ESPaDOnS can be used to do plain spectroscopy (with or without a sky spectrum), or spectropolarimetry (linear or circular polarization). All 3 modes are available in QSO mode.
In the spectroscopic mode 'star only', only the light from the object goes through the instrument. The light is gathered at the f/8 Cassegrain focus of the telescope by the 1.6 arcsec aperture hole and brought to the spectrograph, in the 3rd floor Coude room, through 30m of optical fibers. At the entrance of the spectrograph, the slicer slices the image into 6 slices, which are then dispersed by the grating and the prism cross disperser and recorded on the detector. With 6 slices, the spectral resolution is about 80,000.
In the spectroscopic mode 'star + sky', the light from the star is gathered through the same 1.6 arcsec aperture hole, and the light from the sky located about 7.9 arcsec South is gathered through a bigger aperture hole 2.2 arcsec across. Both spectra are fed to the spectrograph and are recorded simultaneously on the detector. Since 2 spectra are now recorded, the images are sliced into 3 slices only so they fit on the detector without overlapping. This means the resolution is lower than in the 'star only' spectroscopic mode, and is around 68,000.
In the polarimetric mode, the light from the star, going through the same 1.6 arcsec aperture hole, is analyzed by the optical components in the polarimeter, where 2 beams of opposite polarizations are produced and fed to the spectrograph. Once again, 2 interleaved spectra are recorded on the detector, with images sliced in 3, which gives a spectral resolution of about 68,000.
In all modes, 40 orders are present, covering from 3700Å to 10500Å, with 3 very small gaps: 9224-9234Å, 9608-9636Å, 10026-10074Å. Orders actually overlap, except for a few Å in the red.
In addition to these 3 observing modes, ESPaDOnS can be used in one of 4 CCD readout modes. The XSlow mode is NOT offered in QSO mode.
| Mode | Readout noise | Gain | Readout time |
|---|---|---|---|
| Fast | 8.0e | 1.8e/ADU | 25 sec |
| Normal | 4.5e | 1.4e/ADU | 40 sec |
| Slow | 2.9e | 1.2e/ADU | 67 sec |
| XSlow | 2.5e | 0.8e/ADU | 90 sec |
Since late 2005, the CCD's temperature is very well regulated, and the noise is much more stable during a run. Please note that the readout noise values mentioned above are approximate only.
Very bright targets (for which a higher readout noise will not be detrimental) will benefit from the Fast readout mode, while very faint ones could use the Slow. The default is the Normal mode, and targets down to magnitude 12 or 13 have been successfully observed in that mode.
Saturation occurs at about 60,000 ADUs for all modes.
