Instruments

Observatory

Instruments

HIGH RESOLUTION SPECTROGRAPHS

HR spectrographs must accomodate 2,084 spectra within a wavelength range from 360 to 900 nm, as delivered by 0.8″ diameter fibers. A three-arm design is considered and the wavelength range is divided into three segments, one per arm.

At wavelengths shorter than 500 nm, the required spectral resolution is R~40,000, in order to be able to study the many (weak) lines at blue wavelengths without being detrimentally affected by line blending. At wavelengths longer than 500 nm, the required spectral resolution is R~20,000, since here the effects of line blending are reduced and many of the lines of interest are relatively strong. Only relatively small “working windows” in wavelength range can be accessed at high resolution (approximately given by lc/30 at R=40,000 and lc/15 at R=20,000, where lc is the central wavelength of the working window).

LOW MODERATE RESOLUTION SPECTROGRAPHS

The LMR spectrographs suite must accommodate 3249 spectra. At low spectral resolution, the required wavelength coverage is 360 – 1300 nm and 1500 – 1800 nm (optical + J, H). A three-arm design is has been chosen for the optical range (360 – 950 nm) given the design space and the detector real-estate, especially considering the number of resolution elements that are required. This means that the wavelength range covered by each VPH grating is sufficiently narrow that good efficiency is obtained over the range of each. At NIR wavelengths, the entire YJ wavelength range (950 – 1300nm) can be covered by a single detector, given the fiber size (1˝ = 106.7 µm) and a sufficiently fast (f/1.2) NIR camera (the largest NIR detectors available are Hawaii 4RG15, 61mm x 61mm). This helps minimize costs, since the NIR detector drives a major portion of the cost of the spectrograph system.

At moderate resolution, the wavelength coverage requirement is in the range 360 – 950 nm i.e., it only makes use of the optical arms. As such, all the arms are designed to be switchable between two configurations: the optical arms switch between low and moderate spectral resolution, and the NIR mode switch wavelength range between YJ and H. Given the detector sizes and f-numbers, one LMR unit can accommodate ~550 fibers, so six spectrographs are planned for MSE.

Fiber Positioner System

Sphinx, the Fiber Positioner System for MSE, is an evolution of a piezo-actuated tilting spine technology first designed and implemented in FMOS-Echidna (Subaru) and represents a mature, low risk, and high-performance solution to the MSE positioner requirements.

Sphinx includes two main components: the positioner array, which corresponds to an assembly of actuators with their support structure and electronics located at the top end of the telescope structure; and a fiber metrology system, located in the empty central segment position of the primary mirror.

The positioner array is located at the focal surface of MSE and is made of an arrangement of individual “spines”. These carry the FiTS fibers and are moved into position to collect light from science targets by piezo actuators. Sphinx provides full field coverage for all LMR and HR fibers simultaneously.

Fiber Transmission System

The Fiber Transmission System is the set of fibers that collect the light at the focal surface (when set to their position by the Fiber Positioner System) and deliver it to the LMR and HR spectrographs. Fibers are selected, sized and managed maintain the highest possible throughput for the overall MSE system.

High numerical aperture fibers (NA=0.26 – 0.28) capable of accepting the f/1.926 beam delivered by the telescope optics have been selected to avoid additional optics at the fiber input and incurring associated throughput losses. The fibers have an anti-reflection (AR) coating on output ends as the baseline.

Fibers are routed through the Observatory and over the full range of the telescope motion in two axes and the rotation of the field of view, with an emphasis on minimizing fiber stress during observations. This is to reduce the differential focal ratio degradation and far-field effects as the telescope moves across the sky, as well as to maintain uniform and stable throughput among the thousands of fibers.