Spectral response and throughput
As ESPaDOnS, NARVAL provides
complete coverage of the optical spectrum (from 370 to 1,050 nm)
in a single exposure with a resolving power of at least 65,000. The full spectrum spans
40 grating orders (from order #61 in the blue to order #22 in the red). Only very small
gaps are present on the edges of the 3 reddest orders (between 922.4 and 923.4, 960.8
and 963.6nm, 1002.6 and 1007.4nm).
As with ESPaDOnS, three different modes are available.
In the first mode (called
polarimetric mode), one can
measure the intensity and polarisation spectra of the observed star (through sequences
of 4 subexposures). In the second mode (called 'star only' mode), one only measures
the intensity spectrum, with no information derived on the polarisation spectrum. The third
mode (called 'star+sky' mode) allows to determine simultaneously the intensity
spectrum of the object as well as that of the neighbouring sky.
The total throughput of NARVAL as derived from observations of standard stars
with known temperatures and magnitudes
is shown on the right (full line) and compares well with that of ESPaDOnS (dashes).
The efficiency peaks at about 15% in the V band, including
telescope, atmosphere and detector, but excluding injection and guiding losses.
In nominal seeing conditions (ie 1.2 arcsec), injection losses amount to about 10%
(the entrance aperture of NARVAL being a 2.8 arcsec circular hole), while guiding
losses also reach about 10%; the fractional flux entring the instrument roughly
decreases in inverse proportion with the square of the seeing as seeing degrades.
Both the polarimeter (plus the telescope, the fiber link and the slicer)
and the spectrograph (plus the CCD detector) have efficiency curves peaking at about
The measured throughput of the whole instrument is close to the expectations derived
from the estimated transmission of individual components (see
on ESPaDOnS' throughput), with NARVAL being closer than ESPaDOnS to the predictions.
The main difference between NARVAL and ESPaDOnS is thought to result
from the CCD,
presumably better and more sensitive for NARVAL (E2V grade 0) than for ESPaDOnS (E2V grade 1),
especially in the blue range of the spectral domain.
© Jean-François Donati, last update 2007 Jan 20