We present details of the recent trade study on design changes to the Wide Field Optical Spectrometer (WFOS) for the Thirty Meter Telescope (TMT)[1]. WFOS is planned as a first light instrument and will provide highly efficient imaging and multi-slit spectroscopy over the wavelength range 0.31 to 1µm across a field of view of 8.3 by 3 arcminutes. The existing baseline prior to the trade study used a laser cut metal slit mask at the focal plane to enable observation of ~50 to 80 objects simultaneously. The masks would be cut in advance of observing and installed in a cassette, allowing a mechanism to select the mask and move it into place at the focal plane. Each multi-object observation requires a dedicated mask, with a more general single long slit mask remaining in the cassette permanently. The configurable slit unit (CSU) is an alternative approach, and a design that has previously been used in MOSFIRE and FORS. A CSU uses multiple knife edges mounted on computer-controlled bars to create and position slits at the focal plane. In the case of WFOS the CSU will be capable of creating 96 separate slits with the ability to reconfigure them on the fly to adapt to seeing conditions or to respond to targets of opportunity. We detail here the decision criteria, design, and science case analysis used by the WFOS team to decide to change the baseline design of WFOS to incorporate a CSU.
The Thirty Meter Telescope (TMT) will host three science instruments at first light: IRIS (InfraRed Imaging Spectrograph), WFOS (Wide Field Optical Spectrograph), and MODHIS (Multi-Objective Diffraction-limited Highresolution Infrared Spectrograph). IRIS is a workhorse imager and spectrograph coupled to the Narrow-Field InfraRed Adaptive Optics System (NFIRAOS) to exploit the gains possible when working at the diffraction limit on an extremely large telescope. It has an imager field of view of 34 by 34 square arcseconds, and the integral field spectrograph supports a variety of spaxel scales and fields of view at resolutions between 4000 and 10,000. MODHIS, also working behind NFIRAOS, is focused on exoplanet science will deliver precision radial velocity measurements at a spectral resolution of 100,000 for a single object. WFOS is the workhorse optical multi-object imaging spectrograph. It has an 8 by 3 square arcminute field of view and is capable of targeting almost 100 objects at once with resolutions between 1500 and 5000.
We present the conceptual design of the integral field unit (IFU) for Wide Field Optical Spectrograph (WFOS), one of the first-generation instruments on TMT. The IFU is a promising upgrade path of WFOS. The IFU has 4 image slicers with different slice widths of 1.5, 0.75, 0.5 and 0.25 arcsec. The slice length and the number of slices are 20 arcsec and 18 in all slicers. These slicers offer the field sizes of 27, 13.5, 9 and 4.5 × 20 arcsec2, respectively. This field variation covers sizes of galaxy, circum-galactic medium and inter-galactic medium. In the 0.25-arcsec width mode, the spectral resolution reaches R=13,635 without slit loss. Multilayer dielectric reflective coating with high reflectivity (> 98% at any wavelength) is made on all reflective surfaces, which offers high through put of the IFU (> 80%).
We present the preliminary design for the configurable slit unit (CSU) for TMT’s Wide Field Optical Spectrometer (WFOS). The design consists of 96 bar pairs that can create an arbitrary pattern of focal plane slits. The large number of motorized mechanisms to drive the bars into position requires a high reliance on off the shelf components to reduce cost and design effort. A prototype was completed that shows the selected components will likely meet requirements. The current design nearing completion as WFOS ramps into a preliminary design review in 2025.
We present the current design of WFOS, a wide-field UV/optical (0.31-1.0 µm) imaging spectrograph planned for first-light on the TMT International Observatory 30 m telescope. WFOS is optimized for high sensitivity across the entire optical waveband for low-to-moderate resolution (R ∼ 1500-5000) long-slit and multi-slit spectroscopy of very faint targets over a contiguous field of view of 8′ .3×3 ′ .0 at the f/15 Nasmyth focus of TMT. A key design goal for WFOS is stability and repeatability in all observing modes, made possible by its gravity-invariant opto-mechanical structure, with a vertical rotation axis and all reconfigurable components moving only in planes defined by tiered optical benches parallel to the Nasmyth platform. WFOS’s optics include a linear ADC correcting a 9′ diameter field, including both the science FoV and 4 patrolling acquisition, guiding, and wavefront sensing camera systems; a novel 2-mirror reflective collimator allowing the science FoV to be centered on the telescope optical axis; a dichroic beamsplitter dividing the collimated beam into 2 wavelength-optimized spectrometer channels (blue: 0.31-0.56 µm; red: 0.54-1.04 µm); selectable transmissive dispersers (VPH and/or VBG) with remotely configurable grating tilt (angle of incidence) and camera articulation that enable optimization of diffraction efficiency and wavelength coverage in each channel; all-refractive, wavelength-optimized f/2 spectrograph cameras, and UV/blue and red-optimized detector systems. The predicted instrumental through put of WFOS for spectroscopy averages > 56% over the full 0.31-1 µm range, from the ADC to the detector. When combined with the 30 m TMT aperture, WFOS will realize a factor of ∼20 gain in sensitivity compared to the current state of the art on 8-10 m-class telescopes.
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