Spectral response and throughput

Spectral response

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.

Measured throughput

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 40%.

The measured throughput of the whole instrument is close to the expectations derived from the estimated transmission of individual components (see page 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