Measuring Earth's energy budget from space is an essential ingredient for understanding and predicting Earth's climate. We have demonstrated the use of vertically aligned carbon nanotubes (VACNTs) as photon absorbers in broadband radiometers own on the Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) 3U CubeSat. VACNT forests are some of the blackest materials known and have an extremely at spectral response over a wide wavelength range. The radiation measurements are made at both shortwave, solar-reflected wavelengths and in the thermal infrared. RAVAN also includes two gallium phase-change cells that are used to monitor the stability of RAVAN's radiometer sensors. RAVAN was launched November 11, 2016, into a nearly 600-km sun-synchronous orbit and collected data over the course of 20 months, successfully demonstrating its two key technologies. A 3-axis controlled CubeSat bus allows for routine solar and deep-space attitude maneuvers, which are essential for calibrating Earth irradiance measurements. Funded by the NASA Earth Science Technology Office, RAVAN is a pathfinder to demonstrate technologies for the measurement of Earth's radiation budget that have the potential to lower costs and enable new measurement concepts. In this paper we report specifically on the VACNT growth, post-growth modification, and pre-launch testing. We also describe the novel door mechanism that houses the gallium black bodies.
We describe a new apparatus for measuring the spectral irradiance of the Moon at visible wavelengths. Our effort builds upon the United States Geological Survey’s highly successful Robotic Lunar Observatory (ROLO), which determined a precise model for the time-dependent irradiance of the Moon from six years of observations obtained with an imaging telescope equipped with a set of narrow-band filters. The ROLO Irradiance Model allows the Moon to be used as a radiometric reference for tracking changes in the absolute responsivity of near-infrared to visible satellite sensors as a function of time to better than 1 %. The goal of the present effort is to improve the absolute radiometric accuracy of the ROLO model, presently estimated at 5 % - 10 %, to better than 1 %. Our approach, which uses an integrating sphere at the focal plane of a telescope to direct light from the integrated lunar disk into a stable spectrograph, also eliminates the need to interpolate between the 32 visible and near-infrared bands measured by ROLO. The new measurements will allow weather, climate, land-surface, and defense satellites to use the Moon as an absolute calibration reference, potentially reducing the impact of disruptions in continuous long-term climate data records caused by gaps in satellitesensor coverage.
It is standard practice at many telescopes to take a series of flat field images prior to an observation run. Typically the
flat field consists of a screen mounted inside the telescope dome that is uniformly illuminated with a broadband light
source. These flat field images are useful for characterizing the relative response of CCD pixels to light passing through
the telescope optics and filters, but carry limited spectral information and are not calibrated for absolute flux.
We present the results of performing in situ, spectroradiometric calibrations of a 1.2 m telescope at the Fred Lawrence
Whipple Observatory, Mt. Hopkins, AZ. To perform a spectroradiometric calibration, a laser, tunable through the
visible to near infrared, was coupled into an optical fiber and used to illuminate the flat field screen in situ at the
telescope facility. A NIST traceable, calibrated photodiode was mounted on the telescope to measure the spectral flux
reaching the aperture. For a particular filter, images of the screen were then captured for each laser wavelength as the
wavelength was tuned over the filter bandpass. Knowledge of the incident flux then allows the relative responsivity of
each CCD pixel at each wavelength to be calculated.
Earth's atmosphere represents a turbulent, turbid refractive element for every ground-based telescope. We describe the
significantly enhanced and optimized operation of observatories supported by the combination of a lidar and
spectrophotometer that allows accurate, provable measurement of and correction for direction-, wavelength- and timedependent
astronomical extinction. The data provided by this instrument suite enables atmospheric extinction correction
leading to "sub-1%" imaging photometric precision, and attaining the fundamental photon noise limit. In addition, this
facility-class instrument suite provides quantitative atmospheric data over the dome of the sky that allows robust realtime
decision-making about the photometric quality of a night, enabling more efficient queue-based, service, and
observer-determined telescope utilization. With operational certainty, marginal photometric time can be redirected to
other programs, allowing useful data to be acquired. Significantly enhanced utility and efficiency in the operation of
telescopes result in improved benefit-to-cost for ground-based observatories.
We propose that this level of decision-making will make large-area imaging photometric surveys, such as Pan-STARRS
and the future LSST both more effective in terms of photometry and in the use of telescopes generally. The atmospheric
data will indicate when angular or temporal changes in atmospheric transmission could have significant effect across the
rather wide fields-of-view of these telescopes.
We further propose that implementation of this type of instrument suite for direct measurement of Earth's atmosphere
will enable observing programs complementary to those currently requiring space-based observations to achieve the
required measurement precision, such as ground-based versions of the Kepler Survey or the Joint Dark Energy Mission.
Ground-based telescopes supported by lidar and spectrophotometric auxiliary instrumentation can attain space-based
precision for all-sky photometry, with uncertainties dominated by fundamental photon counting statistics. Earth's
atmosphere is a wavelength-, directionally- and time-dependent turbid refractive element for every ground-based
telescope, and is the primary factor limiting photometric measurement precision. To correct accurately for the
transmission of the atmosphere requires direct measurements of the wavelength-dependent transmission in the direction
and at the time that the supported photometric telescope is acquiring its data. While considerable resources have been
devoted to correcting the effects of the atmosphere on angular resolution, the effects on precision photometry have
largely been ignored.
We describe the facility-class lidar that observes the stable stratosphere, and a spectrophotometer that observes NIST
absolutely calibrated standard stars, the combination of which enables fundamentally statistically limited photometric
precision. This inexpensive and replicable instrument suite provides the lidar-determined monochromatic absolute
transmission of Earth's atmosphere at visible and near-infrared wavelengths to 0.25% per airmass and the wavelengthdependent
transparency to less than 1% uncertainty per minute. The atmospheric data are merged to create a metadata
stream that allows throughput corrections from data acquired at the time of the scientific observations to be applied to
broadband and spectrophotometric scientific data. This new technique replaces the classical use of nightly mean
atmospheric extinction coefficients, which invoke a stationary and plane-parallel atmosphere. We demonstrate
application of this instrument suite to stellar photometry, and discuss the enhanced value of routinely provably precise
photometry obtained with existing and future ground-based telescopes.
In this work, development of a fiber-optically coupled, vacuum-compatible, flat plate radiometric source applicable to
the characterization and calibration of remote sensing optical sensors in situ in a thermal vacuum chamber is described.
Results of thermal and radiometric performance of a flat plate illumination source in a temperature-controlled vacuum
chamber operating at liquid nitrogen temperature are presented. Applications, including use with monochromatic tunable
laser sources for the end-to-end system-level testing of large aperture sensors, are briefly discussed.
The feasibility of developing a network of telescopes to monitor the composition of the nighttime atmosphere using
stellar spectrophotometry is explored. Spectral measurements of the extinction of starlight by the atmosphere would
allow, for instance, quantification of aerosol, cloud, water-vapor, and ozone levels over the full range of elevation and
azimuth. These measurements, when combined with data from solar spectrophotometry derived from other instruments,
would provide continuous day/night monitoring of the atmospheric composition from the ground. The foundation for
such an effort would be a set of stable standard stars with known top-of-the-atmosphere spectral irradiances traceable to
international standards based on the SI system of units. Fully automated, reliable, easily maintained and highly costeffective
replicas of the spectrophotometric telescope used to calibrate the standard stars can be deployed worldwide at
sites such as atmospheric and astronomical observatories.