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ESPaDOnS: spectral domain and resolution http://webast.ast.obs-mip.fr/magnetisme/espadons_new/resol.html
Echelle orders
ESPaDOnS
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 clearly visible.
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.
Wavelength calibration
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).
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