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astromalign

Astrometric alignment of image stacks

This code depends on the Astrometry.net python libraries, which can be installed from the latest release: http://astrometry.net/downloads/astrometry.net-latest.tar.gz or from github: https://github.com/dstndstn/astrometry.net

You can either install it:

  • make
  • make py
  • make install INSTALL_DIR=/place/to/install

  • add /place/to/install/lib/python to your PYTHONPATH

or build a local copy by downloading or cloning into a directory (renamed to astrometry) within the astromalign directory. You need to make && make py in the astrometry directory to build the python libraries.

Using

Super steep learning curve, roughly zero documentation.

People doing HST work using Dolphot will want to use the singles.py script, which takes a set of data directories containing Dolphot star catalogs (in FITS format) and aligns them with each other and with a reference catalog.

Others may want to (try) using the code with generic FITS catalogs. For example, let's align subsets of the 2MASS and NOMAD catalogs:

from astrometry.util.fits import fits_table

# Read catalogs, cut to small subregion
twomass = fits_table('2mass_hp000.fits')
nomad = fits_table('nomad_000.fits')
print(len(twomass), '2MASS and', len(nomad), 'NOMAD stars')

from astrom_common import Alignment
from astrom_inter import findAffine

# Matching radius in arcsec
radius = 1.
# Compute an alignment "from" NOMAD "to" 2MASS
align = Alignment(nomad, twomass, radius)
# this function actually does the work...
align.shift()

# Plot the dRA,dDec cloud
import pylab as plt
from astrometry.util.plotutils import loghist
plt.clf()
loghist(align.match.dra_arcsec, align.match.ddec_arcsec)
plt.savefig('before.png')

# Now compute an affine transformation from nomad to twomass, given the "alignment"
refradec = [np.mean(twomass.ra), np.mean(twomass.dec)]
aff = findAffine(nomad, twomass, align, refradec)

# Apply the affine transformation to the NOMAD stars to bring them onto the
# 2MASS system.
nomad.ra, nomad.dec = aff.apply(nomad.ra, nomad.dec)

# Plot the dRA,dDec cloud after
align.match.recompute_dradec(nomad, twomass)
plt.clf()
loghist(align.match.dra_arcsec, align.match.ddec_arcsec)
plt.savefig('after.png')