THE 182 CM TELESCOPE

AFOSC: Observing modes - SPECTROSCOPY
Three observing modes are possible for spectroscopy:

Low Resolution Spectroscopy
In this mode a grism is used in its first diffraction order and the setup configuration is: long slit and the selected grism in their relative wheels and an empty hole in the filter wheel.
Rasmussen Spectroscopy
In this mode a Rasmussen grism with three diffraction orders is used. An order separator filter selects the desired order. The setup configuration is: long slit and Rasmussen grism in their relative wheels and the order separator in the filter wheel.
Echelle Spectroscopy
In this mode an Echelle grism is used. A cross disperser avoids overlapping of the diffraction orders. The setup configuration is: short slit and echelle grism in their relative wheels and the cross disperser in the filter wheel.

The available devices for each of the three wheels are:

Slit wheel
Nine long slit are available (namely 0.70", 0.84", 1.26", 169", 2.10", 3.00", 4.22", 8.44", 16.57"). The short slit to be used in echelle mode is 2.52" large and 5.0" long. The slit orientation is settable rotating the instrument.
Filter wheel
OS1 and filter i as order separators, grism 2 and grism10 as cross dispersers.
Grism wheel
Seven low resolution and two echelle grisms are available. In the following table their properties are summarized.

Grism central wavelength (Å) dispersion (Å/pxl) resolution range (Å)
2 7200 15.67 241 3720-10200
3 4300 3.5 645 3200-6200
4 5800 4.99 613 3500-8450
6 4000 2.23 954 3200-5200
7 5000 2.40 1161 4000-6500
8 5250 2.02 1883 6000-8100
9+GR10   0.62 3615 3300-11000
10 3900 9.89 206 3134-9300
13+ OS1  5250  0.86 3600 4800-5800
13 + i     3600 7200-8400

In the next figure the grisms are compared by plotting their spectral range in nanometers against their resolution.

Flat Field

Flat fields for spectroscopy can be obtained using the lamps projected on the white screen, as for imaging dome flats.
The 1 kW quartz lamp is recommended for short exposure times. Saturation can occur when using the 1 kW lamp with Grisms 2 and 10; with this configuration it is better to use the 0.3 kW lamp. The resulting spectrum is free of lines, but a very strong gradient along dispersion direction is present when grisms with large wavelength coverage are used, since the lamp emission maximum is in the red region. To obtain a high signal-to-noise flat field in the blue region, avoiding saturation in the red, acquire several flats.

Suggested exposure times (in seconds) for the different slit/grism configurations are listed in the following table:

 

Grism/slit 0.70 0.85 1.26 1.69 2.10 3.00 4.22 8.44 ech slit
2                  
3                  
4 9   5   3 3 2 1 3
6 20    12    6  6  4  3  6
7 20   12    6  6  4 2  6
8 20        7    4  2  7 
9 + GR10   - - - - - - -  
10                  
13+ OS1         40         
13 + i                  

Wavelength calibration

The  following lamps are available for the wavelength calibration exposures: Argon (Ar), Helium (He), Neon (Ne), Mercury -Cadmium (Hg-Cd) and Thorium (Th). In most cases it is not possible to obtain a good wavelength coverage using a single  lamp and it is therefore recommended that the spectra of two or three lamps are then summed together.  The calibration exposures should be taken consecutively (without moving the telescope or changing the AFOSC set-up) to avoid the occurrence of instrumental shifts. An useful atlas of comparison spectra is available.

 

lamp
number of lines
Remarks
Efficiency
2 Hg-Cd ~20 Ar lines -
3       efficiency
4 Hg-Cd + Ne  ~40 Ar lines efficiency
6 Hg-Cd ~20   efficiency
7 Hg-Cd + Ne ~40   efficiency
8 Ne + Th ~20   efficiency
9 + GR 10 Th many line blending efficiency
10 Hg-Cd ~20 Ar lines efficiency
13 + OS1 Th many line blending efficiency
13 + i Ar ~10  L efficiency

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