[AstroPy] Changing table column from float64 to float32

[Simon Conseil]

Indeed the BinTableHDU object doesn't have a method to easily replace a
column (which is defined as float64 by the format='D' attribute in your
example, so data assigned to the column will always be recast to
float64).

One option would be to use table_hdu.columns.del_col and
table_hdu.columns.add_col but this isn't the most practical. 

In [3]: a = table_hdu.data['a'].astype('float32')

In [4]: cols = table_hdu.columns.del_col('a')

In [6]: cols = table_hdu.columns.add_col(fits.Column(name='a', array=a,
format='E'))

In [8]: table_hdu = fits.BinTableHDU.from_columns(cols)

In [9]: table_hdu.columns
Out[9]: 
ColDefs(
    name = 'b'; format = 'E'
    name = 'a'; format = 'E'
)

The easiest here would be to use astropy.table.Table:

In [3]: t = Table.read(table_hdu)

In [4]: t['a'] = t['a'].astype('float32')

In [5]: table_hdu = fits.table_to_hdu(t)

In [6]: table_hdu.columns
Out[6]: 
ColDefs(
    name = 'a'; format = 'E'
    name = 'b'; format = 'E'
)


Simon

On Tue, 2022-07-12 at 00:13 +0000, Homeier, Derek wrote:
> On 11 Jul 2022, at 11:14 pm, Michael Brewer <brewer at astro.umass.edu>
> wrote:
> > 
> > Please try this:
> > 
> > table_hdu.replace_column('a',
> > table_hdu.data['a'].astype(np.single))
> > 
> > Michael
> > 
> replace_column is a [astropy.table.]Table method; this won’t work on
> a FITS BinTable –
> I think the most direct way is indeed to recreate the HDU from the
> appropriately cast
> Columns as outlined by Peter. Note that the HDU also contains the
> corresponding header
> information characterising the data section formats, so direct
> manipulation of the data
> parts is limited for a reason.
> > 
> > On 7/11/22 12:17 PM, Peter Weilbacher wrote:
> > > Hi Ivan,
> > > 
> > > I have no particular insight into this, but it seems to me that
> > > since
> > > you created the Column objects with a specific format, assigning
> > > any
> > > data to it will cause it to be casted to match that. (Otherwise
> > > the
> > > underlying memory buffer does not match.)
> > > 
> > > Instead, I guess you could create a new column with the desired
> > > type and
> > > format and recreate the table using that:
> > > 
> > > c1_single = fits.Column(name='a',
> > > array=table_hdu.data['a'].astype(np.single), format='E')
> > > table_hdu2 = fits.BinTableHDU.from_columns(c1_single +
> > > table_hdu.columns[1:])
> > > 
> > > (It appears that you can even remove the .astype() from this and
> > > it
> > > still works.)
> 
> Yes, the `format=‘E’` specifier will already recast to the respective
> FITS (= Fortran) format.
> 
> Cheers
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