EUROPEAN SOUTHERN OBSERVATORY
At the entrance pupil of EMMI, a wheel is mounted containing five different mirrors. Each mirror is optimised for one particular light path of EMMI.
The RLD mirror reflects the light towards the starplate wheel in the RILD mode. This light path is used for red imaging and grism spectroscopy. The mirror is optimised for red reflectivity, giving a reflectivity of 99% between 400 and 1000nm.
The BMG mirror, optimised for blue reflectivity, sends the light to the blue imaging mode (BIMG).
The three remaining mirrors all send the light to the long-slit unit used in grating spectroscopy. Two mirrors are optimised in terms of their reflectivity curves for either red-arm (REMD) or blue-arm (BLMD) spectroscopy.
The last is used for dichroic spectroscopy (DIMD) and had good reflectivity over the entire wavelength region (300-1000nm), although lower than the other mirrors for their optimised wavelengths. Note that the DIMD mode is no longer available, starting for P71.
In the grating spectroscopy modes, a 6-acrminutes long slit is mounted in the focal plane. The slit is located in front of the prism wheel where the beam is split, so that the same slit is used for both arms. The image scale at the focal plane is 156 micron per arcsec (f/11).
The slit width is continuously adjustable between 0.5 and 9 arcsec. Different slit widths can be used for consecutive exposures.
A movable decker is mounted on the slit to allow the length of the slit to be reduced. This is necessary in echelle spectroscopy to match the slit length to the interorder separation. The shortest possible slit is 2.5 arcsec.
A Technical CCD camera provides images of the central field of view of the telescope as reflected off the slit jaws. Under good circumstances, 18-magnitude stars can be seen on the slit viewer. The field is about 1.5 arcminutes.
The default slit orientation is North-South: the angle on the sky can be adjusted by applying a rotator offset to EMMI. Note that the slit units used in grism spectroscopy and grating sepctroscopy are rotated with respect to each other by 90 degrees.
A Technical CCD camera provides images of the central field of view of the telescope as reflected by the MEDIUM DISPERSION slit jaws when in REMD, or BLMD (or DIMD) modes. The images are displayed in a Real Time Display.
Below the long-slit unit the beam is split between the blue and red arm (BLMD and REMD respectively). For this purpose, a wheel is mounted containing three prisms. Two of the prisms reflect the light either to the red grating unit or blue grating unit, depending on which prism. The third prism acts as a Dichroic, sending the light to both grating units simultaneously.
The size of the prisms is limited to the maximum field of the long-slit unit, 1 by 6 arcminute.
Following the prism wheel containing the beam splitters, both the blue and th red arm have a filter wheel containing four positions of which one is always kept free. These are used for neutral-density filters, to attenuate the light from internal calibration lamps. Available filters are: 0.3, 0.5, 1.0, 2.0, 3.0, 4.0, where the number indicates the log of the attenuation factor
All other filters, including order-sorting filters used in red spectroscopy, are mounted in the filter wheels in front of the camera/detector.
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 facing back to back mounted on a rotary support. Eight housings, five for the red arm and five for the blue, are presently available. The housings cannot be exchanged during the night, however for housings containing two gratings both gratings can be used.
A mask can be mounted in the REMD mode to limit the amount of stray light entering the camera. The mask limits the field of view, and therefore the slit length, to 30 arcseconds. Once installed, it can not be removed during the night.
The mask is used mainly for echelle spectroscopy where it reduces the interorder light by 30%. It can in principle also be useful for grating spectroscopy on small objects.
The mask does not affect the blue arm in any way, so that long-slit spectroscopy in BLMD is still possible when the mask is mounted.
Between the collimator and the camera/detector in the red arm, two wheels are inserted in the parallel beam. The first is the red filter wheel (FILR), the second the grism wheel. The blue arm has only one wheel in front of the camera, inserted in the converging beam and used for filters.
The filter wheels take 80-mm circular filters; adaptors are available for some smaller filters but they will produce severe vignetting even in the centre of the field. Seven filters can be mounted at any one time; an eight position is kept free. The red filters are mounted under an angle to avoid reflections with the CCD. The blue filters, mounted in the converging beam, do not show this effect and are mounted under zero-degree inclination.
The grism wheel contains nine positions of which one is kept free. Five positions are used to mount grisms for low-dispersion spectroscopy. Two additional positions are used to mount grisms as cross disperser to be used in the echelle mode. The last position is used for the focus wedge, which is used to focus EMMI in the RILD mode.
In the imaging/grism spectroscopy mode (RILD), the star-plate wheel is mounted within the focal plane. The wheel contains 5 positions, of which one is always kept free. Star plates, in the EMMI language, are dismountable plates containing fixed slits. These plates are mounted in the star-plate wheel to be used with grism spectroscopy. It is also possible to mount a coronographic plate or blank star plates: the latter are used in multi-object spectroscopy. The free position is used for imaging or for slitless spectroscopy. The image scale at the position of the star-plate wheel is 156 micron per arcsecond (f/11).
There is no TV camera for the star-plate unit. Acquisition is done using the possibility to take direct images through the slit. First the projected position of the slit on the CCD is determined using one of the internal lamps. An image of the field is taken through the 'free' position, and the position of the target is determined. The target can now be moved to the projected slit position by offsetting the telescope. A final check can be made by taking a direct image through the slit.