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Long-slit Spectrograph

was decommissioned on 27/1/12


CARELEC is a long-slit (5.5 arc-min) spectrograph featuring all-reflecting optics and aspherized gratings, giving the benefit of broad-band sensitivity, limited only by the spectral response of the detector. It has been available at the OHP 1.93-m telescope since January 1986. It is used at the f/15.5 Cassegrain focus bonnette, where the plate scale is 150 µ/arc-sec. The CCD currently in use is a thin, back-illuminated e2V 42-20 CCD chip with 2048x1024 pixels of 13.5 µ. The f/2.35 camera optics give an optical reduction factor of about 6 and yield a nominal scale of 23 µ/arc-sec on the detector.

Pointing and Guiding

The spectrograph is equipped with an auxiliary CCD camera , specially developed in-house, which is used for target acquisition and guiding. The field of view is approximately 5.5 arc-min, covering the full length of the slit. A variable integration time allows guiding on bright/faint objects. Telescope coordinates are digitized using encoders on both axes and a pointing model allows accurate pointing and setting. Coordinates, equinox, UT and ST are displayed both in the control room and the dome. There are limits on the pointing of the telescope due to its asymmetrical mounting (see diagram).

Slit rotation

The alignment of the spectrograph slit on the sky can be changed by rotating the Cassegrain bonnette. The control paddle is on the telescope itself but the read-out is also displayed in the control room.

Spectrograph control

All spectrograph control functions are managed through LIDO software, developed in-house, on the alix Sun workstation. Users should be aware that data acquisition is still done via an HP A900 computer through a CAMAC interface and an old CCD controller, resulting in long read-out times. The details of the spectrograph configuration are displayed on a "spectrograph status" window. Data reduction is done on alix using MIDAS software from ESO.

Optical Layout

The layout diagram is taken from the article by Lemaitre et al. (1990).

Improved optical coatings

The gratings used in Carelec are all coated with aluminum. The four mirrors were coated with unprotected aluminum until January 1997, when the coating was changed to protected silver enhanced in the violet. The spectrograph efficiency improved by a factor between 1.5 and 2 in the interval 4000Å and 1µ with respect to the previous coatings. The over-all efficiency has not changed below 3900 Å.

Slit and sampling

The slit width can lie anywhere between 50 and 2000 µ, corresponding to 0.3-13 arc-sec on the sky. The total length of the slit is 50 mm in the focal plane, or 5.5 arc-min on the sky. With the e2V CCD, the projected pixel width is 1.7 pixel/arc-sec along the slit and varies slightly depending on the grating angle.


This table gives the characteristics of the Jobin-Yvon gratings available:

Lines/mm λ blaze
order Resolution
R = λ / Δλ
for λ blaze (*)
Optimal efficiency
interval (Å)
Wavelength interval
order overlap
[3600 - 7300]
3000 to 3600 Å **
[6000 - 11000]
3600 to 4600 Å **
[3600 - 7000]
3000 to 3600 Å **
[3600 - 6500]
1844 Å
[3600 - 6500]
916 Å
[5500 - 10300]
916 Å
[3700 - 5300]
426 Å

(*) for a slit width of 2 arc-sec.
(**) depending on spectral region. The selection of the useful wavelength interval is done using various filters
(depending on the central wavelenth) :
GG375 (5500-6500Å) or GG435 (6500-7000Å). No filter needed if <5000Å.
BG38 (3700-4800Å) or BG38+GG395 (4500-5300Å).

Neutral density and order-separating filters

There are two filter wheels located below the slit with eight positions each, one for the order-separation filters and the other for the neutral density filters for attenuation (see available list in the dome). The LIDO software allow the use of an attenuating density for exposures on the sky.

Collimator focusing

The collimator focal length is 1550 mm. It images the telescope entrance pupil on the grating. Focusing on the detector is done by moving the camera mirror under LIDO software control.


Flat-fielding is done using an internal tungsten lamp which can illuminate the slit. Although this gives excellent results, the over-all response of the system must be determined through observations of spectral standard stars. Wavelength calibration is done using Helium, Argon, Neon and Iron (hollow cathode) lamps. Useful FITS files for the different calibration lamps and for selected standard stars are available here.

Detector and Signal-to-Noise ratio

The CCD in use at Carelec is an e2V chip with 2048x1024 pixels of 13.5 µ. The read-out noise is 4.2 e- and the gain is 1.54 e-/ADU. With this detector Mira Véron has obtained a signal-to-noise ratio of 40 on a V~15 quasar on a 60-min exposure using the 66Å/mm grating at 6500 Å with a 2 arc-sec slit under 2.5 arc-sec seeing. Please read a more detailed description of the caracteristics of this chip. See more information about other CCDs in use at OHP.

Quick-look data reduction using MIDAS

Once the CCD chip has been read-out, the data are available on the alix worsktation for display and immediate evaluation using MIDAS. Typical procedures are bias substraction, flat-fielding, wavelength calibartion and flux calibration (if standard stars were observed). See also MIDAS On-line Help pages

Night assistant

A night assistant is on duty at the 1.93-m telescope at all times (except for meal time around midnight) and is responsible for telescope safety. He will open, close and turn the dome, point the telescope, start and set up the auto-guider. Observing is the responsibility of the astronomer. The night assistant is normally able to deal with most problems in the course of the night, but in very difficult cases he might call up resident staff for help. Phone numbers are listed in the observing room.

Reference : Lemaitre, G. et al. 1990, A&A 228,546 :
"Reflecting aspherized grating spectrographs for the Haute-Provence and Nanjing Observatories : MARLY and CARELEC"

Dernière mise à jour : 10 Mar 2012