European Southern Observatory
New Technology Telescope

General :
Observations :

EMMI Low Dispersion spectroscopy (RILD)

Generalities

Low-resolution long-slit spectroscopy is done in the RILD mode by combining one of the grisms with one of the fixed slits (starplates) and, if needed, an order-sorting filter. The grisms cannot be rotated: the achievable wavelength range is fixed for each grism. Due to the larger size of the new Red CCD, the wavelength coverage is larger than what it was with the old chip. As a consequence, many grisms show a second order contamination, hence the use of an order sorting filter is recommended.

There are six fixed slits available, with widths of 0.5'', 1.0'', 1.5'', 2'', 5'' and 10'', and each with a length of 8'. The 0.5'' slit will cause undersampling of your spectra, except when used in 1 x 1 binning mode, under exceptional seeing conditions.

The orientation of the slit can be adjusted by applying an offset to the rotation angle of EMMI.

Grisms and filters can also be combined to obtain slitless spectra imaged on the CCD. The use of filters in combination with a grism reduces the sky background intensity. It also selects the wavelength region of interest and limits the length of the spectrum, thus reducing crowding. The spectral coverage within this wavelength region depends on the position of the object in the field.

Grisms characteristics

The following table summarizes the Grisms available on EMMI and their characteristics.

Grism	g/mm	Blaze angle	Blaze Lambda	Eff. [a]	Dispersion		Rs [b]	Wavelength range
		(deg)	(nm)	(%)	(nm/mm)	(nm/pix)		(nm)
1	150	8.6	560	79	24.5	0.59	270	385 - 1000 [c]
2	300	14.6	490	78	11.6	0.28	580	385 - 900 [c]
3	360	15.0	460	77	9.4	0.23	710	385 - 845 [c]
4	300	22.0	650	72	11.6	0.28	760	550 - 1000
5	600	34.0	530	66	5.5	0.13	1100	400 - 660
6	600	54.0	650	55	5.0	0.12	1620	595 - 835
7	150	10.8			24.0	0.58	280	490 - 1000 [c]

[a] Efficiency at blaze.
[b] Resolution with 1'' slit at 600 nm (at central lambda for grisms 4,5 and 6)
[c] Second overlap occur beyond 800 nm if not using an order-sorting filter . See the manual for details.

Grisms efficiencies

The efficiency curves for the grisms is shown below, and also available in PS format.

Grism throughputs

The following figure summarizes the grism throughputs for a star with an AB magnitude of 12 (in e-/s/nm). The values quoted here were taken during the commissioning in May 2002. Empty fields indicate that the wavelength is not covered by this grism; Grisms #5 and #6 are not characterized with the new CCD yet.

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Last Update: 23 Sep. 2002 (cf.)