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 15µ 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
To view the full frame click
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").
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
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
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 pixel
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"x24", 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
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
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.
The position angle at the Coudé focus of the telescope is given by: