[ ESO ] La Silla

La Silla - Science Operation Department


CES - The image slicers

The image slicers

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.

[Profile of the CES spectrum with the high-res. image slicer]

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
Image slicer # slices Relative slicer efficiency Equivalent "slit" width Profile width on CCD Resolving power Line FWHM @ 5400 Å
High-res. 10-12 1.00 108 µm 350-420 pixel 220,000-235,000 2.65-2.83 pixel
Medium-res. 7 1.20 216 µm ~245 pixel 135,000-150,000 4.15-4.62 pixel
Low-res. 5 1.25 341 µm ~175 pixel 80,000-100,000 6.23-7.79 pixel

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.

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