ESO Faint Object Spectrograph and Camera
EFOSC2 Star-plate Set
Spectroscopic Long slits
MOS slitlet masks
Coronographic mask
Wollaston Masks   (linked to another page)

The terms Star-plates, slits and masks are used interchangeably - they all refer to elements which block light from some region of the field of view of EFOSC2. All these are mounted on the Aperture wheel of EFOSC2.

The aperture wheel is located on the focal plane of the telescope - it is the only optical element in the instrument located before the collimator. The scale at the aperture wheel is the same as the telescope scale (= 7.12 arcsec/mm).

The Slit wheel contains 10 positions of which

Thus between 5 and 7 positions are available for user selected elements at any given time. The full list of star-plates is given below.
Spectroscopic Long slits
The following spectroscopic long slits are available for use on the EFOSC2 instruments :
Slit name
Slit width

The length of the slits are about 5 arcmin.

The 1.5" moveable slit   is permanently mounted on the EFOSC2 instrument. It can be displaced by 10 mm on either side of the centre position by means of a screw. A scale with 0.5 mm markings is located under the slit for measuring the displacement. Moving the screw clockwise shifts the spectral lines towards the blue, i.e. the spectral coverage shifts towards the red. The change in spectral coverage is given by

dL  = 45.5 D * dY
dL is the shift in wavelength in Angstroms,
D is the dispersion of the grism in Angstrom/pix and
dY is the displacement of the slit in mm

However, one should always check the shift by taking a He-Ar spectrum (request the support astronomer to obtain this directly from the OS panel) and visually determining the wavelength range.

The y-pixel location of a slit varies from one installation to another by a few pixels. Observers should obtain all the required calibrations before  a slit is dismounted from the wheel.

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MOS Slit Masks
These are user defined and punched plates for multi-object spectroscopy. EFOSC2 recognises 5 MOS names  - Mos#1, Mos#2, Mos#3, Mos#4 and Mos#5  though jP2PP recognises 3 more MOS names. More information on MOS mask creation can be found in the Multi-Object Spectroscopy page.

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Coronographic Mask
There is coronographic mask available. It is called by the name Cor_mask in the jP2PP templates. The sizes for the large and small spots for the mask are 10.8 and 4.2 arcsec respectively.

Coronographic Mask

Additionally, there is another mask, the Lyot stop  which has to be used in coronographic observations to eliminate scattered light from bright stars. This however is mounted on the grism wheel even though it is a mask (since it must be mounted in the pupil plane of the instrument). The Lyot stop has to be carefully aligned using a very bright star. This alignment procedure requires an hour or two to execute.

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