The Wide Field Optical Spectrograph (WFOS) is one of the first-light instruments of Thirty Meter Telescope. It is a medium resolution, multi object, wide field optical spectrograph. Since 2005 the conceptual design of the instrument has focused on a slit-mask based, grating exchange design that will be mounted at the Nasmyth focus of TMT. Based on the experience with ESI, MOSFIRE and DEIMOS for Keck we know flexure related image motion will be a major problem with such a spectrograph and a compensation system is required to mitigate these effects.
We have developed a flexure Compensation and Simulation (FCS) tool for TMT-WFOS that provides an interface to accurately simulate the effects of instrument flexure at the WFOS detector plane (e.g image shifts) using perturbation of key optical elements and also derive corrective motions to compensate the image shifts caused by instrument flexure. We are currently using the tool to do mote-carlo simulations to validate the optical design of a slit-mask concept we call Xchange-WFOS, and to optimize the flexure compensation strategy. We intend to use the tool later in the design process to predict the actual flexure by replacing the randomized inputs with the signed displacement and rotations of each element predicted by global FEA model on the instrument..
The Wide Field Optical Spectrometer (WFOS) is a seeing limited, multi-object spectrograph and first light instrument for the Thirty Meter Telescope (TMT) scheduled for first observations in 2027. The spectrograph will deliver a minimum resolution of R~5,000 over a simultaneous wavelength range of 310 nm to 1,000 nm with a multiplexing goal of between 20 and 700 targets. The WFOS team consisting of partners in China, India, Japan, and the United States has completed a trade study of two competing concepts intended to meet the design requirements derived from the WFOS detailed science case. The first of these design concepts is a traditional slit mask instrument capable of delivering R~1,000 for up to 100 simultaneous targets using 1 x 7 arc second slits, and a novel focal plane slicing method for R~5,000 on up to 20 simultaneous targets can be achieved by reformatting the 1 arc-second wide slits into three 0.3 arc-second slits projected next to each other in the spatial direction. The second concept under consideration is a highly multiplexed fiber based system utilizing a robotic fiber positioning system at the focal plane containing 700 individual collectors, and a cluster of up to 12 replicated spectrographs with a minimum resolution of R~5,000 over the full pass band. Each collecting element will contain a bundle of 19 fibers coupled to micro-lens arrays that allow for contiguous coverage of targets and adaptation of the f/15 telescope beam to f/3.2 for feeding the fiber system. This report describes the baseline WFOS design, provides an overview of the two trade study concepts, and the process used to down-select between the two options. Also included is a risk assessment regarding the known technical challenges in the selected design concept.
An overview of the current status of the science instruments for the Thirty Meter Telescope is presented. Three first-light instruments as well as a science calibration unit for AO-assisted instruments are under development. Developing instrument collaborations that can design and build these challenging instruments remains an area of intense activity. In addition to the instruments themselves, a preliminary design for a facility cryogenic cooling system based on gaseous helium turbine expanders has been completed. This system can deliver a total of 2.4 kilowatts of cooling power at 65K to the instruments with essentially no vibrations. Finally, the process for developing future instruments beyond first light has been extensively discussed and will get under way in early 2017.
The Multi-Object Broadband Imaging Echellette (MOBIE) is the seeing-limited, visible-wavelength imaging multiobject
spectrograph (MOS) planned for first-light use on the Thirty Meter Telescope (TMT). The MOBIE project to
date has been a collaboration lead by UC Observatories (CA), and including the UH Institute for Astronomy (HI), and
the NAOJ (Tokyo, Japan). The current MOBIE optical design provides two color channels, spanning the 310–550nm
and 550-1000nm passbands, and a combination of reflection gratings, prisms, and mirrors to enable direct imaging and
three spectroscopic modes with resolutions (λ/triangle λ) of roughly 1000, 3000, and 8000 in both color channels, across a field of view that ranges from roughly 8x3 arcmin to 3x3 arcmin, depending on resolution mode. The conceptual design phase for the MOBIE instrument has been underway since 2008 and is expected to end in 2015. We report here on developments since 2010, including assembly of the current project team, instrument and camera optical designs,
instrument control systems, atmospheric dispersion corrector, slit-mask exchange systems, collimator, dichroic and fold
optics, dispersing and cross-dispersing optics, refracting cameras, shutters, filter exchange systems, science detector
systems, and instrument structures.
By July 2014, the Automated Planet Finder (APF) at Lick Observatory on Mount Hamilton will have completed its first year of operation. This facility combines a modern 2.4m computer-controlled telescope with a flexible development environment that enables efficient use of the Levy Spectrometer for high cadence observations. The Levy provides both sub-meter per second radial velocity precision and high efficiency, with a peak total system throughput of 24%. The modern telescope combined with efficient spectrometer routinely yields over 100 observations of 40 stars in a single night, each of which has velocity errors of 0.7 to 1.4 meters per second, all with typical seeing of < 1 arc second full-width-half-maximum (FWHM). The whole observing process is automated using a common application programming interface (API) for inter-process communication which allows scripting to be done in a variety of languages (Python, Tcl, bash, csh, etc.) The flexibility and ease-of-use of the common API allowed the science teams to be directly involved in the automation of the observing process, ensuring that the facility met their requirements. Since November 2013, the APF has been routinely conducting autonomous observations without human intervention.
The Ken and Gloria Levy Spectrometer is now operational at a new 2.4 meter telescope on Mt. Hamilton. Together the
spectrometer and telescope comprise the Automated Planet Finder (APF), a radial velocity instrument. A catastrophic
failure occurred during transit as the instrument was being shipped to the observatory. Several struts buckled in the space
frame that supported the echelle grating. This event has caused UCO/Lick to re-evaluate design methodology and how
engineering safety factors apply to this type of structure. This paper describes the shipping container design, events
during shipment, the failure mechanism, testing and analysis of a remedy, and its implementation. We also suggest
design changes to prevent similar failures in the future.
We report on the on-going effort at University of California Observatories Astronomical Coatings Lab to develop robust
protected-silver coatings suitable for telescope mirrors. We have identified a very promising recipe based on YF3 that
produces excellent reflectivity at wavelengths of 340 nm and greater, has ~1.5% emissivity in the thermal IR, and does
not contain problematic materials for the Mid-IR, such as SiO2 and Al2O3. The recipe holds up extremely well to
aggressive environmental testing (80C and 80% RH; high-H2S atmosphere), and currently is being evaluated under real
observatory conditions. This coating may satisfy the need for telescope mirror coatings that are long-lasting (~5 years or
more) and have good reflectivity into the UV. We also evaluate and compare some other silver-based coatings developed
elsewhere that should be useful in the same role.
In addition, we describe recent upgrades to our coating facilities allowing us to deposit ion-assisted e-beam coatings on
optics up to ~1m. This novel arrangement places the e-gun and ion source on a pivoting "swing-arm", allowing the
position to move radially without changing the e-gun/ion source/ substrate geometry. Large substrates can be coated with
good uniformity using single-axis rotation only. This technique is scalable to arbitrarily large substrate sizes.
The Ken and Gloria Levy Spectrometer is being constructed at the Instrument Development Laboratory (Technical
Facilities) of UCO/ Lick Observatory for use on the 2.4 meter Automated Planet Finder Telescope at Mt. Hamilton. The
mechanical design of the instrument has been optimized for precision Doppler measurements. A key component of the
design is the space-frame structure that contains passive thermal compensation. Determinate hexapod structures are used
to mount the collimator, prism, and echelle grating. In this paper we describe the instrument mechanical design and some
features that will help it detect rocky planets in the habitable zone.
Within the general astronomical community as well as at the University of California Observatories, there has been a
long history of using epoxy to mount optics within instruments such as spectrometers and telescopes. The Ken & Gloria
Levy Spectrometer, part of the Automated Planet Finder (APF) telescope located at Mt. Hamilton's Lick Observatory,
relies on epoxy-bonded joints to attach the instrument's large cross-dispersing prism and echelle grating to its Invar
space-frame structure. Design constraints dictated that these large optics each be attached at only three points, and that
the bond areas be as small as possible while maintaining an adequate strength factor of safety. Previous UCO
instruments, such as the Keck Telescopes' primary mirror segments and the ESI Spectrometer, used Hysol's 9313 epoxy
product for this purpose. Concerns over long-term reliability of such joints led us to re-examine this issue. We
empirically investigated the roles played by epoxy selection and techniques such as surface preparation and the use of a
primer, in creating a robust metal-to-glass bond. Bond strength data was generated, leading us to select a previously
unused epoxy, and to implement particular techniques to ensure bond quality. Most notably, we found that bond strength
data as typically reported on adhesive manufacturers' datasheets was not a reliable indicator of long-term bond reliability
between metal and optical glass.
Two recent Keck optical imaging spectrographs have been designed with
active flexure compensation systems (FCS). These two instruments utilize different methods for implementing flexure compensation.
The Echellette Spectrograph and Imager (ESI), commissioned at the Cassegrain focus of the Keck II Telescope in late 1999, employs an open-loop control strategy. It utilizes a mathematical model of gravitationally-induced flexure to periodically compute flexure corrections as a function of telescope position. Those
corrections are then automatically applied to a tip/tilt collimator
to stabilize the image on the detector.
The DEep Imaging Multi-Object Spectrograph (DEIMOS), commissioned at the Nasmyth focus of Keck II in June 2002, implements a closed-loop control strategy. It utilizes a set of fiber-fed FCS light sources at the ends of the slitmask to produce a corresponding set of spots on a pair of FCS CCD detectors located on either side of the science CCD mosaic. During science exposures, the FCS detectors are read out
several times per minute to measure any translational motion of the
FCS spot images. Correction signals derived from these FCS images
are used to drive active optical mechanisms which steer the spots back to their nominal positions, thus stabilizing the FCS spot images as well as those on the science mosaic.
We compare the design, calibration, and operation of these two systems on the telescope. Long-term performance results will be provided for the ESI FCS, and preliminary results will be provided for the DEIMOS FCS.
All Cassegrain spectrographs suffer from gravitationally- induced flexure to some degree. While such flexure can be minimized via careful attention to mechanical design and fabrication, further performance improvements can be achieved if the spectrograph has been designed to minimize hysteresis and has active compensation for any residual flexure. The Echellette Spectrograph and Imager (ESI), built at UCO/Lick Observatory for use at Cassegrain focus on Keck II, compensates for such residual flexure via its collimator mirror. The collimator is driven by three actuators that provide control of collimator focus, tip, and tilt. The ESI control software utilizes a mathematical model of gravitationally-induced flexure to periodically compute and apply flexure corrections by commanding the corresponding tip and tilt motions to the collimator. In addition, the ESI control software provides an optional, manual, closed-loop method for adjusting the collimator position to compensate for any non-repeatable errors. Such errors may result from mechanical hysteresis or from thermally-induced structural deformations of the instrument and are thus not accounted for by the gravitational flexure model. This method relies on measuring the centroid position of fiducial spots within each echellete image. The collimator is adjusted so that the positions of these spots match those in a reference image. These spots are produced by a small round hole in the slit mask located near one end of the slit. We discuss the design and calibration of this flexure compensation system and report on its performance ont he telescope.
The Echellete Spectrograph and Imager (ESI), currently being completed for use at the cassegrain focus of the Keck II telescope, employs two moderate size translating fold mirrors. These mirrors are used to shift between the three instrument modes; medium resolution echellete mode; low resolution prismatic mode; and imaging mode. In order to maintain the optical stability and calibration of these three modes the mirrors must be removed and repeatably located to within 1.3 arcsecs of tip and tilt. In addition, the mirrors must maintain a fixed orientation relative to the telescope axis under a variety of gravity and thermal loads. In this paper we describe a novel concept for moving and locating these mirrors. Analytical analysis of the mounts is presented. Optical and mechanical testing is described.
The Echellette Spectrograph and Imager (ESI) is being built at UCO/Lick Observatory for the Cassegrain focus of the Keck II telescope. The collimator mirror is optimally constrained by a space-frame structure. It will be actively moved to provide the focus and flexure (tip and tilt) control for the instrument. Careful attention to space-frame geometry has simplified the mechanical design. Analytical and Finite Element Analysis (FEA) are presented to demonstrate how a simple but very stiff structure is used to provide support, flexure control, and focus.
The Echellette Spectrograph and Imager (ESI), currently being developed for use at the Cassegrain focus of the Keck II 10-m telescope, employs two large (25 kg) prisms for cross dispersion. In order to maintain optical stability in the spectroscopic modes, these prisms must maintain a fixed angle relative to the nominal spectrograph optical axis under a variety of flexural and thermal loads. In this paper, we describe a novel concept for mounting large prisms that has been developed to address this issue. Analytical and finite element analyses (FEA) of the mounts are presented. Optical and mechanical tests are also described.
We describe the conversion of an existing f/8 Cassegrain spectrograph to a floor-mounted spectrograph fed by 94 fibers from the f/5 prime focus of the Shane 3-meter telescope at Lick Observatory. The spectrography forms part of the automated Multi- Object Spectrograph system developed as a collaboration between UCO/Lick Observatory and the Lawrence Livermore National Laboratory. Fibers from a robotic fiber-positioner at prime focus degrade the f/5.5 beam from the telescope (after it has passed through a wide-field prime focus corrector) into roughly a f/4.5 beam. If the 4/8 spectrograph were fed directly with this f/4.5 beam approximately 68% of the light would be lost. A simple optical system has been designed that converts the light from the fibers into the f/ratio expected by the spectrograph. The conversion optics are mounted at the entrance to the spectrograph. We describe focal ratio degradation tests of a variety of optical fibers and the design of the `pseudoslit' which mounts the fibers in a line at the input to the conversion optics.