TMT’s wide field optical spectrograph is a multi-object, first-light instrument with broad continuous wavelength coverage (0.310 – 1.0 m) at a moderate spectral resolution of R = 5000. The international WFOS design team has recently completed the downselect of two design approaches: a slicer-based monolithic architecture and a fiber-based modular concept. We present here the end-to-end conceptual design for the fiber-based optical spectrograph. Included are the front-end focal reduction optics for coupling light into the fibers, the spectrograph collimator and camera optics, and the dispersive architecture for each color channel. The highly multiplexed fiber-WFOS presents a unique design challenge in keeping costs for the modular spectrographs low while maintaining performance gains afforded by the TMT, and in particular the TMT plus ground-layer adaptive optics (GLAO). A full performance analysis including predicted spectral resolution and throughput is presented for the design.
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.
The LAMOST completed its first five years of operation in June 2017, and 9 million low resolution spectra are obtained. The spectrographs have been upgraded in 2017, and the resolution can reach up to 7500(with 2/3 slit). In the midresolution mode, the wavelength can cover 495nm-535nm(blue band) and 630nm-680nm(red band). The LAMOST will carry out the middle resolution spectroscopic survey in September 2018, and 3 million middle resolution spectra will be obtained. This paper describes the requirements, optical design and mechanical design of the LAMOST-MRS (the LAMOST middle resolution spectrograph)
The design and performance of a three-channel image and long-slit spectrograph for the new 4-m telescope in China are described. The direct imaging covers a 3 arcmin by 3 arcmin field of view and a large wavelength range 370-1,600 nm, it has two optical channels and one near infrared channel with different filters. The spectrograph with a long slit is to provide two observing modes including the following spectral resolutions: R1000 and R5000. For dispersing optical elements it use volume-phased holographic grisms (VPHG) at each of the spectroscopic modes to simplify the camera system. The low resolution mode (R1000) is provided by consecutive observations with the spectral ranges: 360-860 nm, however it adopts only one VPHG for the first light. The spectral range of medium resolution mode (R5000) is 460- 750nm, it is constrained with the use of a 4k × 4k CCD detector of 15 μm pixel size. Peak efficient in the spectrograph are achieved to be higher than 50% in different resolution mode.
Design a best light-weighting collimator to conform to the requirements of opto-mechanical design. Good surface accuracy is our aim, based on a less mass. The ratio of diameter to thickness, the type, size and thickness of pocket, the thickness of the mirror, the support size and position, the thickness of the wall and so on is concerned. Besides, comparing two kinds material is also discussed. In addition, we consider the situation that the orientation vary in support plane. Use the orthogonal table to analyze these elements, and find the better methods. According to the analysis in ANSYS, the collimator mass can reduce to 103 kg, below 159 kg; the ratio of light-weight can reach 70%; the peak-valley value is below 100 nm, that meets the request of below 200 nm.
High accuracy radial velocity measurement isn’t only one of the most important methods for detecting earth-like
Exoplanets, but also one of the main developing fields of astronomical observation technologies in future. Externally
dispersed interferometry (EDI) generates a kind of particular interference spectrum through combining a fixed-delay
interferometer with a medium-resolution spectrograph. It effectively enhances radial velocity measuring accuracy by
several times. Another further study on multi-delay interferometry was gradually developed after observation success
with only a fixed-delay, and its relative instrumentation makes more impressive performance in near Infrared band.
Multi-delay is capable of giving wider coverage from low to high frequency in Fourier field so that gives a higher
accuracy in radial velocity measurement. To study on this new technology and verify its feasibility at Guo Shoujing
telescope (LAMOST), an experimental instrumentation with single fixed-delay named MESSI has been built and tested
at our lab. Another experimental study on multi-delay spectral interferometry given here is being done as well. Basically,
this multi-delay experimental system is designed in according to the similar instrument named TEDI at Palomar
observatory and the preliminary test result of MESSI. Due to existence of LAMOST spectrograph at lab, a multi-delay
interferometer design actually dominates our work. It’s generally composed of three parts, respectively science optics,
phase-stabilizing optics and delay-calibrating optics. To switch different fixed delays smoothly during observation, the
delay-calibrating optics is possibly useful to get high repeatability during switching motion through polychromatic
interferometry. Although this metrology is based on white light interferometry in theory, it’s different that integrates all
of interference signals independently obtained by different monochromatic light in order to avoid dispersion error caused
by broad band in big optical path difference (OPD).