How to output the DATE_OBS

[simonlucalandi]

Hi all.
In the last few months I collected several hours of sky data using my simple setup and manually extracted the photometry for a few variable stars.
But I don't want to waste all the other data captured... so I'm thinking to build a kind of "survey" tool, using Sourcextractor++ to analize images and create a catalog with the photometry for all the objects found, and than import the catalog in a database.
How to add the date of the observation to the output catalog? What is the best practice here?
Any suggestion is really appreciated.

[ebertin]

Hi @simonlucalandi,
sorry for the late reply.
You can have access to FITS header keyword using a constant parameter and the meta dictionary of the image object (see example below).
Unfortunately, I don't think that you can presently copy string variables to the output SourceXtractor++ catalog. However you can use Julian dates for examples. Please find attached an example of an SX++ configuration file, the very one used for Figure 1 in Bertin et al. 2020. It stores the Modified Julian Date in a column for every exposure. It actually does much more than that, as it fits proper motions straight out of a series of exposures taken at different epochs (and through different filters, but this is irrelevant here):

from sourcextractor.config import *
from glob import glob
import numpy as np

top = load_fits_images(
    sorted(glob('im_?_??.fits')),
    sorted(glob('im_?_??.psf')),
)

# Split the top list by filter
top.split(ByKeyword('FILTER'))

# Split each band by image filename (one image per subgroup)
for n,filter in top:
  filter.split(ByKeyword('IMAGE_FILENAME'))

mesgroup = MeasurementGroup(top)

MAG_ZEROPOINT = 32.19
 
i = 0
flux = {}
mag = {}
x,y = get_pos_parameters()
pmx = FreeParameter(0.0, Range((-100.0,100.0), RangeType.LINEAR))
pmy = FreeParameter(0.0, Range((-100.0,100.0), RangeType.LINEAR))
add_output_column('pmx', pmx)
add_output_column('pmy', pmy)
for band, group in mesgroup:
  flux[i] = get_flux_parameter()
  mag[i] = DependentParameter(lambda f: -2.5 * np.log10(f) + MAG_ZEROPOINT, flux[i])
  add_output_column(f'mag_{band}', mag[i])
  n = 1
  for filename, ima in group:
    tp = ConstantParameter((float(ima[0].meta["MJD-OBS"]) - 6000.0) / 365.25)
    add_output_column(f'mjd_{band}_{n}', tp)

    xr = DependentParameter(lambda x, dx, tp: x + tp*dx, x, pmx, tp)
    yr = DependentParameter(lambda y, dy, tp: y + tp*dy, y, pmy, tp)
    ra,de = get_world_position_parameters(xr, yr)
    
    add_model(ima, PointSourceModel(xr, yr, flux[i]))
    add_output_column(f'ra_{band}_{n}', ra)
    add_output_column(f'dec_{band}_{n}', de)
    n += 1
  i += 1

SX++ was run with

$ sourcextractor++ --detection-image=im_detection.fits --output-properties=FlexibleModelFitting --python-config-file=pm.py --output-catalog-filename=pm.cat

I hope this helps.