Thanks a lot for bringing this up. This is an interesting format to keep columns with similar values in a FITS table without having to increase the official column number.

Following your task report, it just occurred to me that we can specify sub-columns with a `.N' added to the column name for example in the example you sent, if you wanted the third sub-column of the `apflux_g' column, you can run a command like this:

asttable tractor-0397m080.fits -capflux_g.3

We can similarly extend the concept to Gnuastro's plain-text table format (to easily print on the command-line or create FITS tables with this feature), so the sub-column metadata would be printed like this (the `.M' part is new):

# Column N.M: NAME [UNIT, TYPE, BLANK] COMMENT

For example, following the example in that section of the book, such, the meta-data for sub-column 3 of column 5 in a table would look like this:

# Column 5.3: column name [km/s,f32,-99] Redshift as speed

Unfortunately I am too busy to implement this now, but it is a very good suggestion to add this feature, I can see many scenarios where such array-column can be very useful. So I just tried to build upon the initial suggestion with some possibly useful implementation details.

Until I get the chance to implement this, if anyone is interested to try implementing it, any help would be very welcome.