Spectral resolution and stability
The spectral resolution of NARVAL is 65,000 in polarimetric
mode and 'star+sky' mode,
and reaches 75,000 in 'star only' mode. This is very similar to what is achieved
with ESPaDOnS. The curve on the right presents the resolving power of NARVAL
in polarimetric mode (full line) compared with that of ESPaDOnS (dashed line) as
a function of order number (red to the left and blue to the right).
Optical aberations within the spectrograph were estimated by taking images with
different pupil sizes within the instrument. They are found to be nominal, with
a spot diagram featuring a full width at half maximum smaller
than 8 um at detector level (ie half a CCD pixel) through the whole spectrum.
However, spectral resolution should be higher if it were limited by the slit width
and optical aberrations only. The same problem is seen with ESPaDOnS. We suspect
the CCD (and in particular the vertical charge transfer efficiency) to be the
cause of this degradation.
The spectrograph is equipped with a double layer thermal enclosure. The outer
enclosure is thermally controlled and ensure that the inner temperature is constant
within 0.1 K. The second, inner, enclosure is passive and contains the optical
bench and all optical components of the spectrograph. This design ensures that
the thermal stability with the spectrograph is of order of
a few 0.01 K on all time scales (curve to come soon).
This is typically a factor of 10 better than what is achieved for ESPaDOnS.
Residual shifts within the instrument throughout individual nights are still
present, at a level of a few 100 m/s and likely reflect pressure fluctuations.
We correct them by tracking shifts of telluric lines throughout the spectrum.
The acccuracy on the radial velocity variations after this correction is of
of order 10 m/s (rms) throughout one night, and about
20 m/s (rms) on a timescale of a week.
© Jean-François Donati, last update 2007 Jan 20