No abstract

No abstract

Radiometric calibration of the Dual-Wavelength Echidna® Lidar (DWEL), a full-waveform terrestrial laser scanner with two simultaneously-pulsing infrared lasers at 1064 nm and 1548 nm, provides accurate dual-wavelength apparent reflectance (ρapp), a physically-defined value that is related to the radiative and structural characteristics of scanned targets and independent of range and instrument optics and electronics. The errors of ρapp are 8.1% for 1064 nm and 6.4% for 1548 nm. A sensitivity analysis shows that ρapp error is dominated by range errors at near ranges, but by lidar intensity errors at far ranges. Our semi-empirical model for radiometric calibration combines a generalized logistic function to explicitly model telescopic effects due to defocusing of return signals at near range with a negative exponential function to model the fall-off of return intensity with range. Accurate values of ρapp from the radiometric calibration improve the quantification of vegetation structure, facilitate the comparison and coupling of lidar datasets from different instruments, campaigns or wavelengths and advance the utilization of bi- and multi-spectral information added to 3D scans by novel spectral lidars.

The PICTURE-B sounding rocket mission is designed to directly image the exozodiacal light and debris disk around the Sun-like star Epsilon Eridani. The payload used a 0.5[Formula: see text]m diameter silicon carbide primary mirror and a visible nulling coronagraph which, in conjunction with a fine pointing system capable of 5[Formula: see text]milliarcsecond stability, was designed to image the circumstellar environment around a nearby star in visible light at small angles from the star and at high contrast. Besides contributing an important science result, PICTURE-B matures essential technology for the detection and characterization of visible light from exoplanetary environments for future larger missions currently being imagined. The experiment was launched from the White Sands Missile Range in New Mexico on 2015 November 24 and demonstrated the first space operation of a nulling coronagraph and a deformable mirror. Unfortunately, the experiment did not achieve null, hence did not return science results.

The PICTURE-C mission will fly a 60 cm off-axis unobscured telescope and two high-contrast coronagraphs in successive high-altitude balloon flights with the goal of directly imaging and spectrally characterizing visible scattered light from exozodiacal dust in the interior 1-10 AU of nearby exoplanetary systems. The first flight in 2017 will use a 10´4 visible nulling coronagraph (previously flown on the PICTURE sounding rocket) and the second flight in 2019 will use a 10´7 vector vortex coronagraph. A low-order wavefront corrector (LOWC) will be used in both flights to remove time-varying aberrations from the coronagraph wavefront. The LOWC actuator is a 76-channel high-stroke deformable mirror packaged on top of a tip-tilt stage. This paper will detail the selection of a complementary high-speed, low-order wavefront sensor (LOWFS) for the mission. The relative performance and feasibility of several LOWFS designs will be compared including the Shack-Hartmann, Lyot LOWFS, and the curvature sensor. To test the different sensors, a model of the time-varying wavefront is constructed using measured pointing data and inertial dynamics models to simulate optical alignment perturbations and surface deformation in the balloon environment.