[ ESO ] La Silla


La Silla Science Operations: NTT

EMMI - ESO Multi Mode Instrument

General Description

EMMI is a multipurpose instrument allowing observations in the visible wavelengths. This page is intended for providing a short and concise description of the instrument and its capabilities.



EMMI is a versatile instrument, allowing a wide range of observations in the visible wavelength (between 300 and 1000 nm). Precisely:

EMMI is permanently mounted on an adaptor-rotator at the Nasmyth B focus of the NTT.


Observing modes:

EMMI is divided in two "arms" defined by optical elements coated for high efficiency from 300 to 500 nm for the "blue arm" and from 400 to 1000 nm for the "red arm" and with separate detectors. Each arm has two possible light paths: one for imaging and one for grating spectroscopy. In the red arm only, the imaging mode also supports low-resolution spectroscopy using grisms. Each of the four possible light paths is called an "Observing Mode". They are called:

Switching between modes is possible at any time, limited only by the optical elements which can be mounted simultaneously. The figure below shows schematically the EMMI instrumentation. A Close-up of EMMI shows the optical components including mirrors, filter wheels etc.



What mode for what observations?

Imaging can be done in the RILD mode for observations redder than 400 nm and in the BIMG mode for wavelength bluer than 500 nm. In the region of overlap, it is advisable to use the blue arm (although the red arm has a larger field size and smaller pixel size), since it has about 65% better sensitivity and gives more accurate B-band photometry.

Low-dispersion spectroscopy for lambda > 400 nm is done in RILD. Long-slit, slitless and multi-object spectroscopy are possible with resolutions ranging from 280 to 1670.

Medium-dispersion spectroscopy is done in REMD for lambda > 400 nm and in BLMD for lambda < 500 nm. In the region of overlap, REMD gives a larger wavelength coverage and better spatial sampling, while BLMD gives better throughput.

Finally, echelle spectroscopy can be done in REMD with a large wavelength coverage and a resolution up to 70 000. One can also choose to do echelle spectroscopy without the cross-disperser. In this case, one has to choose a filter to sort the order.

This following table summarizes the correspondence between type of observations and EMMI mode (and SUSI), and lists the main specifications. It is meant to help identify which mode would be most appropriate for your planned observations.

Observation Type
Wavelength range
Wide-field imaging
< 500 nm
> 400 nm
6.2' x 6.2'
9.1' x 9.9' **
High-resolution imaging
2.2' x 2.2'
Low-dispersion longslit spectroscopy
> 400 nm
slit 8'
Low-dispersion slitless spectroscopy
> 400 nm
9.1' x 9.9' **
Low-dispersion multi-object spectroscopy
> 400 nm
5' x 8'
Medium-dispersion spectroscopy
< 500 nm
> 400 nm
R=600-70 000
slit < 6'
slit < 6'
Echelle spectroscopy
> 400 nm
R=7700-70 000

* combined BLMD (lambda<410nm( and REMD (lambda>490nm). Not available anymore, starting of P71.
** As of May 2002, with the new Red CCD.

The following figure summarizes the different wavelength ranges and resolution reached by the different Grating/Grisms on EMMI. Grisms: solid lines. Grating: short dashes. Echelle gratings: long dashes.

For detailed description of each mode, start with in the main page.


A short summary of EMMI capabilities

[Observing] [ESO La Silla]  [The SciOps Home Page]  [ESO]  [Index]  [Search]  [Help]  [News]