Spectral domain and resolution
The image on the right represents an example flat field frame taken with ESPaDOnS in
polarimetric mode (using light from a combination of tungsten lamps and filters so that
all orders get a reasonable illumination level). Orders are clearly visible on this
image, where they show up as bright slightly curved strips running vertically, successive
orders being stacked next to each other from the left to the right of the ccd. As obvious
from this image, the order separation varies with wavelength, being largest in the blue (right
side of image) and smallest in the red (left side of image) as expected from a prism
crossdisperser. A close up view of the small scale structure of the orders is displayed
in the insert
(bottom right of image), where the two spectra associated to each order in polarimetric
mode (one spectrum per orthogonal state of the selected polarisation to be measured) are
Up to 40 orders are visible on the image, the first one
being order #22 (centred at 1029nm) on the left side of the chip and the last one being
order #61 (centred at 372nm) on the right side of the chip. Apart from very small gaps
on the edges of the 3 reddest orders (between 922.4 and 923.4, 960.8 and 963.6nm, 1002.6
and 1007.4nm), the wavelength coverage is complete from 369 to 1048nm
and can be obtained in a single exposure.
When reducing the data, the first operation consists at tracking the location and shape of
all orders across the whole chip to a rms accuracy of better than 0.1pxl.
The image on the right represents an example calibration frame taken with ESPaDOnS in
polarimetric mode (using light from a combination of a thorium/argon and a thorium/neon
lamp with filters to minimise the amount of strong red lines blooming the chip). As
obvious from this image, a very large number of lines are present in each order, from
which the accurate relation between pixel number along and across each order can be
derived. The spectral resolution achieved is derived from the width of these lines.
A close up view of the individual thorium lines is shown in the insert (bottom right
of image) where one can see again the dual structure of each order (the gap between the
two spectra as well as the instrumental width of the lines (slightly lower than 2pxl).
An average number of about 50 lines per order (about 2,000 lines
in total) are automatically searched for by the reduction routine and identified using
reference lists of thorium line wavelengths; from these, wavelength calibration
polynomials are produced over the full spectral range. The typical accuracy of this
calibration is found to be of order of 0.06pxl or 150m/s at each
The few remaining neon lines blooming the ccd in the red part of the domain do not really
affect the precision of the calibration procedure.
By measuring the full width at half maximum of the individual thorium lines (reflecting
mostly the instrumental broadening), one can determine the spectral resolution of
ESPaDOnS in the selected instrument configuration (the reduction code does it
The graph on the right shows an example of such thorium lines (the
strongest of the 3 being the ThI line at 550.75385nm). The full line indicates the
wavelength calibrated spectrum around this line derived with ESPaDOnS being set in
polarimetric mode, while the dashed line depicts that obtained in the 'object only'
spectroscopic mode. The respective line widths (at half maximum) are respectively
equal to 8.3 and 6.9pm, in agreement with the spectral resolutions of
68,000 and 81,000 associated to these modes.
These resolutions correpond to velocity elements of 4.4 and
3.7km/s respectively, to be compared to the 2.6km/s ccd pixel size and the
1.8km/s bin size on which the spectra are recovered.
© Jean-François Donati, last update May 5 2004