Echelle spectroscopy is done in the REMD mode by combining the medium-dispersion slit, one of the echelle gratings, and one of the grisms as cross-disperser. The echelle gratings can not be rotated.
A quicklook tool called PyQuick is available for EMMI echelle. See PyQuick page.
The following figure illustrate the light path (same as the REMD light path).
Three echelle gratings (#9, #10 and #14) are presently offered. They can be used in combination with a cross-dispersing grism to obtain data in an echelle format, Grisms #3 and #4 are used with the echelle grating #9, grisms #3, #4, #5 and #6 with the echelle grating #10 and #14. The properties of the echelle spectra obtained using the different cross-dispersers are given in the table below.
The new table below includes updated values and informations for the new Red CCD.
1. The Wavelength range is computed from the reduction of a standard star spectrum (i.e. a bright star). Users may find different values depending on the faintness of their objects and their reduction choices. Fore more informations about the reduction (and hence the values found here) see the Wavelength Calibration Atlas for the REMD-Echelle mode (contain Hints & Tips for the reduction).
2. The "CCD gap" orders column indicates the echelle orders that cross the Red CCD gap. Tracing and Wavelength calibration are more difficult/less precise in that part of the spectrum, but the strength of the influence of the CCD gap depends a lot on the Grating/Grism combination used. The "orders" here are the simply numbered orders, starting from the blue. An estimate wavelength range of where the gap occurs is indicated. (Again, see the Wavelength Calibration Atlas for the REMD-Echelle mode for more information)
3. It is advised to use binning 1x1 for all echelle modes, given the pixel element resolution with a slit of 1.0'' (see Table below).
For echelle spectroscopy, a mask can be mounted in order to reduce the inter-order scattered light by 30%. The presence of the mask limits the field of view to about 30'' in the slit direction. The mask can not be removed during the night. The echelle gratings can also be used without a cross disperser by using a filter to separate the order that you are interested in. See the filter page.
Echelle grating #14 is the grating which gives the highest resolution (up to 70 000 with a slit width of 0.8'', corresponding to a line width of 2.1-2.2 pixels at the blue end of the order) in combination with a wide wavelength range.
The order separations for the echelle gratings are shown below, for all cross-dispersers. (Updated - Dec 03)
Below is the picture of the efficiencies (in %) of the various Echelle gratings with their corresponding grisms. This picture is also available in Postscript format here. (Updated - Dec 03)
Send comments to: ls-emmi at eso dot org