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La Silla Science Operations: NTT

EMMI Spectroscopy Characteristics

EMMI supports spectroscopy with either grisms or gratings, using two different light paths.

Grisms are available in the red arm only and are mounted in the RILD mode: they provide low resolution spectroscopy and allow for either long-slit observation or MOS.

Gratings provide higher resolution but can only be used with a long slit. Separate gratings are mounted in the blue arm (BLMD) and in the red arm (REMD).

An echelle grating can be mounted in the red arm (REMD-HighRes), using one of the grisms as cross disperser.

The following table gives an overview of the characteristics of EMMI spectroscopy. The second table gives the characteristics of the MOS mode.

Spectroscopic modes

Observation type wavelength Mode resolving power field of view
Low-dispersion long-slit spectroscopy >400nm EMMI red R theta=280-1670 slit length 8 arcmin, width 0.5-10 arcsec
Low-dispersion slitless spectroscopy > 400nm EMMI red R theta=280-1670 9.15 times 8.6 arcmin
Low-dispersion multi-object spectroscopy (a) > 400nm EMMI red R theta=280-1670 5 times 8.6 arcmin
Medium-dispersion long-slit spectroscopy < 500nm EMMI blue R theta=840-9000 slit < 6 arcmin
> 400nm EMMI red R theta=600-50000
echelle spectroscopy > 400nm EMMI red R theta=7700-70000 inter-order spacing

(a) no. of slits of order 25
(b)combined BLMD (lambda<410nm) and REMD (lambda>490nm)

MOS capabilities

Wavelength range 380-1000nm
Accessible field 5 times 8 arcmin
Aperture shape Slit
Slit width 0.8, 1.0, 1.34, or 1.8 arcsec
Slit length 4 or 8 arcsec per slitlet
No. objects per field 10-30 (if one spectrum per CCD row)
Punching machine On-line (inside EMMI) or EFOSC2 punch


Emmi has a long light path inside the instrument; as a consequence its spectroscopic flats show absorption lines of water vapour and molecular oxygen, see here and example for grism #2. Here there is the one-dimensional plot for the wavelength calibrated normalized flat; the absorption at 7600A is indicated with an arrow. As a consequence of this, the flat in this area will not be accurate, with a variation in the normalized flat of the order of 3.5%.

Something else you should be aware of is that EMMI long slit is not uniformly illuminated; if you plan to take spectra of extended objects, it is advisable to take one to three spectroscopic sky flats, to correct for the slit illumination function.

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