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 collected light beam passes through a fused Silica window. The telescope focal plane lays inside the cryostat, where the cold aperture masks are located.
The optical design for LIRIS contains the following main components (see graph below):
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%.
The detail optical design and the conceptual mechanical design were subcontracted to the ROE (Royal Observatory of Edinburgh).

                         Slit wheel 
Pupil  wheel
LIRIS schematic of the optical train (provided by UKATC/ROE)
The mechanical design is based on a modular concept, integrated by the following modules: the aperture wheel (slit wheel), the collimator assembly, the central wheel assembly (formed by two filter wheels, the pupil wheel and the grism wheel), the camera wheel and finally the detector assembly with its focusing mechanism.

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.


External schematic view
The LIRIS mechanism are driven by cryogenic stepper motors. The control system is based on a VME system with a Motorola CPU card running VxWorks operating system and two Oregon Micro Systems stepper motor controller boards, allowing a total number of twelve motors to be controlled, each one of them with its own home, limits and power off signals. The VME system is connected to the SUN through an Ethernet network and an RS232 line. An additional 19" 3U rack with the motor drivers are installed under the VME rack. The drivers are from the same manufacturer as the motors' Phytron.

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 detector is a Rockwell Hawaii  1024x1024 HgCdTe array. The pixel size is 18.5 mm, which corresponds to a plate scale of 0.25 arcsecond on the sky.
The quantum efficiency of the detector is larger than 60% according to the manufacturer. The minimum readout noise is less than 10e- using double correlated sampling.  The dark current is very low, less than 0.03 e-/s, which implies less than 100e-/hour. In most observing conditions the LIRIS sensitivity would be limited by background photon noise. The readout noise will be a limiting factor only in the high resolution spectroscopy mode.

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.

The LIRIS Software system is being designed to be fully integrated in the observering environment available at the WHT. A common observer will have access to the following software packages:
Instrument Simulator Software: Given the source characteristics (expected brightness, extended or point source, sky brightness) and an observing mode (imaging or spectroscopy, dithering or beam switching), a  LIRIS simulator will be used to obtain the S/N ratio for a given integration time or viceversa. It will also provide the instrument configuration suitable to carry out the desired observation, i.e. it will be able to generate the templates which can be interpreted by the Template Generator Software (described below).  It will  also  have access to whole sky standard catalogues (POSS, 2MASS, IRSKY) in order to simulate what is included within the LIRIS aperture (it is especially important for background positions and spectroscopic observations). This component may be accessed by general observers at any place, as an HTML application.
Templates Generator Software:  This package will be used to define the LIRIS observing parameters. It will have a set  of predefined  modes, called templates, adequate for the most common types of LIRIS observations.  Each of them will have a number of configurable parameters (exposure time, detector reading mode, filter selection,  etc). This system will generate a sequence of commands or scripts which can be interpreted by any of the systems available at the Instrument Support Platform.
Instrument Support Platform User Interface: This package will be the main User Interface for LIRIS. It will provide the support infrastructure required to operate the instrument, including facilities to set up exposure parameters, data acquisition from the instrument detector, pointing the telescope, and also the execution of LIRIS Observing Templates.
LIRIS Mechanism Control Software: This system can receive commands from the Instrument Support Platform and return status information. It comprises the following 2 subsystems:
LIRIS EPICS Mechanism Control Software: This system is the responsible of the direct control of  the LIRIS  mechanisms  (slit, filter, pupil, grism, camera wheels and detector focus). This software will run on the VME EPICS rack.
Temperature Controller Software: This system will be used to control and monitor the detector temperature. ..
Real Time Display: This tool will be use to monitor the newly acquired images.  It will basically process and display them using  Ximtool / SAOtng as  display device. These tools offer facilities to change color tables, zoom, several image frames. It will also provide some facilities: basic statistics, different  scaling of the representation, and additionally there will be a seeing and background monitor.
Quick Look Data Analysis: This package will provide facilities for a quick analysis of the recently acquired  data. This should include facilities to perform sky subtraction, coaddition and alignment of images, extraction of spectra, default wavelength calibration. It will be based on existing astronomical data analysis packages, like IRAF.
Pipeline Data Reduction: This package will be used to process blocks of images obtained through the LIRIS Observing Templates. It will receive notifications of  available data blocks from the Instrument Support Platform in order to start the processing. The following steps will be done: detector effects removal, flat-fielding, sky subtraction, coaddition and alignment of images. For spectroscopy there will be a default wavelength calibration.

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Last modified: 08/03/02

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