Optical and mechanical design of a telescope for lunar spectral irradiance measurements from a high-altitude aircraft.

We have designed a non-imaging telescope for measurement of the spectral irradiance of the moon. The telescope was designed to be integrated into a wing pod of a National Aeronautics and Space Administration ER-2 research aircraft to measure lunar spectral irradiance during flight. The telescope and support system were successfully flown in August 2018 at altitudes near 21 km and at speeds of ∼760 km/h. The wing pod in which the telescope is mounted has an opening through which the moon can be observed. The mount exposes the telescope to high winds, low pressures, temperatures near -60 °C, and vibrations both due to flight and due to the motion of the aircraft on the ground. This required a telescope design with high thermal stability and high resistance to shock. The optical design of the telescope is optimized to have high throughput and spatially uniform transmission from 380 nm to 1000 nm over a field of view about three times the angular size of the moon as viewed from the Earth. The final design resulted in a telescope with singlet design incorporating a 139.7 mm lens with an effective focal length of 377 mm and a field of view of 1.6°. The light from the telescope is introduced into an integrating sphere, which destroys the image and the polarization for measurement by a fiber-coupled spectroradiometer. Herein, we present an overview of the instrument and support system with emphasis on the telescope design.

Nanokelvin DC and AC Meissner-transition-edge temperature detectors.

Based upon a superconducting transition edge sensor (TES), the Meissner-TES is a relatively new type of high resolution cryogenic thermometer which employs the magnetic transition of a superconductor to measure temperature. We have improved the signal-to-noise for DC sensing by a factor of 30 compared to our prior effort and developed a new AC mode which uses an oscillating magnetic field and a lock-in technique with much lower magnetic noise than the DC mode. The thermometer was tuned in situ over a range of operating temperatures 10-50 times larger than the transition width of the superconductor, using an applied persistent magnetic field. The DC mode can have sensitivity better than 1 nK for 100 s averaging, and the AC mode has sensitivity better than 120 nK for very small applied magnetic fields near 14 nT and 100 s averaging. The Meissner-TES can be applied to high resolution temperature control, high sensitivity infrared sensing, optical power scale realization, and the study of temperature-dependent phase transitions.

Wideband infrared trap detector based upon doped silicon photocurrent devices.

We have designed, fabricated, and measured infrared trap detectors made from arsenic-doped silicon (Si:As) blocked impurity band (BIB) photodetectors. These trap detectors are composed of two detectors in a wedge geometry, with an entrance aperture diameter of either 1 or 3 mm. The detectors were calibrated for quantum efficiency against a pyroelectric reference detector using a Fourier transform spectral comparator system, and etalon effects and spatial uniformity of the traps were also quantified. Measurements of the traps at a temperature of 10 K show that nearly ideal external quantum efficiency (>90%) can be attained over much of the range from 4 to 24 µm, with significant responsivity from 2 to 30 µm. The traps exhibited maximum etalon oscillations of only 2%, which is about 10 times smaller amplitude than those of the single Si:As BIB detectors measured under similar conditions. Spatial nonuniformity across the entrance apertures of the traps was about 1%. The combination of high detectivity, wideband wavelength coverage, spectral flatness, and spatial uniformity make these trap detectors an excellent reference detector for spectrally resolved measurements and radiometric calibrations over the near- to far-infrared wavelength range.