution to this problem. With Table, FITS tables (ASCII or binary) are directly accessible to the Unix-like operating systems power-users (those working the command-line or shell, see Command-line interface). With Table, a FITS table (in binary or ASCII formats) is only one command away from AWK (or any other tool you want to use). Just like a plain text file that you read with the cat command. You can pipe the output of Table into any other tool for higher-level processing, see the examples in Invoking Table for some simple examples.


Footnotes

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In “The art of Unix programming”, Eric Raymond makes this suggestion to programmers: “When you feel the urge to design a complex binary file format, or a complex binary application protocol, it is generally wise to lie down until the feeling passes.”. This is a great book and strongly recommended, give it a look if you want to truly enjoy your work/life in this environment.


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