Resampling HST images onto the pixel grid of a MUSE image

Both of the photometry fitting algorithms depend on having an HST image that is sampled on the same pixel grid as the MUSE image that is being characterized. A suitable HST image can be obtained by applying a script called regrid_hst_to_muse to a higher resolution HST image. The regrid_hst_to_muse script resamples the high-resolution HST image onto the same pixel grid as the MUSE image, and scales its pixel fluxes from HST electrons s^-1 to the flux units of the MUSE image (usually 1e-20 erg cm^-2 s^-1 Angstrom^-1).

The arguments of the function can be obtained by running it with the -h option:

% regrid_hst_to_muse -h
usage: regrid_hst_to_muse [-h] [--quiet] [--field [name]]
                          muse_image hst_images [hst_images ...]

positional arguments:
  muse_image      The filename of a template MUSE image or MUSE cube in FITS
                  format.
  hst_images      The filenames of one or more HST images with 30mas pixels.

optional arguments:
  -h, --help      show this help message and exit
  --quiet         Suppress the messages that report each step as it is
                  performed.
  --field [name]  When this option used, it specifies a field name to use in
                  the output filename instead of using the input filename. The
                  output files are called hst_<filter>_for_<field>.fits, where
                  <filter> is the name of the filter taken from the header of
                  the HST FITS file, and <field> is either the basename of the
                  MUSE input file (minus its .fits extension), or the value of
                  this optional parameter.
%

The regrid_hst_to_muse script gives each resampled output file a name, hst_<filter_name>_for_<muse_filename>, where <filter_name> is the name of the HST filter found in the FILTER keyword of the FITS header, and <muse_filename> is the name of the input MUSE FITS file, including its .fits suffix.

In the following example, the script is used to resample HST images taken through 2 different filters, called F606W and F775W:

% regrid_hst_to_muse IMA-R-MUSE-2014-09-24T05\:49\:36.365.fits hlsp_xdf_hst_acswfc-30mas_hudf_*.fits
Reading MUSE image: IMA-R-MUSE-2014-09-24T05:49:36.365.fits
Reading HST image: hlsp_xdf_hst_acswfc-30mas_hudf_f606w_v1_sci.fits
Resampling the HST image onto the MUSE pixel grid.
Changing the flux units of the HST image to match the MUSE image
Writing the output file: hst_F606W_for_IMA-R-MUSE-2014-09-24T05:49:36.365.fits
Reading HST image: hlsp_xdf_hst_acswfc-30mas_hudf_f775w_v1_sci.fits
Resampling the HST image onto the MUSE pixel grid.
Changing the flux units of the HST image to match the MUSE image
Writing the output file: hst_F775W_for_IMA-R-MUSE-2014-09-24T05:49:36.365.fits
%

By default the output filenames are based on the HST filter name and the name of the input MUSE file. If the MUSE file is just one exposure of multiple observations of the same field, then the regridded HST image will be usable for any of them. In this case it may be less confusing to indicate a name for the field, such that this gets used in the output filename, rather than the name of one exposure. This can be done using the optional --field argument, as follows:

% regrid_hst_to_muse --field UDF01 IMA-R-MUSE-2014-09-24T05\:49\:36.365.fits hlsp_xdf_hst_acswfc-30mas_hudf_*.fits
Reading MUSE image: IMA-R-MUSE-2014-09-24T05:49:36.365.fits
Reading HST image: hlsp_xdf_hst_acswfc-30mas_hudf_f606w_v1_sci.fits
WARNING: MpdafUnitsWarning: Error parsing the BUNIT: 'ELECTRONS/S' did not parse as unit: At col 0, ELECTRONS is not a valid unit. Did you mean electron? [mpdaf.obj.data]
Resampling the HST image onto the MUSE pixel grid.
Changing the flux units of the HST image to match the MUSE image
Writing the output file: hst_F606W_for_UDF01.fits
Reading HST image: hlsp_xdf_hst_acswfc-30mas_hudf_f775w_v1_sci.fits
Resampling the HST image onto the MUSE pixel grid.
Changing the flux units of the HST image to match the MUSE image
Writing the output file: hst_F775W_for_UDF01.fits
%

Note that the output filenames in this case, with the addition of the option --field UDF01, changed to hst_F606W_for_UDF01.fits and hst_F775W_for_UDF01.fits.

Resampling from within a python script

The resampling operation can alternatively be performed from within an arbitrary python script using the imphot.regrid_hst_like_muse() function, followed by imphot.rescale_hst_like_muse(). For example:

from mpdaf.obj import Image
import imphot

hst = Image(hst_fits_filename)
muse = Image(muse_fits_filename)
imphot.regrid_hst_like_muse(hst, muse, inplace=True)
imphot.rescale_hst_like_muse(hst, muse, inplace=True)