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ESPaDOnS: thermal response and spectral stability http://webast.ast.obs-mip.fr/magnetisme/espadons_new/stability.html
Spectral stability
By taking calibration frames during a complete night (at a rate
of one every 10min) and by correlating all images with respect
to the first one in the series, it is possible to see how the position
of the spectrum with respect to the ccd varies with time; this
experiment is very useful to estimate how much spurious
spectral radial velocity changes are induced by thermal and
mechanical relaxation within the spectrograph. The graph on
the right show the changes in the radial velocity of the thorium
spectrum (in km/s, full line) with respect to the first spectrum
of the series, while the 2 other curves depict the corresponding
temperature and pressure changes (in deg and mbar, dashed
and dash-dot line respectively) throughout one night.
We find that the position of the thorium spectrum with respect
to the ccd varies by typically:
-3.5 km/s per deg change in the spectrograph
temperature;
0.3 km/s per mbar change in external pressure.
Once the temperature and pressure effects are subtracted off, the residual changes in radial velocities, equal to about 20m/s
rms, indicate what the true absolute stability of the spectrograph is. Note that this experiment demonstrates clearly the need for
an outer enclosure with thermal regulation to reduce the shifts with temperature as much as possible and make them depend
mostly on pressure.
From such a series of thorium frames, we can also estimate the relative stability of the instrument (with respect to a given
spectral reference). Using the even thorium frames as the reference and the odd thorium frames as the test spectrum whose
stability is to be checked, we obtain that the relative stability is better than 10m/s rms, for a time lag of less than 10min between
the object and reference measurements.
© Jean-François Donati, last update May 10 2004
2 of 2 08/07/04 11:30 PM