EUROPEAN SOUTHERN OBSERVATORY
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.
|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)
|Accessible field||5 times 8 arcmin|
|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
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.