Download High-Resolution Optical Spectrometer (HROS)

High-Resolution Optical Spectrometer (HROS)
Science Objectives
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Mapping variations in physical constants over cosmological timescales
Baryons at the epoch of peak galaxy formation
Abundance of oldest stars in the Milky Way
Chemical evolution in Local Group Galaxies
Diffusion and mass loss in stars
Doppler radial velocity monitoring of exoplanets around low mass stars
Characterization of exoplanetary atmosphere (e.g., oxygen as a bio-marker)
(Figure 1)
Figure 1: Detecting oxygen in exoplanetary atmosphere. Simulated high-resolution
spectrum of the oxygen A-band absorption feature in the atmosphere of a transiting
planet. The feature can be detected in a 3-hour integration by TMT/HROS (adapted
from Webb & Wormleaton 2001)
Top-level Observatory Requirements
Requirement ID
Description
[REQ-1-ORD-4740] Wavelength range
[REQ-1-ORD-4745] Field of view
[REQ-1-ORD-4750] Length of slit
[REQ-1-ORD-4755] Image Quality
[REQ-1-ORD-4760] Spatial Sampling
[REQ-1-ORD-4765] Spectral Resolution
(slit)
[REQ-1-ORD-4770] Spectral Resolution
(imager slicer)
[REQ-1-ORD-4775] Sensitivity
[REQ-1-ORD-4780] Stability
Requirement
0.31-1.0µm (required); 0.3-1.3µm
(goal)
10 arcseconds
5 arcseconds, with this separation
between orders
 0.2 arcsec FWHM at detector
< 0.2 arcsec per pixel
R = 50,000 (1 arcsec slit)
R  90,000
Must maintain 30m aperture
advantage over existing similar
instruments
Long term stability required to
achieve radial velocity measurement
repeatability and accuracy of 1 m/s
over time spans of 10 years.
Description
Feasibility studies were awarded in 2005 to two separate groups with two very different
HROS concepts (Figure 2). These studies were completed and externally reviewed in
March 2006. The University of California – Santa Cruz team led by Steve Vogt has
proposed a classical Moderate- to High-Resolution Spectrometer (“MTHR”) echelle
concept, and the University of Colorado team led by Cynthia Froning has proposed a
multiplexed 1st order spectrograph concept5 (“CU-HROS”).
The MTHR concept builds upon the heritage of the VLT/UVES and Keck/HIRES
spectrographs as it combines the best advantages from both: the dual-white-pupil/dualarm configuration of UVES to limit the sizes of the echelle, cross-disperser and camera
and a HIRES-style camera to allow for a much larger camera size as the spectrometer is
scaled up to match TMT. After passing through an atmospheric dispersion corrector, the
converging f/15 TMT beam is directed to a fold mirror. Two elements reside on this path:
an iodine cell for very precise wavelength calibration and a fused-silica total-internalreflection image de-rotator prism. Another fold mirror may be a fast tip-tilt mirror that
would remove telescope guiding errors caused by windshake or other sources of
vibrations. A pupil is formed downstream from this fold mirror by a CaF/fused silica
doublet located at the output face of the image de-rotator. This pupil location can be
utilized for filters, polarizers, and/or a small pickoff mirror that could send a small
fraction of the light to a wavefront sensor. The pupil is followed by a dichroic
beamsplitter that directs the light to the blue and red arm slits. Each slit has a useable
length of 20. The beam is collimated before reaching an echelle that is 1.0  3.5 m in
size. Based on the largest gratings currently available, the echelle would have to be a 3 
5 mosaic. The beam is cross-dispersed by a second 2  3 mosaic of 305 mm  406 mm
gratings. The camera is a scaled up version of the HIRES catadioptric camera. The
gratings will be supported by a 330mm-thick piece of Zerodur, 3.3 meters long and 1
meter wide. The predicted throughput efficiency of MTHR will exceed 20% (~1.5
HIRES and UVES), and this efficiency coupled with the TMT aperture should lead to a
20-40-fold improvement in relative observing speed over existing spectrographs. The
MTHR concept also includes an interesting lower resolution mode (“MODRES”) in
which a large fiber positioner capable of patrolling the full 20 field of view of TMT
would feed MTHR with the light from up to 667 different targets to deliver spectra with a
resolution of R=2300-11000.
Figure 2: (Left) Schematic diagram of the CU-HROS concept (Right) Top view of
MTHR’s high-resolution mode
The HROS concept from the University of Colorado (“CU-HROS”) uses an array of
high-performance (>95%) dichroic mirrors to direct light into 32 narrow-band firstorder spectrographs that covers the wavelength range 310-1100 nm at R=100,000
in a single integration. Each channel will go through five dichroic reflections.
Although all spectrographs will be identical, gratings and detectors will be
optimized at each wavelength to maximize performance. The CU-HROS fore-optics
include an atmospheric dispersion compensator, an image de-rotator as well as
stops and baffles. An acquisition camera, an absorption cell for wavelength
calibration and pickoff mirrors for an AO system are also part of the fore-optics
subsystem. A Fiber Integral Field Unit (FIFU) with a microlens array (0.1/lenslet)
is used to dissect the output beam from the fore-optics and feed fibers that are then
reformatted to create a 0.1  10 pseudo-slit. Rather than a single, contiguous slit,
CU-HROS will have an array of 5-7 one square arcsec IFUs to allow sampling
multiple points of an extended object and/or to provide well-separated sky
channels. The dichroic tree will need to be shielded from ambient thermal
fluctuations and from acoustical vibrations. The collimator, grating and camera
optics in each channel are greatly simplified by the narrow wavelength range over
which they have to operate.
References
1. Froning, C. et al., “Conceptual design for the High Resolution Optical
Spectrograph on the Thirty-Meter Telescope: a new concept for a groundbased high-resolution optical spectrograph”, 2006, SPIE,
2. Osterman, S. et al., “A high-resolution optical spectrograph for the Thirty
Meter Telescope: design and performance”, 2006, SPIE,