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EMMI Medium Dispersion Spectroscopy
(REMD - BLMD)

Generalities

Medium-Dispersion Spectroscopy is done in modes REMD or BLMD by combining one of the gratings with the adjustable medium-dispersion slit and, if needed, with an order-sorting filter. The grating can be rotated to achieve the desired wavelength range. The following figures illustrate the light-paths of both modes.

 

Slit and gratings

The width of the medium-dispersion slit can be continuously adjusted between 0.5'' and 10''. Note that the medium-dispersion slit is oriented North-South, when EMMI is at 0 degree rotator offset (this is in contrast of the orientation in the RILD mode). The slit can be rotated at any given angle on the sky by applying an offset to the rotator. This can be especially important in the blue, because of the differential atmospheric refraction. In that case, it is recommended to align the slit with the parallactic angle.

Four gratings are available in the red arm, and five in the blue arm. Their properties are described below.

 

Grating characteristics

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

Red arm
Grating
g/mm
Blaze angle
Blaze Lmbda
Eff. [a]
Dispersion [b]
Rs [c]
Wavelength range (nm)
(deg)
(nm)
(%)
(A/pix)
(nm)
D/6
1200
21.0
550
72
0.2
5000
64
D/7
600
11.3
600
68
0.41
2600
130
G/8
316
6.8
620
70
0.79
1500
252 [d]
G/13
150
2.2
550
68
1.67
700
540 [d]
Blue arm
Grating
g/mm
Blaze angle
Blaze Lmbda
Eff. [a]
Dispersion [d]
Rs [b]
Wavelength range (nm)
(deg)
(nm)
(%)
(A/pix)
(nm)
A/1
Mirror
-
-
-
-
-
-
A/4
300
3.6
400
72
1.84
840
170
B/3
1200
13.9
380
65
0.45
3400
46
B/5
158
1.8
370
72
3.5
400
360
C/11
3000
-
350
~50
0.15
9000
15
C/12
600
-
350
~65
0.92
1700
90

[a]Efficiency at blaze
[b]Dispersion with binning 1
[c]Resolution with 1" slit at 600nm and binning 2 (red) or 400nm and binning 1 (blue)
[d]Second order overlap occurs in the red beyond 780nm; use order sorting filter #645 or #646

The grating are permanently mounted two by two in five housings. Only one housing can be mounted at a time on each arm, thus only two gratings in the same housing are available at a given time for each arm. The holographic grating #11 is designed to approach the resolution (but not the wavelength coverage) of the echelle gratings on the red side, with its resolution of 0.1 nm

 

Grating throughput

System efficiency (telescope+EMMI) for the different gratings are monitored during technical time, below you can find the efficiency plots for all the red gratings and the individual plots.
Please be aware that these are the measured system efficiency at BLAZE wavelength for each grating. Curves away from blaze will be added as time permits.
Currently we have complete coverage for gratings #8, #13, and almost complete for grating #6, while grating #7 is only measured at BLAZE.
In the blue arm we have gratings #3 and #5 at BLAZE







Grating
gif image
ps file
6
gif image
ps file
7
gif image
ps file
8
gif image
ps file
13
gif image
ps file
3
gif image
ps file
5
gif image
ps file


For additional details concerning the gratings, see the manual.

 

 

 

Send comments to: ls-emmi at eso dot org

11 Dec. 2006 (Ela)
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