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CARELEC
Long-slit Spectrograph
was decommissioned on 27/1/12
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Introduction
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.
Gratings
This table gives the characteristics of the Jobin-Yvon gratings available:
| Lines/mm |
λ blaze
(Å) |
order |
Resolution
R = λ /
Δλ
for λ blaze (*) |
dispersion
(Å/mm) |
Optimal efficiency
interval (Å) |
Wavelength interval
without
order overlap
|
| 150 |
5000 |
I |
450 |
263 |
[3600 - 7300] |
3000 to 3600 Å ** |
| 150 |
8000 |
I |
727 |
263 |
[6000 - 11000] |
3600 to 4600 Å ** |
| 300 |
5000 |
I |
900 |
133 |
[3600 - 7000] |
3000 to 3600 Å ** |
| 600 |
5000 |
I |
1800 |
67 |
[3600 - 6500] |
1844 Å |
| 1200 |
4000 |
I |
2900 |
33 |
[3600 - 6500] |
916 Å |
| 1200 |
7500 |
I |
2700 |
33 |
[5500 - 10300] |
916 Å |
| " |
" |
II |
5400 |
15 |
[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.
Calibrations
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"
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