The MUSICOS spectrograph
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It is a cross-dispersed fiber-fed spectrograph, covering most of the visible domain in 2 exposures (380-540 nm and 540-870 nm), with a resolving power of 35000. The spectrograph can be fed either by a single fiber, or by 2 fibers, giving the possibility to record simultaneously 2 spectra with interleaved orders. The 2 fibers are separated by 180 microns at the spectrograph entrance, while the spacing between 2 successive orders on the detector is 360 microns at minimum (on the red side of the spectrum) and 640 microns at maximum (on the blue side of the spectrum). The double-fiber mode is necessary for certain types of observations, like spectropolarimetry (one polarization channel per fiber), or very precise radial velocity monitoring (one stellar channel and one simultaneous spectral calibration channel)
This figure sketches the optical layout of the spectrograph.
The stellar light enters the fiber in a f/2.5 beam. The 50 micron fiber corresponds to 2.1 arcsec on the sky. The spectrograph entrance aperture is f/4. The photon loss resulting from this aperture mismatch is of the order of 30%, but is compensated by the quasi-absence of loss at the fiber entrance. After the collimator, the parallel beam hits the grating (Milton-Roy; 31.6 gr/mm, blaze 63.5 deg), then the prism. There are 2 different prisms, one (F5) for the blue domain (380-540 nm), the other one (SF5) for the red domain (540-880 nm).
The change from one wavelength domain to the other is entirely automatic, and requires four motions inside the spectrograph:
- rotation of the prism plate
- rotation of the plate holding the camera objective and the detector, in order to accomodate the different mean deviation angles
- translation of the CCD detector to compensate for the focus difference between the two wavelength domains
- tilt of the CCD with respect to the optical axis, in order to compensate at first order the small chromatism of the objective
The setup can be shifted from one domain to the other thanks to a single button control unit in the control room. The setup is complete when all lights of a given color (blue or red) are on while all lights of the other color are off. After each intervention on the spectrograph or the detector, e.g. filling in the dewar, it is recommended to have all four motions go back and worth once, and re-do flat-field and wavelength calibrations.
In each one of the two domains, the spectrum contains about 46 orders: orders 64 to 109 for the red domain, orders 100 to 146 for the blue domain. The height of the orders is about 3 pixels of 24 microns, while the FWHM in the direction of the dispersion is about 2 pixels of 24 microns.
The spectrograph also features a photon-counting device, which can be used as an expose-meter. A small fraction of the incident flux (about 4%) is tapped before the collimator, and sent to an avalanche photo-diode. The measured flux is displayed in real time in the control room. This device is based on a silicon detector, and therefore has its maximum efficiency in the red, near 650-700 nm.
The table below indicates the performances of this instrument, as obtained at Pic du Midi 2m ``Bernard Lyot" telescope, for an A0 star with mV=7, with a thinned Tektronix 1024 x 1024 CCD, in one hour.
|lambda (nm)||S/N per resolution element (8.5 km/s)|
The spectrograph can also be used in conjunction with a polarimeter,
and thus provide the possibility to obtain spectra in both circular and
linear polarization. The MUSICOS
polarimeter was developped in 1995/1996, and is available at TBL since
The fiber is optically conjugated with the hole through a focal reducer f/25 --> f/2.5. With this entrance aperture the 50 micron fiber corresponds to 2.1 arcsec on the sky. With this setup, the stellar flux falls on the fiber as soon as it disappears in the hole.
The guiding is provided by a small CCD camera, imaging the central part of the mirror. This camera is operated by a PC, installed in the control room. Its field of view is of the order of 40 arcsec. A variable neutral density can be inserted in front of the guiding camera for the observation of the brighest stars.
The fiber unit also includes a tungstene lamp for flat-field exposures and a Th/Ar lamp for wavelength calibration. A blue filter may be used in front of the Tungstene lamp, in order to reduce the strong wavelength dependence of the flat-field flux in the blue range of the spectrograph. If needed, this filter may be used only in the blue range, and only in front of the flat-field lamp.
The fiber unit is entirely controlled from the control room. A small control unit allows the observer to:
- control the neutral density wheel in front of the guiding camera. This is done with the buttons marked "attenuation TV". Note that the calibration lamps can be switched on only when this wheel is in a park position (corresponding to maximum attenuation), and that requesting one of the lamps to be switched on will automatically cause the wheel to go to this park position.
- switch on and off the calibration lamps. Switching off the lamps is obtained by pressing both CC (Th/Ar) and TU (Tungstene) buttons simultaneously.
- position the blue filter or a neutral density in fornt of the calibration lamps. This is done with the buttons marked "position attenuation". The encoder positions for the various possibilities are listed on the unit.
The guiding camera is controlled with a PC from the control room. The
corresponding software is in directory d:TICS. When you are there, just
type "op". When op is running, type ACQ, then choose an integration time.
The integration time unit is 1/40 sec (1 frame). This software also provides
side functions, like e.g. the measurment of the seeing (program MANU).
Please consult the detailed documentation available at the summit.