Instead of a slit the CES is used with an image slicer, which
slices up the spot of light
from the fibre and rearranges the slices in a slit-like fashion. As this
"slit" is composed of various slices it has a varying intensity profile
(along the "slit").
The intensity profile of the spectrum perpendicular to the dispersion
direction is shown in the figure below.
In this figure both the profiles obtained with the flatfield lamp and
the profile of a stellar exposure are compared.|
The following table summarizes the properties of the image slicer.
For historical reasons, also data for the two lower-resolution slicers are shown. The second
column lists the number of slices typically present with the respective
slicer; column 3 gives the slicer efficiency relative to the
high-resolution slicer; column 4 lists the width
of the slicer produced equivalent "slit"
at the entrance to the spectrograph; the width of the slicer profile
on the CCD (perpendicular to the dispersion) is given in
column 5; column 6 gives the achievable resolving power for each slicer;
and column 7 lists the line width corresponding to this resolving power at
a selected wavelength of 5400 Å (to show an example). Note that this
line width is wavelength dependent.
The CES image slicers
||Relative slicer efficiency
||Equivalent "slit" width
||Profile width on CCD
||Line FWHM @ 5400 Å
Note also that the number of slices, profile width, achievable
resolving power, and line width are given here as ranges reflecting the
typical uncertainties encountered during instrument setup. Instrumental
setup is a time consuming and delicate procedure and in practise the
optimum spectral resolution
cannot always be guaranteed. At any rate an attempt is made to achieve
resolution values within the listed ranges.