CFHT operating manual - Homepage Usask

ESPaDOnS: data reduction routines http://webast.ast.obs-mip.fr/magnetisme/espadons_new/reduction.html
distance from order center only (for the slit shape, assumed to be identical for all orders).
The previous two pieces of information are then merged together to derive a new curvilinear coordinate system for each order,
with one coordinate being the distance from order center and the second one the distance along the order from the slit position
at the first pixel of the order. The comparison frame is then extracted within this curvilinear system to obtain a ThAr spectrum,
with flux as a function of distance along each order.
Finally, this ThAr spectrum is used to derive automatically the details of the wavelength to pixel relationship at order centre (ie
dispersion relation); to achieve this, the code starts by searching, fitting and identifying thorium lines iteratively in each order
with no human help, then fits with a 2d polynome the position of all lines successfully identified (up to several thousands
typically) as a function of both order number and distance along the order. With this scheme, each line effectively participates,
not only in the wavelength calibration of a single order, but also in the wavelength calibration of all orders simultaneously,
making this process very robust and accurate. The typical rms precision of the derived wavelength calibration at any given pixel
is about 150m/s.
Optimal extraction of stellar spectra (step #2)
In the second step, optimal extraction of each order in each
polarisation spectrum of each subexposure is performed, using
the curvilinear coordinate system set up in step #1. The graph
on the right shows an example optimal extraction of a solar
spectrum in the particular case of order #30 (centred on
750nm), in which one group of very strong telluric lines is
clearly visible in the last third of the order.
The optimally extracted spectra from each subexposure and
each polarisation state are then combined together in a specific
way to obtain the intensity, polarisation and check spectra,
along with the error bars associated to each spectrum point.
Finally, automatic continuum normalisation and wavelength
calibration (with the dispersion polynomes derived above) of
the resulting spectra is achieved, and radial velocity corrections
from earth spin and orbit motions are applied to the
wavelength scale before storing the final result into a
multicolumn ascii file.
A complete spectrum obtained with ESPaDOnS and reduced
with Libre-ESpRIT is worth about 190,000 data points, each point corresponding to a velocity bin of 1.8km/s.
© Jean-François Donati, last update May 25 2004
2 of 2 08/07/04 11:33 PM