The AstroGwyn astrometric calibration pipeline is run on the
images. The first step is to run SExtractor on each image. The
parameters are set so as to extract only the most reliable objects (5
sigma detections in at least 5 pixels). This catalog is further
cleaned of cosmic rays and extended objects. This leaves only
real objects with well defined centres: stars and (to some degree) compact galaxies.
This observed catalogue is matched to the USNO astrometric reference catalogue.
The (x,y) coordinates of the observed catalogue are converted
to (RA, Dec) using the initial Elixir WCS.
The catalogues are shifted in RA and Dec with respect to one
another until the best match between the two catalogues is
found. If there is no good match for a particular CCD
(for example when the initial WCS is unusually erroneous),
its WCS is replaced with a default WCS and the matching
procedure is restarted. Once the matching is complete,
the astrometric fitting can begin. Typically 20 to 50
sources per CCD are found with this initial matching.
Elixir provides a first order solution for the WCS with
typical errors on the order of 1 arcsecond.
AstroGwyn improves on this to provide a higher order solution
with an accuracy of typically 0.2 arcseconds.
As the accuracy of the WCS improves, the observed and reference
catalogues are compared again to increase the number of
matching sources. A larger number of matching sources makes the
astrometric solution more robust against possible errors (proper motions, spurious detections, etc.)
order terms are determined on the scale of the entire mosaic - that is to
say, the distortion of the entire focal plane is measured.
This distortion is well described by a polynomial with second and
fourth order terms in radius measured from the centre of the mosaic.
The distortion appears to be stable over
time, even when some of the MegaPrime optics are flipped.
Determining the distortion in this way means that only
2 parameters need to be determined (the coefficients of r2
and r4) with typically (20-50 stars per chip) * (36 chips) =~ 1000
observations. If the analysis is done chip-by-chip, a third order solution
requires (10 parameters per chip)*(36 chips)= 360 parameters. This is
From the global distortion, the distortion local to each CCD is
determined. The local distortion is translated into a linear part
(described by the CD matrix) and a higher order part (described by the
PV keywords). The transformation was described in detail in an
appendix of the first draft of the MegaPipe paper. The appendix was
removed but is available here. The
higher order part is 3rd order as well, but the coefficients depend
directly and uniquely on the 2 parameter global radial distortion. The
error introduced by this translation is less than 0.001 arcseconds.
For the first band to be reduced (the i-band, if it exists, otherwise the order of
preference is r, g, z, u), these source
catalogues are matched with the an external astrometric catalogue to
provide an initial astrometric solution.
This external catalogue is either the USNO
A2 or the Sloan Digital Sky
For the other bands, the image catalogues are first matched to the
USNO to provide a rough WCS and then matched to a catalogue generated
using the first image so as to precisely register the different bands.
The final astrometric calibration has an internal uncertainty of about
0.03 arcseconds and an external uncertainty of about 0.2 arcseconds,
as discussed here.