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Coudé Echelle Spectrograph


The classical Coudé spectrograph has recently been upgraded to a white-beam Echelle spectrograph. The new spectrograph allows to take spectra with a resolution of up to 67000.

The Echelle spectrograph is situated in a temperature stabilised Coudé room in the cellar of the observatory, and is fed by a Coudé train using five flat mirrors that have been silvered to maximise the throughput.

The spectrograph uses an Echelle grating with 31.6 lines per millimetre (the blaze-angle is 65 degrees) and an f/46 collimator which produces a beam of 150mm in diameter. Three different grisms serve as cross-dispersing elements: The so called UV grism covers the wavelength range from 3400 to 5500 Å, the VIS grism covers 4700 to 7400 Å, and the so called IR grism 5380 to 9270 Å. The grisms can be changed by remote control within seconds. The spectrum is finally formed by an f/3 camera of 450mm focal length on to a 2k x 2k CCD chip with 13,5µ pixels. The sensitivity of the CCD is 2.6 e-/DN, and the RON 4.16 e-. 

Fig 1: The figure shows a small part of the spectrum of AD Leo close to Halpha. To view the full frame click here.

Resolution and wavelength coverage

Using a slit width of 0.52mm (corresponding to 1.2"), a two-pixel resolution of about 67000 is achieved. With a slit width of 0.90mm (corresponding to 2.0"), the resolution is about 35000. In the geometry direction, one pixel corresponds to 0.51". The maximum slit length is 13mm (corresponding to 29"). However, in order to avoid overlapping of the orders, the slit height has to be 3mm or less (corresponding to 6.7"). 

UV channel  
Wavelength range   : 3400 to 5500 Å
Dispersion                   : 0.027 to 0.042 Å per pixel, or 1.8 to 2.8 Å per mm
Orders                       : 105 to 166
Distance between the orders : more than 20 pixel

VIS channel


Wavelength range   : 4700 to 7400 Å
Dispersion                   : 0.037 to 0.057 Å per pixel, or 2.5 to 3.8 Å per mm
Orders                       : 77 to 122
Distance between the orders : more than 34 pixel
IR channel

Wavelength range   : 5380 to 9270 Å
Dispersion                   : 0.042 to 0.073 Å per pixel, or 2.8 to 4.8 Å per mm
Orders                       : 62 to 105 
Distance between the orders : more than 21 pixe


The sensitivity of the Echelle spectrograph strongly depends on the seeing conditions, on the amount of extinction, and on the wavelength region used. Fig. 2 shows the limiting magnitude that can be achieved under typical conditions using 6" slit and a slit width of 1.2". Fig. 3 shows the slitloss for various seeing conditions. 

Fig 2: Limiting magnitude of the Echelle spectrograph.

We show the limiting magnitude for two different cases: High resolution, high S/N-ratio spectra, and low resolution low S/N-ratio spectra. The full line show the second case. In this case we show the limiting magnitude for a S/N-ratio of 30 per pixel, if a wide slit, and an exposure time of one hour is used. The dashed line is for the case that detailed line profiles of relatively bright objects are of interest. This line is for a 1.2" slit, the same exposure time, and a S/N-ratio of 100 per pixel under typical observing conditions.

Fig 3: Slitloss for different seeing conditions.


Acquisition and guiding is done by using the slit-view TV camera. The field of view of the camera is 29"x 24", one pixel corresponds to about 0.33". With an integration time of one second, stars down to 16th magnitude can be observed. For more details see manual of the TV guider 

Iodine cell

Accurate radial velocity measurements can be obtained, if the iodine cell is used. The gaseous iodine is used for generating a very dense reference system of absorption lines which are superimposed onto the stellar spectrum. The superimposed iodine lines provide both a highly precise wavelength scale (calibrated with a Fourier-transform spectrum) and a specification of the spectrograph PSF in situ over the spectrum. Please note that the resulting spectrum is almost useless for any other purpose then radial-velocity measurements if the cell is used.

Fig 4: The figure shows one extracted echelle order of a spectrum of HD35296
Fig 5: Same as above but taken through the iodine cell. The wiggles are not noise but iodine lines! Please note that exposure time of this spectrum is only a 5th of that of the spectrum above.

Position angle

The position angle at the Coudé focus of the telescope is given by:

                                   P.A. = 151° + δ - τ

where δ the Declination and τ the hourangle.