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ING Newsletter No. 6, October 2002
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LIRIS: A Long-Slit Intermediate Resolution Infrared Spectrograph for the WHT

José Acosta-Pulido1, E. Ballesteros1, Mary Barreto1 (Project Manager), Santiago Correa1, José M. Delgado1, Carlos Domínguez-Tagle1, Elvio Hernández1, Roberto López1, Arturo Manchado1 (Principal Investigator), Antonio Manescau1, Heidy Moreno1, Francisco Prada2, Pablo Redondo1, Vicente Sánchez1, Fabio Tenegi1.

1: Instituto de Astrofísica de Canarias; 2: Isaac Newton Group of Telescopes

LIRIS is an Instituto de Astrofísica de Canarias (IAC) project that consists in a near-infrared (0.9–2.4 microns) intermediate resolution spectrograph, conceived  as a common user instrument for the WHT.

LIRIS will have imaging, long-slit and multi-object spectroscopy observing modes (Å~1000–3000). Coronography, and polarimetry capabilities will eventually be added. Image capability will allow easy target acquisition for spectroscopy.

1. Scientific Drivers

Given its common-user status, it should be possible to use LIRIS 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 potential scientific applications: 2. General Description

The optical system is based on a classical collimator/camera design (Figures 1, 2 and 3). The expected throughput (averaged across the wavelength range) for the optics is 80% and 64% in imaging and spectroscopic modes, respectively. The total throughput including filters, grisms and detector is 35% and 30% in imaging and spectroscopic modes, respectively. Grisms are used as the dispersion elements (the grism transmission is assumed to be 80%). Low resolution grisms are manufactured in Corning 9754 (Figure 4) while medium resolution will be manufactured in ZnSe. A set of filters (broad band, Z, J, H, Ks and narrow band Br-α, K-continuum, H-continuum, [FeII], H2 (v=1–0), H2 (v=2–1), CH4 and HeI) have been acquired through a consortium headed by Alan Tokunaga.

Figure 1
Figure 1. LIRIS schematic representation. [ JPEG | TIFF ]

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 focussing mechanism. The detector will be mounted in a cold translation mechanism to compensate for non-achromaticity along the observing spectral range.

Figure 2, 3, 4
From left to right: Figure 2. Collimator. Figure 3. Camera. Figure 4. Corning 9754 Gris. [ JPEG | TIFF ]

The detail optical design and the conceptual mechanical design were subcontracted to the ROE (Royal Observatory of Edinburgh).

The slit wheel (Figure 5) contains 16 positions: one blank position, five long slits and ten multislit positions. The two filter wheels contain 12 positions each, and will hold the filters and the Wollaston prisms. The pupil wheel (Figure 6) contains 12 positions and will hold the pupil masks, plus an optional apodization mask with rotation mechanism for coronography capabilities. The grisms wheel has 10 positions for grisms. The camera wheel (Figure 7) has four positions and will carry the camera and the optics to re-image the pupil onto the detector plane, as well as an aperture and a black aperture. All mechanisms use Phytron cryogenic stepping motors and the control system is based on a VME system.

Figure 5, 6, 7
From left to right: Figure 5. Slit wheel. Figure 6. Pupil wheel. Figure 7. Camera wheel integrated in the test cryostat (dummy on camera position). [ JPEG | TIFF ]

The instrument is pre-cooled with LN2, and the cooling system is a two-stage closed-cycle refrigerator (Figure 8) (CTI model 1050C), which works on the Gifford-McMahon cycle.

The detector is a Hawaii 1024×1024 HgCdTe array using a SDSU controller, which communicates with the control computer (SUN workstation) using the SBUS card.

An agreement has been established between the IAC and the ING to develop jointly the detector control system and the Mechanism Control Software for the two infrared instruments (LIRIS/IAC and INGRID/ING). 

The LIRIS Software system is being designed to be fully integrated in the observer environment available at the WHT. A common observer will  have access to the following software packages: Instrument Simulator Software, Templates Generator Software, Instrument Support Platform User Interface, LIRIS Mechanism and Thermal Control Software, Real Time Display, Quick Look Data Analysis and Pipeline Data Reduction.

3. Current Status

At present LIRIS is in the calibration phase at IAC. Assembly, integration and verification phases were carried out in summer 2001.

The collimator, camera, slits wheel mechanism and the main central wheel (filter 1 and the pupil wheel) mechanism have been successfully tested at test cryostats (Figure 9) in cryogenic conditions. They have also been pre-integrated on LIRIS to check the interfaces (Figures 10, 11 and 12).

Figures 8, 9
Left: Figure 8. Closed-cycle refrigerator with the two-stage thermal links integrated. Right: Figure 9. Mechanism cryostat on the Mitutoyo during the functional verification of the main central wheels module. [ JPEG | TIFF ]

Figure 10, 11, 12
From left to right: Figure 10. Main central wheel module (filter wheel side) pre-integrated on LIRIS. Figure 11. Bottom view of the slit wheel mechanism. Figure 12. Pre-integration of the slit wheel on the LIRIS optical bench. [ JPEG | TIFF ]

Test multi-slits masks have been manufactured by Electric Discharge Machining (EDM) and successfully tested achieving a roughness of 1.15±0.15microns.
 
The engineering and the scientific detectors have been tested in cryogenic conditions on a purpose-built detector test bench (Figures 13 and 14). The main characteristics of the science degree array at 80K are as follows: readout noise ~20e–, dark current 0.065e–s–1, bad pixels <1.5% and the detector behaves linearly within 2% up to 50% of the full-well (175,000e–). The signal offset was found to vary 670e–/K with the detector temperature. The current temperature controller permits a stability of better than 0.005K, which implies a signal offset variation of less than 4e–.

Figure 13, 14
Left: Figure 13. Internal view of the detector test cryostat. Right: Figure 14. Image of an USAFSR pattern taken with the scientific grade detector, through the H-band filter. [ JPEG | TIFF ]

In November 2001 the LIRIS Cryostat integration was started (vacuum tank, optical bench, closed-cycle cooler, radiation shields, etc.) (Figures 15, 16, 17 and 18) and in December the first cool-down was successfully completed (Figures 19 and 20).

Figure 15, 16, 17, 18
From top to bottom: Figures 15 and 16. Positioning the optical bench on the trusses in the vacuum tank central ring (cables provisionally fixed for this cycle). Figures 17 and 18. Optical bench with the collimator and reticules placed in specific positions to check their behaviour during the first cycle (these reticules were visible at all time through a very useful cryostat auxiliary window, see Figure 18). A simulation of the detector module was placed outside the beam line to test the detector thermal control system. [ JPEG | TIFF ]

The following LIRIS cool-down took place in March and it included the slit wheel mechanism, collimator, central wheels mechanisms (two filter wheels, pupil and grisms wheels) and the camera mechanism. First light and commissioning at the telescope is expected at the beginning of  2003.

Figure 19, 20
Left: Figure 19. LIRIS cryostat ready for the first cycle. Right: Figure 20. Pre-cooling with liquid LN2. [ JPEG | TIFF ]

For on-line information about the LIRIS project, please visit our web site at: http://www.iac.es/proyect/LIRIS/.

Email contact: Arturo Manchado (amt@ll.iac.es)


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