Long-slit Intermediate Resolution Infrared Spectrograph for the WHT
LIRIS is an Instituto de Astrofísica de
Canarias (IAC) project that consist a near-infrared (0.9-2.4 microns) intermediate
resolution spectrograph (R=1000-3000), conceived as a common user instrument
for the WHT at the Observatorio del Roque de los Muchachos (ORM La Palma).
LIRIS will have imaging, long-slit and multi-object spectroscopy, coronography and polarimetry working modes. Coronography and polarimetry, will be upgrades not available at first phase. Image capability will allow easy target acquisition.
The optical system is based on a classical collimator/camera design. Grisms are used as the dispersion elements. The plate scale (0.25 arsec/pixels) matches the median seeing (0.5 arcsec in the K band) at the ORM. The detector is a Hawaii 1024x1024 HgCdTe array operating at 65 K.
The optics of LIRIS are all refractive, with the exception of a single folding flat mirror in the collimator assembly. The expected throughput (averaged across the wavelength range) for the optics is 80% and 64% in imaging and spectroscopic modes, respectively. The grism transmission is assumed to be 80%.
- A slit wheel is introduced at the telescope focal plane for spectroscopy.
- A refractive collimator forms an image of the primary mirror near the last element and produces a collimated beam where the filters, grisms and Wollaston prisms are inserted.
- A cold stop: The entrance pupil of LIRIS is the primary mirror of the telescope, given the fact that the secondary is oversized with respect to the primary in the WHT, contrary to an optimized infrared telescope.
- The collimated beam passes through the pupil.
- A refractive camera is used to focus the light onto the detector.
Collimator Pupil wheel Camera LIRIS schematic of the optical train (provided by UKATC/ROE)
The slit wheel contains 16 positions:
1 blank position, 5 long slits (widths 0.65”, 0.75”, 1”, 2.5” and 5” )
plus 10 multislit positions.
The two filter wheels contains 12 positions each, and will hold the filters and the Wollaston prisms.
The pupil wheel contains 12 positions and will hold the pupil masks, plus an optional apodization mask with rotation mechanism for coronography capabilites.
The grims wheel has 10 positions for grisms.
The camera wheel will carry the camera and the optics to image the reimage the pupil onto the detector plane.
The detector will be mounted in a cold translation mechanism (focus mechanism) to compensate for non-achromaticity along the observing spectral range.
The vacuum vessel is a welded cylindrical vessel.
It is made in three sections, a central ring and two end covers. The front
cover is a circular plate which carries the window and its mounting and
a cover over the port giving access to the entrance wheel. It is not necessary
to remove the front cover to change focal plane masks. The rear cover could
be removed to gain access to all the other internal modules of the instrument.
It has an auxiliary access port to facilitate the first integration &alignment
steps without detector.
The center section carries all the semi-permanent access ports. These carry ports for electrical wiring, cooling and vacuum pumping
The optical bench is an aluminium welded structure, supported from the front ring of the central section of the vacuum vessel by three trusses made from G10 glass-epoxy composite. It has a cylindrical shape and its plates acts as a reference surfaces and mounting for all the cold modules. The bench will be also used as a tank for LN2 for pre-cooling the instrument. The front plate supports the entrance wheel. The rear plate supports the rest of the cold parts. The optical bench is fitted with a panel heater to allow a controlled warm-up while still under vacuum.
The instrument is precooled with LN2, and the cooling system is a closed-cycle refrigerator (CTI model 1050C), which works on the Gifford-McMahon cycle. There two stages which provides cooling powers of 45W at 60K and 4W at 15K. The first stage cools all the internal parts, except for the detector and its housing. The second stage mantains cool the detector at the working temperature of 65K. The detector temperature is stabilized using the commercial PID controller Lakeshore 340.
An agreement has been established between the IAC and the ING to develop jointly the Mechanism Control Software and the detector control system for the two infrared instruments (LIRIS/IAC and INGRID/ING).
The controller system uses the SDSU controller,
which is a commercial product developed by the San Diego State University
(SDSU) and supplied by IRLabs (Tucson, AZ). It is based on the 56200 DSP
by Motorola. The current code was developed by P. Moore of the Isaac Newton
Group for the INGRID project. The detector is read through four channels
at a rate of 3 msecs/pixel, leading to a time
of 0.9 seconds for a complete frame readout. The SDSU controller communicates
with the control computer through optical fibers.
The available read out modes are double correlated (DC), Multiple non destructive reads (MNDR) and reading up the ramp.
A temperature controller is required due to the strong dependence of the signal offset level of the detector with the substrate temperature. For this reason the detector temperature has to be stabilized below 0.005K. The temperature controller Lakeshore 340 has been selected, which guarantees a stabilization below 0.005K, sufficient to constrain the offset variation to 0.1e- RMS.
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