[AstroPy] fits table column values
aldcroft at head.cfa.harvard.edu
Mon Dec 23 12:40:25 EST 2013
On Mon, Dec 23, 2013 at 11:16 AM, Slavin, Jonathan
<jslavin at cfa.harvard.edu>wrote:
> Hi all,
> Maybe I'm being dense today, but I'm not clear on how to do something
> pretty basic. The issue is that I have essentially 3-D data wherein the
> data is not known on a uniform grid. That is, although the data is on an
> x-y grid, neither the x values nor the y values are uniform. I want to
> save the values (i.e. z values) at each point in the grid in a fits file.
> The natural thing to do is either create an image or a table, but
> specifying the grid is not quite so straightforward in either case. For a
> table one could have one column be an axis (e.g. 'y') and save the y
> values, but then the x values are the table column 'names', which I think
> have to be strings. Nothing is fundamentally unworkable about that, it's
> just a little awkward to have string versions of the numeric axis values.
> If I output an image instead, then I'm left with specifying the grid
> locations in the header.
If I understand, you have data values Z on a rectilinear 2-d grid (X, Y).
If so one solution I've seen before  is storing an image in the primary
HDU and then the X and Y grid values in the second HDU as a table with two
columns X, Y and just one row. Each of those two columns is actually a
vector of the appropriate length. This is a little contorted, but FITS is
not an ideal match for this.
 I forget exactly where, but there is a FITS standard for calibration
data products that uses this layout for N-d data defined on rectilinear
> So, I do have ways of doing what I want to do, but it just seems that the
> ways I've thought of are not the simplest ways. And I wonder if there
> isn't some other way that I haven't thought of. Any hints on this would be
> appreciated. Maybe some other output format (e.g. hdf5) would be better?
Certainly HDF5 is more flexible and would let you store the two vectors of
X and Y grid locations in a more natural way.
> Jonathan D. Slavin Harvard-Smithsonian CfA
> jslavin at cfa.harvard.edu 60 Garden Street, MS 83
> phone: (617) 496-7981 Cambridge, MA 02138-1516
> fax: (617) 496-7577 USA
> AstroPy mailing list
> AstroPy at scipy.org
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the AstroPy