The Instituto de Astrofísica de Canarias (IAC)
is undertaking the design and construction of a common-user near infrared
spectrograph (LIRIS) for the Cassegrain (f/11) focus of the 4.2 m William
Herschel Telescope (WHT) sited at the Observatorio del Roque de Los Muchachos
(ORM, La Palma). LIRIS will be a near infrared (0.9-2.5 mm)
intermediate-resolution spectrograph designed to operate over a spectral
resolution range between 1000 and 5000, with added capabilities for coronographic,
multiobject and polarimetric observations. The instrument allows the combination
of an adequate spatial resolution (0.25 arcsec/pixel) with a large useful
field of view (4.2 arcmin) across the slit, thanks to the use of the new
1024x1024 pixel HgCdTe Hawaii detector manufactured by Rockwell. All the
optics and mechanisms situated inside the cryostat will be cooled to below
100 K. The detector will operate at 77 K. Calibration and tracking will
be made with the existing Cassegrain A&G Box, into which a near infrared
calibration system will be incorporated.
The arrival at the beginning of the 90s of new large-format and low noise level infrared detectors (e.g. NICMOS3) marked a great advance in the development of astrophysics at wavelengths greater than 1 m m and enabled observing programmes that had hitherto been restricted to the visible to be extended into the infrared. The majority of observatories since then have obtained such detectors for direct imaging, although very few have the necessary instrumentation available for carrying out wide-field spectroscopy in the near infrared.
Furthermore, measurements taken in the wavelength range 1- 2.5 m m on the William Herschel Telescope (4.2 m) at the Roque de los Muchachos Observatory (ORM, La Palma) over the last two years have demonstrated the quality of the WHT and of the ORM sky for observing in the near infrared (see Table 1)
LIRIS would enable to carry out infrared spectroscopy in the near-infrared bands with a long slit and spectral resolutions as high as 5000.
LIRIS aims to attain the following objectives:
LIRIS will be used, mainly in bright time, at the Cassegrain focus of the WHT. Given its common-user status, it should be possible to use it for a wide range of astrophysical disciplines, including the areas of stellar, planetary, extragalactic and cosmological physics. We list below some of the most relevant possible scientific applications:
2.1 Infrared spectroscopy of proto-planetary nebulae
Proto-planetary nebulae (PPNe) are nebulae in which the
central star is highly obscured by dust, there being no visible counterpart
in the majority of cases. For this reason, they can be observed only in
the infrared. By means of K-band spectroscopy it is possible to investigate
the distribution of ionized and molecular gas. This permits an understanding
of this rapid transition phase that later gives rise to the formation of
a planetary nebula.
2.2 Detection of very low mass secondaries, planets and brown dwarfs
The IR is the most favorable spectral domain for detecting the contribution to the integrated light of a multiple system (binary or planetary) from the least massive components. IR intermediate-dispersion spectroscopy can permit the detection of molecular bands (H2O and CO) from very cool objects superimposed on the continuum of the primary. A particularly interesting case is that of the recently discovered planets at less than 0.5 AU from their stars. The temperature of these planets must be high enough for them to have their emission peak in the near IR, i.e. in the spectral range of LIRIS.
Furthermore, red colours do not constitute a unique identification
of brown dwarfs, whereas infrared spectroscopy in the wavelength range
of LIRIS would allow the methane bands characteristic of an atmosphere
sufficiently cool for a brown dwarf to be stand out.
2.3 IR spectroscopy of nearby and distant galaxies with massive star formation: nearby HII regions and distant starbursts
The existence of a population of galaxies at around z=1
with active star formation, probably of low metallicity and with blue colours
has recently been demonstrated by the spectroscopic surveys carried out
on the Keck telescope. The possibility that these objects constitute a
population of dwarf galaxies in the process of developing the first bursts
of star formation is extremely interesting, since they would enable us
to find objects of metallicities < 1/100 solar, which might reflect
the optimum representation of the pregalactic abundances necessary for
deriving a precise value of the primordial abundances. In this way we could
intercompare the spectroscopy of our local surroundings with that at other
redshifts and thus unravel some of the so far unresolved problems of their
2.4 Stellar kinematics and populations in spiral galaxies
The presence of dust in the center of spiral galaxies
means that the ideal spectral range for their studies is the infrared.
In particular, we shall be able to determine kinematic peculiarities and
their correlation with the stellar populations at the centers of galaxies
without the uncertainty that arises in the visible range. This will enable
us to understand the nature and mechanisms of the formation of bulges,
discs, bars and dynamical subsystems such as counterrotating discs and
2.5 Ultraluminous infrared galaxies
Galaxies with high luminosity in the far infrared, denominated
"ultraluminous infrared galaxies" (ULIRGs) exhibit violent star formation
owing to interaction processes. Such galaxies have huge quantities of dust
thereby rendering their study in the visible quite impracticable. LIRIS
will enable us to study such galaxies in order to investigate whether they
really constitute the progenitors of elliptical galaxies.
2.6 Physical conditions of the gas, and the stellar kinematics and populations of starburst galaxies and AGNs
With LIRIS it is intended to study the physical conditions
of the gas, and the kinematics and formation of stars in galaxies with
massive star formation and high extinction in their centers. With this
project two objectives may be addressed: first, the determination of the
origin and age of the bursts of star formation in isolated dwarf irregular
galaxies, and secondly the study of nuclear starbursts in galaxies that
show bars and/or AGNs. In this context, the study of the underlying stellar
populations may be undertaken using the CO absorption band at 2.3 m
m, as well as the study of the physics of the gas with lines such as FeII
at 1.65 m m, Brg
and HeI at 2.06 m m and H2 at 2.12
m m. With the same spectral lines we can determine
the gas stellar kinematics, which will give us the dynamics of the nuclear
zones since such measurements in the visible wavelength range are severely
affected by dust. Such observations will help us to understand the starburst
2.7 The fundamental plane of high-redshift elliptical and S0 galaxies
The Hubble Space Telescope has shown that the morphological
classification of galaxy clusters at redshift z=0.5 can be undertaken.
The colour-magnitude diagram of one of these clusters reveals elliptical,
S0 and a large population of late spiral and irregular galaxies. LIRIS
will allow the study of the stellar dynamics and populations of elliptical
and S0 galaxies in such clusters. Hence, spectroscopic measurements in
the IR could provide the velocities of dispersion and the spectral indices
of the stellar component of such galaxies using such species as the CaII
triplet. With such measurements, we could obtain the fundamental plane
of the high-redshift elliptical and S0 galaxies and compare it with the
nearby galaxies in order to understand their formation and evolution.
2.8 Redshift measurements of infrared sources
The large-scale infrared surveys such as 2MASS and ISO
will provide an enormous quantity of infrared sources, from stellar objects
to galaxies, which will have to be classified. In this context, LIRIS provides
us with a very useful tool, since with the expected limiting magnitude
we could obtain the spectroscopic counterparts of the said sources. For
example, given the limiting magnitude of the 2MASS survey (K=15) we could
obtain an S/N=20 spectrum on the WHT with some 10 minutes? integration
The aim of this project is to build a common-user near infrared instrument for the Cassegrain (f/11) focus of the WHT. The proposed instrument will have the following characteristics:
Cold coronographic and apodization masks will be used in the focal and pupil planes to reduce the effects of scattered and diffracted light around bright point sources. Slits, coronographic masks and multislit masks will be situated at the telescope focal plane. All the slits and masks will be mounted on the same cold wheel to avoid the problems related with the small depth of focus at the Cassegrain position. Wheel diameter will be designed as large as possible to allocate a minimum of 5 slits, 1 coronographic mask, 9 multislit masks and 1 blank with the required size for imaging.
Grisms will be used to attain spectral resolutions of 1000 and 5000 over the full spectral range. At resolution 1000 it will be possible to observe simultaneously both the Z and J bands and the H and K bands. Grisms will be mounted near the pupil plane. Positions to allocate simultaneously a maximum of 10 grisms will be provided. A total of 19 filters, including narrowband filters and the standard Z, J, H, K and K? broadband photometric filters, will be provided. Filters will be mounted in a maximum of two filter wheels.
A warm half-wave and quarter-wave modulator will be situated in a rotating assembly outside the instrument window and will be used in combination with two Wollanston prisms and four wire-grid analyzers mounted in a cold wheel situated behind the grism wheel for spectropolarimetric measurements. An apodization mask with a central obscuration and spider vanes to block the thermal emission from the telescope structure will be situated at the Lyot position for coronographic measurements. A cold mechanism will be developed to switch between the Lyot stop and the apodization mask. The mask should rotate to compensate for the rotation of the image of the secondary mirror in an altazimuthal telescope.
Since all the mechanisms will be motorized, observers could switch rapidly between spectroscopic, polarimetric, coronographic and direct imaging modes.
One of the great challenges presented in the design of LIRIS is to be able to reach the maximum possible efficiency, which will necessitate a careful optimization of the instrument. A magnitude 17 should be reached in K for a point source with S/N=3, 1800 s of integration, seeing of 0.5 arcsec and a spectral resolution of 1000.
We expect to have the first comissioning at the telescope
before the end of 2000.
LIRIS will be a classical collimator/camera design. The optical components of LIRIS can be divided in five subsystems: warm polarimetric plates, window, slits and focal plane masks, collimator, pupil optics (filters, grisms, apodization masks, Wollanston prisms and wire-grid analyzers), camera and detector. The image quality over the entire field-of-view must ensure that the size of the mean seeing disk observed on the ORM (0.5 arcsec in K) will increase less than 10% in the imaging mode and that the FWHM of the 0.5 arcsec slit image will be lesser than 2 pixels in spectroscopic mode. A single camera system will be used to obtain the desired plate scale of 0.25 arcsec/pixel, selected to provide a good sampling of the typical seeing disk at 2.2 m m. Optical system will be based on lenses rather than mirrors to facilitate the design, manufacturing and alignment.
The detector will be mounted in a cold translation mechanism to compensate for non-achromaticity in the observing spectral range. Total translation range will be ± 2.5 mm along the optical axis.
A primary goal for the design of the lens system is to attain the maximum optical efficiency. Lens design (including the selection of materials and anti-reflection coatings) and lens manufacturing will fulfil this main objective. Optical efficiency values for LIRIS optics (including grisms) larger than 30% at a spectral resolution of 1000 in the K band are expected to be obtained.
A pupil lens will be included in the design for direct imaging of the entrance pupil for alignment purposes in the lab and during the first test run at telescope. A Hartmann mask will be included in the grism wheel to focus the instrument.
A stand-alone calibration unit will be designed for LIRIS.
All cold mechanisms will be based on wheels and will be as identical as possible to minimize costs and time of design, manufacturing, integration and verification. The cold plate and the structural support connecting it with the warm walls of the dewar must have the highest possible mechanical stiffness to guarantee a maximum flexure of ±4 m m/hour measured at the detector plane with respect to the telescope focal plane. A detailed finite element analysis will be made to minimize the flexure and the heat conduction of the structural support and to model the gravity induced deflections of the cold plate under operating conditions.
A single wheel should be used at the telescope focal position to mount the slits, coronographic masks and multislit masks. The diameter of this wheel will be optimized during the design to allocate the maximum number of holes. Minimum number of positions is 16, according to §3
LIRIS mechanisms will be completely modular. All mechanisms
within LIRIS will be interfaced to the control electronics using the same
components: motors, microswitches and connectors. All mechanisms will be
driven by stepping motors mounted inside the cryostat to avoid the presence
of any mechanical feedthrough.
The cold plate assembly containing the optics, vacuum getter, slit wheel, analyzer wheel, filter wheels, grism wheels, detector focusing mechanism and associate electrical and electronics components will be situated inside the cryostat and cooled to below 100 K. The array will be cooled down to 77 K. The dewar will be designed to make optimum use of the space available at the Cassegrain position of the WHT.
A Gifford-Mc Mahon mechanical cooler will be used for long-term cooling stability of the cold plate, detector and radiation shield. An anti-vibration mounting will be designed to isolate the vibration of the cold head from the instrument cold plate.
The use of a LN2 can or a LN2 open circuit for cooling
down LIRIS will be decided during the mechanical conceptual design phase.
A heating system would be provided at the cold plate to speed the service
LIRIS control system can be divided into two areas, control of mechanisms position and control of the array readout and timing. Although there has been some previous debate over which controller should be used to operate the detector, the decision has been taken to use the San Diego State University (SDSU) CCD controller2 supplied by IR Labs. (Tucson, AZ). This decision has been based in an agreement established between the Instituto de Astrofísica de Canarias (IAC) and the Isaac Newton Group (ING) to develop a common system for the two infrared instruments (LIRIS/IAC and INGRID/ING) that will be sharing the Cassegrain focus of the WHT. Both instruments will interface to the common user instrumentation control system of the telescope.
The data acquisition system and mechanism control will
be adapted as far as possible to the basic architecture used in the ING
telescopes. This architecture is based on a standard model for control
systems: host « network «
controllers « I/O «
instruments. In particular:
Several software subsystems will be required for LIRIS. The following is brief functional description of these subsystems:
Data Acquisition Software: The implementation of the Data Acquisition System (DAS) will be based on the SDSU solution. Their software will run on the Image Acquisition VME and on the CCD controller. This software will perform the following general functions: control the array operation, read image data from the array in the different modes and send it to the VME memory, image pre-processing on the VME, store images in VME-local disks (accessible through the network) and send images to the host computer for viewing and processing. The system will have the following features:
Image Processing Software: This software will run on the host computer and will perform the following general functions: data reduction, offset application and obtain statistics.
Access Catalogues Software: The Access Catalogues Software will run on the host computer. This software will be used to select objects for acquisition and guiding in the most important catalogs such as: Digital Sky Survey, SAO, SKYVIEW, Flux standards, 2MASS (when available), DENIS (when available).
Telescope Control Software: LIRIS will interface to the WHT Telescope Control System for acquisition (pointing and offset in all observing modes) and guiding (connection with the WHT autoguider).
Graphic User Interface: The user interface will run on the host computer and will be responsible for reading user commands and displaying acquired infrared data and instrument current status.
User commands will be required for mechanism control, selection of the readout mode, selection of the observing and calibration modes, control of the image, processing and logging systems, accessing catalogues and telescope control
The display will show to the user the image, the spectrum, the statistics, the mechanism status, the current readout mode, the current observing mode, the status of the image processing and logging systems, the information from catalogues, the telescope information and the errors and alarms
An engineering user interface with low-level control capabilities
will also be developed.
The authors wish to thank E. Atad-Ettedgui, P. Hasting
and T. Purkins (Royal Observatory of Edinburgh), R. Rutten and P. Moore
(Isaac Newton Group of Telescopes), C. Aspin (Nordic Optical Telescope)
and R. Lenzen and P. Bizenberger (Max-Planck-Institut für Astronomie)
for the stimulating discussions and their valuable advice during the early
stages of this project.