EUROPEAN SOUTHERN OBSERVATORY La Silla Science Operations: NTT
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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.

Introduction:

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

• Wide-field imaging,
• Low-resolution multi-object or long-slit grism spectroscopy,
• Medium-resolution long-slit grating spectroscopy
• Echelle spectroscopy.

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:

• RILD : Red Imaging and Low Dispersion Spectroscopy
• BIMG : Blue Imaging Mode
• REMD : Red Medium Dispersion Spectroscopy
• BLMD : Blue Medium Dispersion Spectroscopy

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	Mode	Resolution	FOV
Wide-field imaging	< 500 nm > 400 nm	BIMG RILD	0.37''/pix 0.1665''/pix	6.2' x 6.2' 9.1' x 9.9' **
High-resolution imaging		SUSI	0.13''/pix	2.2' x 2.2'
Low-dispersion longslit spectroscopy	> 400 nm	RILD	R=280-1670	slit 8'
Low-dispersion slitless spectroscopy	> 400 nm	RILD	R=280-1670	9.1' x 9.9' **
Low-dispersion multi-object spectroscopy	> 400 nm	RILD	R=280-1670	5' x 8'
Medium-dispersion spectroscopy	< 500 nm > 400 nm .	BLMD REMD DIMD*	R=840-9000 R=600-70 000 BLMD+REMD	slit < 6' slit < 6' .
Echelle spectroscopy	> 400 nm	REMD	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

• For each observing mode of EMMI there are a number of optical elements that can be installed in order to configure the instrument in your specifications. Not all of these components (grisms, gratings, filters...) can be mounted together and therefore you must specify the instrumental configuration required for your observations. The setup can not be changed during the night. The NTT team does not guarantee the availability of configurations that differs from the ones mentioned in your Phase I proposal.
  
  

• EMMI has a grism wheel in the red arm only. It has 9 positions of which 1 is kept free for direct imaging and 1 is used for the focus wedge. Two of the remaining positions are rotated by 90 degrees to be used as cross-dispersers for the echelle gratings. Thus, for low-resolution spectroscopy, five grisms can be mounted for a single night. Changing the grisms during the night is not possible.

• EMMI contains two grating units, one in each arm, which are used for (long-slit) medium-resolution spectroscopy. The gratings are mounted in special housings, and one such housing can be installed on each arm of EMMI. The housings can contain two gratings back to back. 8 housings, five for the red arm and 3 for the blue arm are presently available. The housings cannot be changed during the night.

• There are 3 echelle gratings available for the red arm. Two of these, #10 and #14 (the ones with the highest resolution), have a special housing containing only one grating. Exchanging between these two gratings of the other housings in an observing run is normally not possible, unless requested in the Phase I proposal.

• EMMI has a slit unit mounted in the red arm and used for grism spectroscopy. It contains a wheel which has 4 positions to insert a starplate, i.e. a dismountable plate containing a slit. Thus, up to 4 slits can be mounted at any given time. There are 6 fixed slits available, with widths of 0.5'' to 10'', each with a length of 8'. It is also possible to make your own starplate, to get an off-center slit or slitlets for MOS. For this purpose, a punching machine is mounted inside EMMI. A coronographic glass plate can also be installed in the starplate wheel. It can be used in conjunction with a filter and/or a grism (either imaging or slitless spectroscopy is possible), but not with a long slit. The plate contains 6 black dots of size ranging from 1'' to 5''.

• EMMI contains a slit unit used for grating spectroscopy. It is mounted in front of the beam spliter so that the same slit is used for both arms. The slit length can be adjusted between 3'' and 330'' and its width between 0.2'' and 10''. A technical CCD camera provides images of the central field of view of the telescope as reflected off the slit jaws in grating spectroscopy. It should be noted that the two slit units are rotated over 90 degrees with respect to each other. The grating slit unit is oriented North-South, whereas the starplate slits (RILD) are oriented East-West. This often causes confusions because on the CCD, the spectra are always oriented the same way, the dispersion being parallel to the X-axis. EMMI can be rotated as a whole to align the slit with any particular angle on the sky, but this rotation angle differs by 90 degrees between the two modes.

• For the echelle spectroscopy, a mask can be mounted which limits the field of view to about 30'' in the slit direction. It reduces the inter-order scattered light by 30%. This mask is used for echelle gratings #10 and #14, but not for echelle grating #9.

Apr 21 2004 (Ela)