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A high-resolution optical spectrograph for the Thirty Meter Telescope: design and performance Steve Osterman, Cynthia Froning, Matthew Beasley, James Green, Stephane Beland Center for Astrophysics and Space Astronomy, University of Colorado, Campus box 389, Boulder CO 80309 ABSTRACT We have completed a conceptual design study of the High Resolution Optical Spectrograph for the Thirty Meter Telescope project. We propose the use of a fiber fed integral field unit and a dichroic tree to achieve R=100,000 spectroscopy from 310 to 1100 nm independent of AO performance. The system relies on the dichroic tree to provide coarse wavelength selection, and 32 first order spectrograph benches. This approach allows for simultaneous optimization of grating and detector performance for all wavelengths, resulting in high efficiency, near uniform dispersion, and reduced program risk and cost due to the high degree of component commonality. We present projected performance and design details. Keywords: Optical, spectroscopy, high resolution, dichroic, Thirty Meter Telescope, HROS,CU-HROS 1. INTRODUCTION This paper presents the results of a feasibility study performed for the Thirty Meter Telescope project1, and is an extension of Froning, et al. (2006), “Conceptual design for the High Resolution Optical Spectrograph on the Thirty-Meter Telescope: a new concept for a ground-based highresolution optical spectrograph”.2 Froning, et al. discuss the scientific motivation, top level design and advantages of the CU-HROS design. This paper, in turn, focuses on the design details as presented in the feasibility study. As laid out in Froning, et al., it is clear that the large physical scales of extremely large telescopes (ELTs) and the need for ever higher resolution and sensitivity necessitate a fundamentally new approach to instrument design to break the cost-growth paradigm inherent to these ambitious projects. In breaking with the simple scalings of existing instruments, we developed an alternative to the conventional cross-dispersed echelle design and achieve high resolution, high efficiency broad band optical spectroscopy by using a dichroic tree to sort the science spectrum into 32 individual narrow band channels which then feed simple first-order spectrographs. In this paper we discuss the design at the component level, and provide system level performance estimate. Our organization is as follows: Section 2 presents an overview of the concept and its principal components. In Section 3, we discuss the design in detail, and in Section 4 we present performance estimates and design constrains. Science drivers, signal to noise estimates, system level advantages and upgrade paths are addressed in Froning, et al. The instrument described in this paper is referred to as CU-HROS to distinguish it from the generic HROS instrument intended for TMT. 2. INSTRUMENT OVERVIEW We present a design concept for a high resolution optical spectrograph which is suitable for large aperture, long focal length ELT projects – a design which combines new and emerging technologies to limit the growth in size, cost and risk as telescope apertures continue to grow. This design features: ___________________________ * [email protected] voice 1.303.492.3656 fax 1.303.492.5941 A fiber-optic integral field unit (FIFU) to decouple throughput and spectral resolution from telescope seeing or adaptive optics performance. An array of high performance dichroic mirrors to perform primary wavelength selection and reduce the spectral grasp needed by the individual first order spectrograph benches to roughly 3% of the total band pass. High efficiency coatings, detectors and diffraction gratings, made possible by the narrow band over which these components must be optimized. Small, low cost, low risk components with a high degree of replication, reducing risk and cost. Full spectral coverage in a single observation with no mechanisms after the foreoptics. 2.1. Instrument requirements Our view of HROS is that of a large, multi-purpose high-resolution optical spectrograph. In order to meet these requirements (Table 1), we have several issues to confront. First, the large size of TMT requires either very large optics or small spatial sampling. Second, the telescope must provide an R > 50,000 spectrum from 310 nm to 1100 nm (ideally in one exposure to maintain efficiency). Third, the quality of the spectrum has to be high across the band pass to allow science at arbitrary wavelengths. Ideally, the spectrograph must maintain high efficiency over as a large a band pass as possible. Finally, the instrument must maintain the effective area advantage provided by the thirty meter primary mirror. Instrument design and performance requirements are either dictated by the project or flow down directly from the science goals as described in Froning, et al. In addition, the nominal Specification Requirement Goal Telescope F/# f/15 f/15 Location Nasmyth platform Nasmyth platform Wavelength Coverage 310 – 1100 nm in a single, or at most two, observations. 310 – 1100 nm in a single observation Spatial Sampling <0.2"/lenslet or image slice at the telescope focal plane 0.1" /lenslet or image slice at the telescope focal plane Spatial Resolution <0.2" FWHM at detector <0.2" FWHM at detector Field of View 10" 70" Effective Slit Length 1"x5" At least 5 discrete 1 square arc second IFUs Sensitivity Must maintain 30m advantage Efficiency > than 15% over 80% of band pass Support Calibrated Spectroscopy? Yes Yes Spectral Resolution R=50,000 R=100,000 Spectral Stability Line center stable to 3 m/s for the duration of an observation Line center stable to 3 m/s for the duration of an observation Spectral Repeatability Wave cal. must provide centroid knowledge to <3 m/s between observations Wave cal. must provide centroid knowledge to <1 m/s between observations Table 1. CU-HROS Performance Requirements requirement for a 1"x5" slit has been modified in favor of a requirement for areal coverage, and instead of a requirement for long term stability, we require wavelength knowledge based on cotemporal or near cotemporal wavelength calibration. Finally, the field of view of the foreoptics has been increased to 70" to ensure guide star acquisition and to accommodate a single laser ground layer adaptive optics system (SL-GLAO). 2.2. Design Overview When we conceived of CU-HROS, we started with the conventional echelle-based design, which has long been the standard for high-resolution optical spectrographs for ground-based telescopes. The echelle has many advantages, in particular making use of the 2D format of the CCD for the high spectral band pass at high resolution. However, as the telescope apertures and focal lengths grow and the instruments are seeing limited or near seeing limited (especially in the optical) the size of the instrument grows dramatically. For an instrument such as the one specified in the original HROS requirements — a slit-limited spectral resolution of 50,000 matched to a 1" slit — the beam size at the echelle is 0.9 m, requiring a very large echelle mosaic, with equally large optics at other points in the system. In addition, since an echelle works over a large band pass, the variation in grating efficiency dominates the system throughput. Prism cross dispersers becomes impractical due to the very large size of the diffracted beam, further reducing efficiency. With these constraints, we chose to move in a new direction to address these challenges. Our solution is to use a multiplexed first order spectrograph, as illustrated in Figure 1. Spectrograph components that have significant efficiency variations with wavelength (detectors and gratings) may be individually optimized over a narrow wavelength range to maximize performance. Broadband components can be either reflective (collimator) or very carefully chosen to minimize loss of efficiency or spectral resolution (atmospheric dispersion compensator, or ADC, and the fiber-optic IFU, or FIFU). The nominal design operates in a single mode with a minimum number Figure 1. Schematic diagram of the CU-HROS Concept. Light from TMT enters the enclosure and is reimaged onto a 0.1" pseudo-slit via a fiber optic IFU. The spectrum is sorted into 32 spectral bins via the dichroic mirror array, feeding 32 narrow band, first order spectrographs with near constant resolution and efficiency. of mechanisms (all in the foreoptics), covering the wavelength range of 310 – 1100 nm at a spectral resolution or R=100,000 in a single integration. Light from the telescope is directed to the Nasmyth platform and into the CU-HROS foreoptics. These include the ADC, derotator, stops, and baffles. A fast steering mirror will be used to increase image stability, and the light will then pass to the focal plane reimaging optics to reform and possibly magnify the focal plane onto the fiber-optic integral field unit (FIFU) array. Foreoptics also include a camera for acquisition, an absorption cell for wavelength calibration, and reserve space for AO pickoff optics. After being directed into the FIFU, the image is dissected in a coherent manner, and the 1"x5" (2x10mm) entrance slit is reformatted to multiple 0.066 x 13 mm pseudo-slits. The FIFU includes additional fibers dedicated to wavelength calibration sources. Light exiting the FIFU is collimated and then passes through 5 banks of dichroic mirrors, subdividing the spectrum into 32 channels of roughly equal spectral grasp. Each channel then feeds a single spectrograph bench operating in first order and optimized for its own narrow wavelength band. A possible physical layout for the dichroic tree and spectrograph feed points is shown in Figure 2. This arrangement uses a single additional fold mirror to place the blue channels directly below the red channels to increase packing efficiency. 3. DESIGN DETAILS 3.1. Instrument location and enclosure The high resolution optical spectrograph for TMT will reside on the Nasmyth platform. This simplifies mechanical design since the gravity vector will remain constant. Image roll will be accommodated by a derotator, as discussed in section 3.2. The stability requirements imposed in Table 1 demand that the instrument be placed in a thermally stable (better than ±1°C) and acoustically isolated environment. Our baseline design accomplishes this by placing the instrument in a vacuum chamber, as indicated in Figure 1. We recognize that the vacuum chamber represents a significant cost, weight and operational impact. If subsequent analysis shows that this chamber is not required, then we will modify our design accordingly. Our design philosophy throughout this project has been to baseline an instrument that we are certain will achieve the design requirements, and to look for cost savings as the program evolves and resources are available for more detailed analysis. The arrangement shown in Figure 2 will compress the instrument footprint to 5x3x3 meters. With vacuum chamber, outer enclosure and clean tent, the footprint on the Nasmyth platform should not exceed 9x10x4.5 meters. Figure 2: Possible layout of the dichroic array. Light enters from the upper left, and either passes through to the upper red arm, or is reflected back and folded into the lower blue arm. The feed points for the individual spectrograph benches are indicated by spheres. Overall size is roughly 5x3x3 meters. 3.2. Foreoptics and focal plane CU-HROS will use a series of foreoptics to derotate the image, correct for atmospheric dispersion, allow target guiding and acquisition, change beam speed to optimize for the fiber optic feed, and allow for an adaptive optics feed to correct the seeing. Light from the telescope enters the foreoptics (ADC and derotator), and passes any pickoff mirrors or beam splitters required for guidance or AO wave front sensor (WFS) feed (see Figure 2, Froning, et al.). The focal plane is reimaged onto the fiber optic integral field unit which reformats the 1" point source field of regard into a long, narrow pseudo-slit: in terms of the 2mm/arc second plate scale, the pseudo-slit is approximately 0.1" wide by 10" long. The fiber-optic integral field unit CU-HROS relies on reformatting the 1" entrance slit to a pseudo-slit with an effective width of less than 0.1". The current design implements this image dissection through the use of a fiber-optic integral field unit (FIFU).3,4 The design could be achieved with a conventional image slicer, but at the cost of reduced performance. The narrow pseudoslit allows us to reduce the width of the entrance slit to no more than 0.10" equivalent plate scale in order to reduce the size of our optics. Also, since the image width is now fixed at output width of the FIFU, spectrograph performance is not limited by telescope seeing or AO performance. If the TMT delivers a 1.0" PSF to the focal plane rather than a 0.4" PSF, then the light will be spread across more fibers, diminishing signal to noise per pixel, but the resolution will not be impacted. Similarly, if a tight PSF is not stable on the focal plane, this will appear as a variable distribution of light in the cross dispersion direction at the spectrograph focal plane, but will not change the line width. The FIFU dissects the image plane and remaps a 2x2mm (1"x1") entrance aperture to roughly 13.7 by 0.066 mm. We accomplish this by placing an array of 91 0.066 mm core fibers behind a microlens array at the focal plane on 0.2mm centers, and remapping that to a vertical array one fiber wide (Figure 3). We are opting to use a hexagonal entrance aperture geometry for two reasons: This allows us to maximize the packing efficiency while minimizing geometrical focal ratio degradation4, and this geometry makes better use of focal plane real estate given the likely PSF from the telescope. Figure 3. (a) Illustration of one possible remapping of the fiber optic IFU input to the pseudo-slit output. Tessellated hexagonal microlenses (used to minimize geometrical FRD) are used to concentrate light onto the individual fibers, which are then remapped into a long, narrow pseudo-slit. Various remapping schemes, such as the spiral mapping shown here, and various raster mappings will be considered prior to implementation. (b) Distribution of light at the pseudo-slit for a 0.5 arc second Gaussian PSF. Spiral image dissection concentrates light towards the center of the pseudo-slit. While the default design for HROS called for a rectangular entrance aperture for a point source spectrograph, we will build an array of several one square arc second hexagonal fiber bundles. In order to take advantage of improved performance when the seeing disk decreases, we are considering using a methodology similar to that described in Kenworthy et al. (2001), where the microlens to pseudo-slit mapping spirals outwards from the center of the microlens array. 5 This would concentrate light towards the center of the output array, would reduce the number of illuminated pixels at the detector, and would allow on-chip binning at the detector to reduce detector read noise. This and other mapping schemes will be evaluated prior to implementation. Note that the individual fibers are resolved in cross dispersion at the detector. Rather than a fixed contiguous 1"x5" slit, we would provide an array of at least 5 one square arc second IFUs in a nominally fixed grid. This would allow for sampling multiple points on an extended object, and for a well separated sky channel. Alternatively, the central IFU could be larger, perhaps 2 or 3 square arc seconds, with fewer sky channels in a fixed array around that. In addition to decoupling CU-HROS performance from telescope seeing, the FIFU allows us to insert dedicated calibration fibers into the IFU and inject the calibration signal directly into the spectrograph while science spectra are being recorded. For example if two fibers at the top and bottom of the pseudo-slit were dedicated to calibration, then the signal from these would be spatially separated from the science signal, but could be correlated to the science signal with very high repeatability since they are being emitted from the same physical structure. Finally, the output geometry of the FIFU can be tailored to the needs of the collimating optics. It is important to note that since the instrument will be designed with the FIFU in mind. As such the collimator and camera optics will be optimized to accommodate the fast beam speed and the additional vertical extent of the pseudo-slit. This will allow us to minimize or eliminate vignetting and resolution degradation that might otherwise occur if such a pseudo-slit were introduced to an optical system originally designed to accommodate a compact, slow source. Wavelength calibration systems We intend to implement at least two, and preferably three, calibration systems: These include an iodine absorption cell, a thorium/argon lamp, and a laser frequency comb. As mentioned above, one of the advantages of the FIFU is that it allows us to inject light from an external source such as a discharge lamp or a frequency comb directly into the FIFU, making it possible to record calibration spectra during an observation. This provides real-time wavelength knowledge and a direct measure of any drift or blurring of the spectra since any motion in the science spectra would also be present in the calibration spectra. Today, iodine absorption cells are standard wavelength references for high precision velocity observations (for planet searches), and have provided reliable and cross-system replicatable absorption spectra, but at the expense of depressing throughput of the target continuum by 50%. Furthermore, an iodine cell will not provide absorption features across the full instrument band pass, leaving some channels uncalibrated. However, a deployable iodine absorption cell is included in the CU-HROS concept to maintain continuity with current observing programs and to act as a calibration source for certain observations. CU-HROS will also include either a Th/Ar lamp or a suite of emission line lamps to provide emission line calibration spectra across the instrument band pass. The light from these lamps can be injected directly into dedicated fibers in the FIFU to provide simultaneous science and wavelength calibration, or fed into the science beam via a beam splitter to provide knowledge of line center across the pseudo-slit. The first option would allow continuous injection of the wavecal spectrum without contaminating the science spectrum, while the second would provide direct measure of any wavelength offset as a function of position across the IFU output (this could also be obtained from the absorption cell). Th/Ar and He/Ne/Th lamps are dim but well characterized, and provide Figure 4: Consecutive pulses emitted by a mode-locked laser generate a uniformly spaced series of emission lines (“teeth”). Figure based on Udem, et al (2002).6 rich spectra in the region of interest. For example, Th/Ar lamps provide over 4000 stable, identified lines between 310 and 1100 nm, and have provided excellent reference spectra for many instruments. In addition to emission line spectra, recent work in at the National Institute for Standard Technology (NIST) in Boulder, Colorado, has produced laser frequency combs with the potential for use as extremely stable, well characterized line sources.6,7 A frequency comb produces emission lines (‘teeth’) at a regular frequency spacing: ω n = nωγ + ωo, where ωγ is the pulse frequency of the stimulation laser, and the offset frequency ωo can be phase locked to a highly stable atomic source (Figure 4). Current work has generated line spacing as wide as 2GHz (approximately 250,000 λ/Δλ). This line spacing can be increased by using an etalon to filter the unwanted lines, reducing the number of lines by an order of magnitude.8 The advantage of this light source is that it produces regularly spaced, extremely stable emission lines across the instrument band pass: the driving frequency and offset frequency can be phase locked to an off the shelf atomic frequency source, yielding wavelength accuracies as great as 1 part in 1010 when coupled to a rubidium microwave transition source. Even though the output intensity varies over several orders of magnitude, the output is very intense relative to our requirements, so selective filtering could be used to normalize the beam. 3.3. Collimator and Dichroic Tree Collimating optics We will use an all-reflective collimator based on a three-mirror anastigmat to provide an achromatic collimated beam without residual astigmatism or coma (Figure 5).9 A three-mirror anastigmat provides control over coma, spherical aberration, and astigmatism with an arbitrary field curvature. In addition, to preserve the post-grating camera performance, the collimator cannot leave FIFU output and collimator optics Camera optics and spectrograph focal plane Figure 5. The Collimator and all-up system. View from top demonstrating the location of the exit pupil of the collimator, chosen to minimize the size of the dichroics. The large size of the camera is an artifact of the rendering which includes the full size. See Figure 7 for side view. aberration in the beam. The design uses off-axis elements and is designed to accommodate the extended field and fast beam introduced by the fiber bundle in order to eliminate vignetting in the collimator or dichroic array. With the freedom to place the fiber ends at arbitrary positions, a curved input plane is easily created, reducing the constraints of the collimator. We expect the optics to be ~1 meter class. With advances in mirror polishing technology this does not represent a significant increase in cost, and reflective optics remain a cost savings relative to refractive optics. In addition, the use of reflective surfaces instead of refractive optics will minimize chromatic aberrations throughout the system. Dichroic Tree Dichroic mirrors provide the primary wavelength selection for the CU-HROS instrument, and the performance of the instrument is dependent on these mirrors operating at high efficiency. Dichroic mirror technology has undergone substantial advances in the last several years, yielding mirrors with sharp transitions (>5nm from 90% rejection to 90% acceptance) and high efficiency (typically >95% transmission/reflectance). Measured performance of dichroic mirrors already produced by Barr Associates support this contention (Figure 6-a) and exceed our specification of transmission/reflection 95%, out of band transmission/reflection 5%, and 90% to 10% transition edge in less than 5nm. The ability of Barr Associates to deliver optics with the performance shown in Figure 6-a reinforces our confidence in their ability to meet our performance requirements. Note that the fluctuations in transmission/reflection shown in the curve also appear in the theoretical data for the individual HROS dichroic mirrors; however, the locations of the dips in performance are uncorrelated, and tend to average out after 5 filters (Figure 6-b). The dichroic mirrors will have a 200 mm clear aperture, although this may grow if doing so relaxes design constraints on the collimator and camera optics, and be optimized 15° angle of incidence in order to minimize polarization effects. 3.4. Spectrograph Benches Diffraction Gratings Holographic gratings are the preferred option for CU-HROS. Holographic gratings are produced by the interference pattern of two stigmatic (laser) source point sources on a photo-sensitive Figure 6. (a) Measured performance for an existing 125x125 mm dichroic mirror; data provided by Barr Associates. While this optic is designed for longer wavelengths than we will be operating at, the manufacturer considers optics designed for the visible to be less challenging to fabricate. Note the high performance and low ripple show in this figure. Similar performance had been achieved with the filters for the HST Wide Field 3 instrument. (b) Performance of dichroic tree for each channel based on theoretical data provided by Barr Associates with modeled performance reduced to match specification.10 The dip in performance at short wavelengths is due to structure in the first dichroic – since this optic must provide high efficiency across the greatest band width, it is the most difficult to optimize. Overall performance for CU-HROS is discussed in section 4.2. material. The photo- sensitive material is developed and a series of smooth grooves are produced on the surface of the substrate. Due to the smooth nature of the grooves, holographic gratings have very low scatter (measured at better than 10-5 – 10-6/Å at 122nm for the Cosmic Origins Spectrograph gratings).11 Our preferred option is to request a pseudo-sinusoidal groove profile by adjusting the exposure time to produce deeper grooves. This produces high efficiency devices (~65%), although over a relatively narrow bandpass (~5%). With the design for CU-HROS, each channel represents a 3% bandpass and very high groove efficiency can be achieved at all wavelengths. Camera Optics CU-HROS requires fast (f/1.8) cameras with wide fields (14 degrees) and excellent aberration control (spot FWHM = 15 μm). Preliminary investigations imply that several extant solutions meet our needs. In discussions with Zygo Corp and Goodrich Space Optical Branch, both vendors felt that appropriate, costeffective solutions exist and could be produced. A significant advantage to the CU-HROS spectrograph cameras is the narrow wavelength range required for each camera, simplifying design by removing the need for achromatism. Our camera design is aided by the significant anamorphism in the beam from the grating to the camera. By compressing the beam in the spectral direction, the final beam is closer to f/5, increasing performance. The trades that must be examined closely in the future are the relative advantages of a single all-reflective system (simplifying overall system engineering) versus the slightly more complicated refractive. Refractive systems do not have the broadband performance to allow one design to be used in all channels; however, perhaps as few as three different configurations will be needed to accommodate the broadband performance of CU-HROS. Figure 7 is an example of an all-reflective camera suitable for our design. The camera is based on a three-mirror anastigmat system that produces <15 μm images without considering tolerancing or manufacturing error. The camera itself uses three aspheric elements, which will add to the cost of manufacture, although recent advances in figuring technology will mitigate this cost. We are considering options to permit the diamond turning of the mirrors, which presents a significant cost savings. The optics are in the 200 mm size range, increasing availability and reducing cost and risk to the program. We also take advantage of the anamorphism introduced by the grating to help control aberrations in the spectral direction to smaller than the spatial on-chip binning size (5 pixels). The current design has less than 4 pixels of distortion from the middle of the detector to the edge, so binning is limited by cosmic ray rates rather than our ability to bin in a straight line. Light from dichroics Diffraction Grating Spectrograph focal plane Figure 7. Example camera design employing a three mirror anastigmat that performs to specification. Note that while the camera optics appear quite large, we will be using small off axis portions of the two largest optics. Detectors The detectors for CU-HROS represent conventional technology in a large format application. Commercial CCD vendors (e2v, Fairchild) have expressed interest in working with us to produce a series of custom CCDs for our application. Our current design would use 12.5 micron pixel, 9K by 3K CCDs, which will fit on a conventional six-inch silicon wafer. The well depth of such CCDs would be on the order of 100,000 electrons for the 12.5 microns pixel pitch. While there is a trade between larger pixels and reduced read noise, the resulting cosmic ray rate must also be considered. At the faint limit for a seeing limited telescope read noise dominates all other noise sources. This is a function of the aperture and will affect all instruments equally: assuming a best-case F/1.0 final beam speed, the spatial scale of a 30-meter telescope is roughly 150 microns per arc second, which for 15 m pixels translates to >10 pixels per resolution element (total area, including cross dispersion), independent of instrument design. 4. PERFORMANCE ESTIMATES 4.1. Spectral resolution Each spectrograph is designed to provide similar resolution, with the band pass for each of the 32 spectrograph benches increasing with wavelength. Since the spectral coverage increases linearly with wavelength, resolution is virtually unchanged from one channel to the next. Each spectrograph channel will have the same band pass in terms of individual resolution elements so that the resolution will be close to constant across the entire wavelength range (ranging from R = 100,000 to 104,000). One resolution element encompasses 2x2 pixels (30x30μm) on the detectors, and the design 50% encircled energy width in the dispersion direction is >20μm for the full optical train (Figure 8). 4.2. Efficiency HROS efficiency is expected to be uniformly high with limited variation between the red and blue ends of the spectrum. Component level efficiency estimates are summarized in Table 2, and system level performance is illustrated in Figure 9. Figure 8. Ray trace from FIFU output to spectrograph focal plane. FFT diffraction enclosed energy in the dispersion direction (half width) is shown for three fiber locations near the central wavelength for channel 16 (547.5-571.4nm). At the red and blue ends of the channel the enclosed half width increases to 10 μm. Inset shows spot diagrams for two lines separated by λ/Δλ=100,000 (585.000nm (top) and 585.006nm (bottom)). Component Level Efficiency Breakdown Refl/Trans per Qty surface Item Description Derotator ADC Reimaging optics Chamber window FIFU Collimator Vignetting Dichroic tree Grating Camera CCD . 3 mirror 4 surfaces 4 mirror wide band AR Durham best effort 3 mirror Due to finite source size (HROS 100) Optimized for narrow band 3 surfaces, tuned AR/Refl Optimized for narrow band 0.98 0.97 0.98 0.97 0.65 0.98 0.95 0.95 0.65 0.98 0.90 Net Efficiency 3 4 4 2 1 3 1 5 1 3 1 Net HROS Performance: 0.941 0.885 0.922 0.941 0.650 0.941 0.950 0.774 0.650 0.941 0.900 0.18 Table 2. Projected CU-HROS efficiency breakdown. High reflectivities, transmissions and grating and detector efficiencies are achievable due to narrow band over which many of the components must be optimized. Wherever possible, vendor estimates have been used. Since the spectrographs can be optimized for a narrow band, channel to channel variation will be dominated by dichroic mirror performance (Figure 6-b), and that is principally controlled by the first dichroic in the array. High efficiencies have been assigned to the detectors, diffraction gratings and camera optics since these can be optimized for the narrow bands of the individual spectrograph benches – ranging from 13nm at the blue end of the instrument band pass to 46 nm at the red end. Somewhat more conservative estimates are made for optics which must perform across the entire 800nm instrument band pass. The dichroic mirror efficiencies do not reflect the vendor estimates, but rather the more conservative specification, so that higher overall performance may be realized. The overall 18% efficiency is modulated by a sharp drop to 10-14% efficiency at the channel boundaries due to the finite width of the dichroic mirror transitions (Figure 6-b). This effect is mitigated by allowing overlap in the spectral coverage of adjacent channels and then coadding the dispersed data. Figure 9. CU-HROS end-to-end efficiency for the entire instrument band pass including coaddition of overlapping spectral regions. 5. CONCLUSION CU-HROS represents a novel alternative design for high resolution spectroscopy on the next generation of extremely large telescopes. The design presented in this paper decouples spectrograph resolution from telescope seeing and AO performance, provides near uniform resolution and efficiency across the instrument band pass. The design requires modestly sized optics and leverages a high degree of replication to reduce cost, complexity and risk, breaking the cost-growth paradigm for the construction and instrumentation of extremely large observatories. The authors gratefully acknowledge the support of the TMT partner institutions. They are the Association of Canadian Universities for Research in Astronomy (ACURA), the Association of Universities for Research in Astronomy (AURA), the California Institute of Technology and the University of California. This work was supported, as well, by the Canada Foundation for Innovation, the Gordon and Betty Moore Foundation, the National Optical Astronomy Observatory, which is operated by AURA under cooperative agreement with the National Science Foundation, the Ontario Ministry of Research and Innovation, and the National Research Council of Canada. _________________________________________ REFERENCES 1. 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