European Southern Observatory New Technology Telescope

General :
Observations :

EMMI Medium Dispersion Spectroscopy
(REMD - BLMD)

Generalities

Medium-Dispersion Spectroscopy is done in modes REMD or BLMD by combining one of the gratings with the adjustable medium-dispersion slit and, if needed, with an order-sorting filter. The grating can be rotated to achieve the desired wavelength range. The following figures illustrate the light-paths of both modes.

Slit and gratings

The width of the medium-dispersion slit can be continuously adjusted between 0.5'' and 10''. Note that the medium-dispersion slit is oriented North-South, when EMMI is at 0 degree rotator offset (this is in contrast of the orientation in the RILD mode). The slit can be rotated at any given angle on the sky by applying an offset to the rotator. This can be especially important in the blue, because of the differential atmospheric refraction. In that case, it is recommended to align the slit with the parallactic angle.

Four gratings are available in the red arm, and five in the blue arm. Their properties are described below.

The grating are permanently mounted two by two in five housings. Only one housing can be mounted at a time on each arm, thus only two gratings in the same housing are available at a given time for each arm. The holographic grating #11 is designed to approach the resolution (but not the wavelength coverage) of the echelle gratings on the red side, with its resolution of 0.1 nm

Grating throughput

Count rates for the different gratings are monitored during technical time, and scaled to an AB star of magnitude 15 and unit airmass. The values (in e-/s/nm) are given in the following table for data obtained between August 99 and January 2000. Updated values of the count rates can be found on the NTT web pages, as the new CCD becomes better characterized (23 Sep. 2002). For additional details concerning the gratings, see the manual.

Send comments to: NTT Webmasters <nttweb@www.ls.eso.org> Last Update: 25 Sep. 2002 (cf)