The laser guide star facility for the Thirty Meter Telescope

The Thirty Meter Telescope (TMT) will utilize adaptive optics to achieve near diffraction-limited images in the near-infrared using both natural and laser guide stars. The Laser Guide Star Facility (LGSF) will project up to eight Na laser beacons to generate guide stars in the Earth's Na layer at 90 - 110 km altitude. The LGSF will generate at least four distinct laser guide star patterns (asterisms) of different geometry and angular diameter to meet the requirements of the specific adaptive optics modules for the TMT instruments. We describe the baseline concept for this facility, which draws on the heritage from the systems being installed at the Gemini telescopes. Major subsystems include the laser itself and its enclosure, the optics for transferring the laser beams up the telescope structure and the asterism generator and launch telescope, both mounted behind the TMT secondary mirror. We also discuss operational issues, particularly the required safety interlocks, and potential future upgrades to higher laser powers and precompensation of the projected laser beacons using an uplink adaptive optics system

Scalable background-limited polarization-sensitive detectors for mm-wave applications

We report on the status and development of polarization-sensitive detectors for millimeter-wave applications. The detectors are fabricated on single-crystal silicon, which functions as a low-loss dielectric substrate for the microwave circuitry as well as the supporting membrane for the Transition-Edge Sensor (TES) bolometers. The orthomode transducer (OMT) is realized as a symmetric structure and on-chip filters are employed to define the detection bandwidth. A hybridized integrated enclosure reduces the high-frequency THz mode set that can couple to the TES bolometers. An implementation of the detector architecture at Q-band achieves 90% efficiency in each polarization. The design is scalable in both frequency coverage, 30-300 GHz, and in number of detectors with uniform characteristics. Hence, the detectors are desirable for ground-based or space-borne instruments that require large arrays of efficient background-limited cryogenic detectors.Comment: 7 pages, 3 figures, Presented at SPIE Astronomical Telescopes and Instrumentation 2014: Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII. To be published in Proceedings of SPIE Volume 915

Update on the TMT laser guide star facility design

The Thirty Meter Telescope (TMT) will implement a Laser Guide Star Facility (LGSF), which will generate up to nine Na laser beams in at least four distinct asterisms. The TMT LGSF conceptual design is based upon three 50W solid state, continuous wave, sum frequency 589 nm lasers and conventional beam transport optics. In this paper, we provide an update to the TMT LGSF conceptual design. The LGSF top end and the beam transfer optics have been significantly redesigned to compensate for the TMT telescope top end flexure, to adapt for the new TMT Ritchey-Chretien optical design, to reduce the number of optical surfaces and to reduce the mass and volume. Finally, the laser service enclosure has been relocated within the telescope azimuth structure. This will permit the lasers to operate with a fixed gravity vector, but also requires further changes in the beam transport optical path

Aerogel Scattering Filters for Cosmic Microwave Background Observations

We present the design and performance of broadband and tunable infrared-blocking filters for millimeter and sub-millimeter astronomy composed of small scattering particles embedded in an aerogel substrate. The ultra-low-density (< 100 mg/cu cm) aerogel substrate provides an index of refraction as low as 1.05, removing the need for anti-reflection coatings and allowing for broadband operation from DC to above 1 THz. The size distribution of the scattering particles can be tuned to provide a variable cutoff frequency. Aerogel filters with embedded high-resistivity silicon powder are being produced at 40-cm diameter to enable large-aperture cryogenic receivers for cosmic microwave background polarimeters, which require large arrays of sub-Kelvin detectors in their search for the signature of an inflationary gravitational-wave background

Silicon-Based Antenna-Coupled Polarization-Sensitive Millimeter-Wave Bolometer Arrays for Cosmic Microwave Background Instruments

We describe feedhorn-coupled polarization-sensitive detector arrays that utilize monocrystalline silicon as the dielectric substrate material. Monocrystalline silicon has a low-loss tangent and repeatable dielectric constant, characteristics that are critical for realizing efficient and uniform superconducting microwave circuits. An additional advantage of this material is its low specific heat. In a detector pixel, two Transition-Edge Sensor (TES) bolometers are antenna-coupled to in-band radiation via a symmetric planar orthomode transducer (OMT). Each orthogonal linear polarization is coupled to a separate superconducting microstrip transmission line circuit. On-chip filtering is employed to both reject out-of-band radiation from the upper band edge to the gap frequency of the niobium superconductor, and to flexibly define the bandwidth for each TES to meet the requirements of the application. The microwave circuit is compatible with multi-chroic operation. Metalized silicon platelets are used to define the backshort for the waveguide probes. This micro-machined structure is also used to mitigate the coupling of out-of-band radiation to the microwave circuit. At 40 GHz, the detectors have a measured efficiency of 90%. In this paper, we describe the development of the 90 GHz detector arrays that will be demonstrated using the Cosmology Large Angular Scale Surveyor (CLASS) ground-based telescope