Updated 1 March 2012

NIRMOS: a wide-field near-infrared spectrograph for the Giant Magellan Telescope

Daniel Fabricant*, Robert Fata, Warren R. Brown, Brian McLeod, Mark Mueller, Thomas Gauron, John Roll, Henry Bergner, John Geary, Vladimir Kradinov, Tim Norton, Matt Smith, Joseph Zajac

Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA, USA 02138

ABSTRACT

NIRMOS (Near-Infrared Multiple Object Spectrograph) is a 0.9 to 2.5 μm imager/spectrograph concept proposed for the Giant Magellan Telescope1 (GMT). Near-infrared observations will play a central role in the ELT era, allowing us to trace the birth and evolution of galaxies through the era of peak star formation. NIRMOS' large field of view, 6.5′ by 6.5′, will be unique among imaging spectrographs developed for ELTs. NIRMOS will operate in Las Campanas' superb natural seeing and is also designed to take advantage of GMT's ground-layer adaptive optics system. We describe NIRMOS' high-performance optical and mechanical design.

Keywords: Infrared spectroscopy, extremely large telescope, Giant Magellan Telescope, ground layer adaptive optics

1. INTRODUCTION We describe a 0.9 to 2.5 μm imager/spectrograph concept for the Giant Magellan Telescope (GMT) named the Near InfraRed Multiple Object Spectrograph (NIRMOS). NIRMOS, coupled with the huge collecting area of the Giant Magellan Telescope (GMT) and the superb seeing at Las Campanas, would enable GMT scientists to break open fields of research that have remained tantalizingly out of reach for the current generation of 8 m-class telescopes.

The near-infrared (NIR) will play a central role in the scientific programs for ELTs because the peak star formation in the Universe occurs at z~2.5, where our best studied optical emission line tracers of star formation and metallicity are shifted into the NIR. In the ELT era we aim to develop a physical understanding of galaxies at z=2-3 comparable to our current understanding of galaxies at z=0.2-0.3. NIRMOS will detect the rest-frame optical spectral lines emitted by these galaxies that will allow us to study their gas-phase metallicities, dust properties, star formation rates, and AGN activity as well as the continuum emission used to characterize their stellar populations.

Tracing the birth and early evolution of the first galaxies at z>7 falls to NIR and longer wavelengths as Ly α is redshifted to λ≥1 μm. Discovery and characterization of the Ly α emission from the first galaxies at z>7 as well as from the afterglows of gamma ray bursts at z>7 with be within the grasp of ELTS. Spectroscopy of these high redshift sources will enable a fundamental measurement of cosmic reionization, which is expected to be complex, temporally extended, inhomogeneous; the epoch of reionization is currently poorly constrained.

GMT’s compact focal plane scale allows NIRMOS to have a wide field of view (6.5′ by 6.5′) for efficient surveys, a field of view that cannot be easily addressed by instruments at other ELTs. The aperture of GMT combined with the field of view of NIRMOS will also enable precise, time-series differential spectroscopy of bright stars - the technique needed to measure the transmission and emission spectra of transiting exoplanet atmospheres.

NIRMOS would enable break-through discoveries in a broad swath of astrophysical research. For example, 8m-class telescopes can measure the surface composition for only the brightest Kuiper Belt objects ̶ the characterization of outer solar system bodies requires the power of NIRMOS. From high redshift gravitational lenses and z>2 galaxy proto-clusters to the stellar population of the metal-poor Magellanic Clouds and metal-rich Galactic center, wide-ranging impact is the defining characteristic of NIRMOS science.

*dfabricant@cfa.harvard.edu; phone 617 495-7398

2. SCIENTIFIC REQUIREMENTS The GMT project office summarized the requirements for NIRMOS as shown in Table 1.

Table 1. Detailed requirements for CoDR Study from GMT Project Office

Parameter Requirement Goal Notes

Wavelength Range 1 – 2.5μm 0.9 – 2.5μm Coverage of a full band in a single exposure

Spectral Resolution Rφ ≥ 1,500 R = 5000 with narrow slits

Baseline mode is expected to match GLAO image sizes

Multiplex Factor > 100 - For slit MOS mode Field of View Slit MOS 20 sq. arcmin. 25 sq. arcmin. Field of View Imaging Mode 5′ x 5′ -

Field of View Fiber MOS Full 20′ diameter field Only expected to operate in the J & H bands

Image Quality 0.15′′ 80%EE - Don’t degrade images from telescope/site by more than 5%

Velocity Stability < 0.1 - Flexure in units of spectral resolution element per hour

Throughput ≥ 20% 2.2μm ≥ 30% at blaze peaks

Exclusive of slit losses, telescope and atmosphere

The scientific requirements that flow from the NIRMOS science case add a requirement for extended (simultaneous) spectral coverage at R>3000, covering yJ or HK in one observation, as well as accommodation of medium-band filters for imaging. The NIRMOS design meets all of these requirements.

3. OPTICAL DESIGN 3.1 Reflective vs. refractive optics

The simplicity and achromatic nature of reflective optics is always appealing, but wide-field spectrograph designs are rarely possible with reflective optics. In the present case, the required large field of view and fast focal ratio eliminate any possibility of off-axis reflective designs. In order to keep the central obstruction in an on-axis design to manageable levels we would need a very fast camera. We would need a huge beam size to accommodate the restricted angular field of fast Schmidt-style cameras (perhaps 10°); this large beam size would lead to a huge instrument and difficulty in obtaining key components like gratings. A refractive design is the more feasible approach.

3.2 Optical material choices

The palette of optical materials for near-infrared optical design is limited. CaF2 is the obvious choice for supplying positive optical power. BaF2 has been used in smaller NIR optics, but procuring blanks of the size required for NIRMOS is only intermittently possible. ZnSe is available in large, but relatively thin blanks (~25mm) but has not proven to be useful in the NIRMOS optics. Fused quartz is available in large sizes, and by the standards of NIR materials is reasonably priced. Fused quartz can be used for windows and color correction in negative power elements. The Ohara optical glass S-FTM16 has been used with considerable success for color correction with CaF2 but is not available in the sizes required for NIRMOS. A substitute, S-TIM28, is available in large sizes but has a less desirable dispersion curve. The NIRMOS baseline optical design uses just three materials: CaF2, infrared fused quartz and S-TIM28.

3.3 Use of aspherics

Aspheric surfaces typically come at significant cost, but many fast spectrograph cameras use aspherics to obtain better performance with a minimum number of surfaces. In a cryogenic infrared design where every lens is a singlet, scattered light and throughput considerations make aspheric surfaces worthwhile. The first choice is to put the aspherics on a crystalline material like CaF2 because direct diamond turning of the surface is possible. Post-polishing with conventional or magneto-rheological techniques is frequently specified to achieve smoother surfaces.

3.4 Volume phase holographic transmission gratings

Transmission gratings are the most practical option for NIRMOS. There are two families of transmission gratings available: ruled gratings replicated in transparent resin and holographic gratings, including volume phase holographic gratings. Volume phase holographic (VPH) gratings are produced by holographically encoding a sinusoidal index variation in a recording medium, typically dichromated gel. Several studies have shown that these VPH gratings survive operation in cryogenic environments like that required for NIRMOS with no change in efficiency2,3,4,5. VPH gratings have several important advantages over ruled transmission gratings: higher efficiency (peaking near 90% as opposed to just over 70%), higher dispersion (ruled grating lose efficiency due to total internal reflection at steep facets), and lower scattering6 (see Figure 1). For these reasons we have chosen VPH gratings for use in NIRMOS.

Figure 1. Scattering profiles of ruled grating on the left and VPH grating on the right, measured by Ellis and Bland-Hawthorn6 Note that the scattering wings of the VPH grating on the right are lower than the ruled grating by about an order of magnitude.

3.5 Scattered light

The sky brightness in the J and H bands is dominated by strong OH emission lines in the J and H bands. At Las Campanas the reported sky brightness7 is 16.1 mag/sq. arcsec in J and 14.0 mag/sq. arcsec in H, yielding fluxes of 7330 and 24000 photons s-1 m-2 μm-1 arcsec-2. The level of the interline background is controversial, but the best measurement is still that of Maihara8, who measured 590 photons s-1 m-2 μm-1 arcsec-2. Ellis and Bland-Hawthorn6 have suggested that this measurement is dominated by scattered light from reprocessed OH line emission, and that the true interline background light is a factor of seven lower, and is set by the zodiacal light. However, this has not been confirmed to date to the authors’ knowledge.

Table 2. Expected fraction of background in J and H due to Maihara (1993) interline background. OH lines are the dominant background.

Band Sky Brightness (mag arcsec-2)

Photon flux (ph s-1 m-2 μm-1 arcsec-2)

Maihara interline bkgd (ph s-1 m-2 μm-1 arcsec-2)

Maihara interline bkgd fraction of total

J 16.1 7330 590 0.080 H 14.0 24000 590 0.025

Table 2 shows that scattered light from the dominant OH lines has the potential to create problems, and observers report that scattered light from these strong OH lines may have compromised the performance of some existing infrared spectrographs. What is the source of this scattered light? Two sources of scattered light are likely to dominate: the scattering wings of the diffraction grating and light reflected from the detector and returned to the detector after reflecting from a lens surface. The scattering wings from the grating will add to the natural interline background by

spreading the light from OH lines to nearby pixels. Our use of VPH gratings is the most effective design choice to reduce this source of scattered light.

The reflected light from the detector and lens surfaces can be modeled in detail with ray tracing software but here we simply estimate the total amount contributed from each surface. The reflectivity of the detector is ~10% and the reflectivity of each antireflection coated lens surface is ~1%, so the contribution from each lens surface is ~0.1%. This light is on average uniformly distributed over the focal surface, but care must be taken to avoid sharply focused ghost images. Light reflecting from diverging surfaces will only partially reach the focal surface. With a filter and grating in place, NIRMOS has a total of 30 surfaces. The filters and gratings are tilted, so we need consider only 26 of these surfaces. We conservatively estimate that less than 50% of this scattered light returns to the focal plane rather than being absorbed elsewhere, or ~1.3% of the total OH light. If the slitlet length in use for any given observations is 65% of the total slit length (50 slits, each 5ʺ long) about 0.8% of the OH line light will appear uniformly distributed over the observed spectra. This scattered background amounts to ~10% of the Maihara background in the J band, and 30% of the Maihara background in the H band. Note that Ellis and Bland-Hawthorn contend that scattered light accounts for 85% of the Maihara background in H.

All of our sensitivity calculations assume the full Maihara interline background, so if Ellis and Bland-Hawthorn are correct, and if we are able to control the scattered light to better levels than in the University of HAWAII 2.2-m Coudé spectrograph used by Maihara, our sensitivity calculations are conservative.

We have already taken several steps to reduce scattered light in the NIRMOS design. These steps include ghost image analysis and optimization with ZEMAX, tilting the filters to deflect reflected light, and the use of low-scatter VPH gratings. We will use non-sequential ray tracing codes and careful baffle design to control scattered light as the design moves to more detailed phases.

Table 3. Summary of NIRMOS baseline design parameters

Parameter Value Detector Array 2x3 array of HAWAII 4RG, each 4K by 4K pixels Slit Length 6.5'

Field of View 42 square arcminutes for imaging or spectroscopy, 33 square arcminutes with full spectral coverage for spectroscopy

Multiplex Capability ~80 with 5ʺ long slits Scale at Detector 0.049ʺ per pixel Aspheric Lenses 2 in camera Collimated Beam Diameter 270 mm

Collimator Focal Length 2200 mm

Camera Focal Length 665 mm Camera Focal Ratio f/2.4 Lyot Stop Position 220 mm from collimator, 205 mm from camera Filter Diameter 300 mm clear aperture, 325 mm total VPH Grating Diameter 300 mm clear aperture, 325 mm total Filter Tilt 7° CaF2 Field Lens Plano-convex, 4 segments Element Diameters 600 mm window and field lenses, < 390 mm remainder

3.6 Optical design summary

The main parameters for the NIRMOS optics are summarized in Table 3, and the imaging and spectroscopic configurations are presented in Figure 2 and Figure 3, respectively. The camera mechanism can be tilted in one axis

about the grating surface to allow the camera to be placed at the Bragg condition for the VPH gratings without large and impractical prisms to straighten the beam.

Figure 2. Baseline NIRMOS optics in imaging mode; both of the filter positions are indicated although only one filter at a time will be used. All of the lenses are made from CaF2 except those lenses marked with a colored dot. Infrasil , S-TIM28 . All of the lenses are 390 mm in diameter or less except for the entrance window and the two field lenses. These three elements are 600 mm in diameter. The leading surfaces of the first and last camera lenses are aspheric.

Figure 3. Baseline NIRMOS optics in spectroscopy mode. All elements are identical to those for the imaging mode except that a VPH grating is inserted at the correct Bragg angle and the camera/detector array is rotated to maintain the (Littrow) Bragg condition.

Table 4. 80% encircled energy diameters for the baseline design. The specification is 0.15″.

Band 80% Encircled Energy Diameter (Arcsec)

Y (0.9 - 1.1 µm) 0.09 J (1.15 - 1.25 µm) 0.06

H (1.5 - 1.8 µm) 0.08 K (2.0 – 2.5 µm) 0.12

Worst monochromatic image (2.5 µm) 0.15

Telescope Segmented Filter Lyot Detector Focus CaF2 Field Lens Positions Stop Array

VPH Grating

3.68 m

4.01 m

3.7 Superb Image Quality

NIRMOS is designed to achieve superb image quality so that even with ground layer adaptive optics the image quality will be dominated by the atmosphere and not the instrument. NIRMOS provides a worst case 80% encircled energy of 0.15ʺ as shown in Table 4: a 0.2″ FWHM image is degraded by no more than 10%.

3.8 High Throughput

The GMT Project requirements call for >20% throughput at 2.2 µm, with a goal of 30% at the peak of grating blazes. The expected NIRMOS throughput shown in Figure 4 exceeds these goals for both imaging and spectroscopy modes.

Figure 4. Throughput estimate for NIRMOS. The overall thoughput for imaging and spectroscopy are shown as red and black lines, respectively. We do not include atmospheric transmission or aperture losses.

3.9 Pupil Imaging

We have placed emphasis on forming sharp pupil images because the GMT’s pupil is segmented, and significant pupil blur would cause higher background or throughput loss. Maintaining a sharp pupil image across the 0.9 to 2.5 µm band required a doublet (achromatic) field lens. The pupil blur is ±1.2 mm across the YJ bands and ±0.66 mm across the HK bands. The pupil image of one of the GMT primaries is 90 mm in diameter at the cold stop.

3.10 Operating Modes

Table 5. Spectroscopic operating modes, Slit width is 0.5″, except † 0.4″, and * 0.25″.

Y J H K YJ JH HK Mode multislits multislits multislits multislits slits, fiber long slit, fiber long slit λ range (µm) 0.9-1.1 1.15-1.35 1.5-1.8 2.0-2.5 0.9-1.35 1.05-1.70 1.50-2.44 Resolution 3169 3033 3133 2702 3124†,4998* 5133* 3208† Ruling (l/mm) 650 500 390 250 460 390 275 Field for full λ range 5.0′ 6.3′ 5.6′ 5.3′ 2.0′ 0′ 0′ Thickness (µm) 5.0 6.3 8.3 13.0 7.0 8.0 9.8 Index modulation 0.1 0.1 0.1 0.0865 0.08 0.0859 0.1 Incidence angle air 18.77° 18.21° 18.77° 16.33° 13.57° 13.6° 14.0°

4. NIRMOS EXPOSURE TIME CALCULATOR

Figure 5. The NIRMOS exposure time calculator and spectral simulator is on-line at http://hopper.si.edu/saoetc/sao-etc .

As shown in Figure 5, we have written an exposure time calculator and spectral simulator so that prospective GMT observers are able to estimate NIRMOS’ sensitivity for their research (see http://hopper.si.edu/saoetc/sao-etc). The NIRMOS spectral simulations shown in the next section were performed with this tool. Noise sources including atmospheric emission lines, the Maihara interline background (590 ph m-2 s-1 um-1 arcsec-2), dark current, read noise, and source photon statistics have been modeled. The user can select any of the proposed seven grating/filter combinations, and adjust the slit width, seeing, source intensity, and source extent to realistically model source counts and resolution. A fairly extensive library of spectra ranging from stars to z=7 Ly α emitters is available and more spectra can be added easily. The raw flux calibrated spectrum or the spectrum with the regions near strong sky emission lines suppressed can be plotted to get a clearer picture of the information content in the spectrum.

5. NIRMOS MULTIPLEX ADVANTAGE AND SENSITIVITY Table 6. Galaxy counts with photometric z >2 from Chandra Deep Field South from Franx9.

KAB z>2 galaxies per sq. arcmin

z>2 galaxies in 42 sq. arcmin. NIRMOS field

22.7 1.0 42 23 1.7 71 23.5 2.6 109

One of the key questions for a multiple object survey spectrograph is whether it can observe faint enough galaxies to make good use of the multiplex advantage. Table 6 gives an estimate of the z>2 galaxy count in a single NIRMOS field. With a multiplex capability of ~80 with 5ʺ long slitlets, we need to reach to KAB of 23.5 to use most of this multiplex advantage. We use our exposure time calculator to produce Figure 6 and Figure 7 showing simulated four hour integration J band spectra for the faintest and brightest limit in Table 6. These spectra have sufficient signal to noise to measure velocity dispersions.

Figure 6. Simulated J band spectrum of a z=2.3 galaxy with JAB=24.6 (KAB=23.5) observed for four hours in natural seeing. There are ~109 galaxies per NIRMOS field of view to this magnitude limit. The upper panel shows the complete simulation, while the lower panel has the strong skylines excised.

Figure 7. Simulated J band spectrum of a z=2.3 galaxy with JAB=23.8 (KAB=22.7) observed for four hours in natural seeing. The upper panel shows the complete simulation, while the lower panel has the strong skylines excised. About 1/3 of the galaxies in a K selected sample to KAB=23.5 would have spectra of this quality.

6. MECHANICAL OVERVIEW

Figure 8. Left: NIRMOS mounted within the rotating Gregorian Instrument Assembly (GIA). This assembly has four large instrument bays below the upper platform. Right: the GIA location within the stationary portion of the instrument platform.

Figure 9. External overview model of NIRMOS

Upper mounting truss

Lower vacuum housing

Entrance window

Lower mounting truss assembly

Central vacuum structure with box beam frame

MANIFEST fiber feed connector

Focal plane housing Slit mask cassette module

Electronic boxes (6)

Collimator housing

Collimator support trusses

Turbo and roughing pump

Cryo coolers (23)

NIRMOS is a large cryogenic instrument with a total mass of ~9000 kg and a cold mass of ~1500 kg mounted at the direct Gregorian focus of the GMT. Figure 8 (left) shows NIRMOS mounted in the rotating Gregorian Instrument Assembly (GIA) and Figure 8 (right) shows the location of the GIA within the instrument platform behind the primary mirrors.

Figure 9 is an overall external view of NIRMOS. The main vacuum assembly consists of aluminum modular components that are bolted together. The central vacuum structure is reinforced with a stiff box beam frame to provide a rigid common reference surface to mount each of the major optical assemblies, minimizing the relative motion between the lens groups, filters, dispersers and detector array.

The NIRMOS electronics boxes are mounted on top of the central box frame. The electronics provide the mechanism control, cryogenic control, temperature and vacuum control, detector support, active flexure control and instrument interface electronics. The upper and lower mounting truss assemblies attach to the central box frame and collimator housing and at three points to the GMT instrument mounting frame.

Figure 10 is a cross-sectional view of NIRMOS. The six collimator and six camera lenses are mounted to bezels with bonded flexures at the lens perimeters. The flexure mounts are compact, and provide uniform lens support and good thermal conduction between the optic and bezel.

The NIRMOS optics are completely baffled using cold shields that are made from thin aluminum and will be held at the same temperature as the optics ~120K. A radiation shield consisting of 3 layers of gold coated Kapton is mounted between the cold shields and warm structure.

Figure 10. Cross-sectional view of NIRMOS

Collimator lens barrel assembly

Camera lens barrel assembly

Radiation shield, collimator

Cold shield with radiation shield

Cold shield with radiation shield

Radiation shield for slit cassette

Figure 11. Upper mechanisms

Figure 11 shows the upper mechanisms which include the slit mask cassette elevator, the slit mask insertion mechanism and clamp. Figure 12 shows the lower mechanisms including the filter wheels, grating wheel, pupil mask and the tilting camera mechanism. These mechanisms allow the user to select between 10 slit masks, 14 filters and 7 gratings at the beginning of an observation. The science detector array consists of six 4K by 4K HAWAII-4RG imagers. The array is located at the end of the camera lens barrel.

Figure 12. Lower mechanisms and science detectors

7. DETECTOR ASSEMBLY AND FLEXURE CONTROL The NIRMOS focal plane will consist of six closely butted 4K x 4K (15um pixel) imagers in a 2 x 3 array as shown in Figure 13. Each imager has an active area of 61.4 mm square and there is a dead space between image areas of about 3.2 mm. Two short flex cables from one edge of each imager connect to its readout controller. Each controller is a single Teledyne SIDECAR ASIC that contains the downloadable programs to run the imager in various modes, as well as precision analog signal processing for 32 output channels. The controller produces 16-bit digitized video at a speed >300 kpix s-1 per channel. Read noise is estimated at 12-15 e- pix-1, which can be reduced to less than 3 e- by the use of multiple nondestructive samples.

Slit mask cassette elevator

Science detector array

Flex prints for array readout

Hermetic connectors

Grating wheel mechanism

Filter wheel mechanism (2)

Tilting camera mechanism

Pupil mask mechanism

Slit mask insertion mechanism

Figure 13. Views of the components and assembled 3 x 2 focal plane of HAWAII-4RG detectors.

The NIRMOS design includes an active flexure control system. The flexure term that must be considered carefully is differential motion between the slit mask and detector array. For imaging applications and on short timescales for spectroscopy, this term is likely to be negligible. For longer spectroscopy integrations OH lines provide an abundant signal for detecting shifts in the wavelength and spatial directions.

Figure 14. Focal plane array of six HAWAII 4RG detectors mounted on the five axis flexure/focus stage.

The detector array is mounted on a nano-positioning piezoelectric stage that allows lateral, tip-tilt, and focus control to compensate for gravitational flexure and thermally-induced focus changes. The stage’s range of motion is 1 mm laterally in each of two axes, 1.5 mm in focus. The step size in X and Y is 10 nanometers and 15 nanometers in Z. Maximum speed capability is 500 µm/s in each direction. This stage uses Physik Instrumente’s “finger-walker” technology that can hold position when powered off. A similar system was developed for use in Binospec15. The stage manufacturer (PI) has stated that this stage technology is suitable for use at cryogenic temperatures.

8. THERMAL DESIGN NIRMOS must operate at cryogenic temperatures so that the blackbody glow of the instrument is well below the infrared inter-line sky background; an operating temperature of 120 K meets this requirement. Cryogenic operation requires a vacuum dewar with radiation shields to minimize heat transfer.

Preparing NIRMOS for operation requires cooling 1,530 kg from room temperature to 120 K. We have set a limit of seven days for cooling based on the cycle time for service or maintenance. We have considered two types of cryogenic systems: LN2 or cryocoolers. A minimum of 1,053 kg (1,303 liters) of LN2 would be required to cool NIRMOS. Alternately, 23 cryocoolers could cool NIRMOS in the allotted seven days. Once at operating temperature, ~2.5 liters hr-1 of LN2 would be required to maintain temperature, or alternately 11 cryocoolers operating at reduced power.

LN2 advantages include: cooling can be very fast as needed, no vibration is produced, the initial cost of an LN2 system is lower, and no waste heat is generated. Cryocooler advantages include: no LN2 trucks, external tanks, or plumbing infrastructure are required, the potential hazard of venting large quantities of nitrogen gas is avoided, cryocoolers are more controllable for targeted cooling, and colder temperatures can be achieved for the IR array.

We have chosen cryocoolers as the baseline to minimize operational complexity and costs, despite the probable higher capital costs. In subsequent design phases we may elect to use a hybrid system for the slit mask cassette assembly: LN2 for rapid initial cooling and two cryocoolers for maintaining temperature to reduce the large number of cryocoolers required to achieve a daytime mask exchange. However, our conceptual design uses cryocoolers to cool the optics, detector, and slit masks to the desired operational temperatures.

Figure 15 illustrates the thermal design concept. The optical system (labeled “cold mass”) is surrounded by a light-tight cold shroud, designed to eliminate stray light into the system. Radiation shields and G-10 supports minimize the radiative and conductive heat transfer between the cold mass and the room temperature vacuum vessel.

Figure 15. Schematic layout of the thermal system components

We performed initial transient cool-down and warm-up analyses of the flexure-mounted NIRMOS lenses wherein the collimator and camera cold tubes are cooled down from 293 K to the operational temperature of 120 K or warmed up from 120 K to 293 K. The duration of the cool-down (7 days) is dictated by the number of the cryocoolers (23), the warm-up duration is limited by lens stress. Using 500 psi as the allowable stress (both in tension and compression), it is determined that the instrument can be warmed up in 5 days. Radiation coupling, which would reduce thermal gradients and stress, is neglected in the current analysis based only on thermal conduction. The model consists of detailed 3D solid lenses, bonds, nubs, and flexures as shown in Figure 16. Time dependent temperatures are applied to the ends of the flexures.

Figure 16. Left: section view of finite element lens mount model showing flexures. Right: close-up view of flexure

9. FINITE ELEMENT ANALYSIS The system level finite element model of the instrument shown in Figure 17 has >1.5 million degrees of freedom. The lenses are modeled with detailed solid element meshes, the epoxy bonds and nubs as well as the camera yoke are also modeled with solid elements. The flexures, bezels, and most of the vacuum housing are modeled with shell elements. Beams and trusses are modeled with beam elements. Point mass elements are used to represent assemblies and mechanisms that are currently not modeled in detail (radiation shields, racks with electronics, detector assembly, focal plane and fiber feed assembly, etc.).

Figure 17. NIRMOS finite element system model.

Figure 18. Left: Image motion at detector due to gravity. Right: Focus change at detector due to gravity.

Double blade flexure

Nub

Figure 18 shows the image motion at the detector and defocus at the detector at different telescope elevation angles when the entire instrument is subjected to the gravity loads. These plots use the zenith gravity case as a reference: the image shift and the defocus at the detector are defined to be zero at the zenith. At elevations >30° , the maximum image shift is about 5 pixels along the X-axis and about 4 pixels along the Y-axis. The image defocuses <11 µm at elevations >30°. We conclude that the flexure compensation described above is needed to guarantee meeting the flexure requirement of 0.5 pixel hr-1.

10. DISCUSSION Although a cryogenic spectrograph on this scale might seem intimidating, the NIRMOS concept has been refined to minimize technical risks and to maximize design simplicity and elegance. To have a large impact, NIRMOS must be ambitious in scope, but it conservatively relies entirely on demonstrated technology. NIRMOS’ engineering and software design builds upon the heritage of successful instruments built by the CfA instrument team in service at the MMT and Magellan. At Magellan these instruments include Megacam10, a 16K by 18K optical imager, and MMIRS11, a multi-object near-infrared imager/spectrograph. At the MMT these instruments include SWIRC12, a near-infrared imager, and Hectospec13/Hectochelle14, a fiber-fed moderate and high-dispersion optical spectrographs with a robotic fiber positioner. Binospec15, a wide-field multislit optical spectrograph, will be commissioned at the MMT in two years.

NIRMOS’ optics take full advantage of the best wide-field images that ground layer adaptive optics can deliver, but the impressive performance predictions in our science case rely only on Las Campanas’ excellent natural seeing. NIRMOS is an appealing instrument concept for GMT because it addresses broad and fundamental science and because it makes relatively modest demands upon telescope systems.

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