Spectral resolution and stability

Spectral resolution

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