The long-term balance between Earth’s absorption of solar energy and emission of radiation to space is a fundamental climate measurement. Total solar irradiance (TSI) has been measured from space, uninterrupted, for the past 40 years via a series of instruments. The Compact Total Irradiance Monitor (CTIM) is a CubeSat instrument that will demonstrate next-generation technology for monitoring total solar irradiance. It includes novel silicon-substrate room temperature vertically aligned carbon nanotube (VACNT) bolometers. The CTIM, an eight-channel 6U CubeSat instrument, is being built for a target launch date in late 2020. The basic design is similar to the SORCE, TCTE and TSIS Total Irradiance Monitors (TIM). Like TSIS TIM, it will measure the total irradiance of the Sun with an uncertainty of 0.0097% and a stability of <0.001%/year. The underlying technology, including the silicon substrate VACNT bolometers, has been demonstrated at the prototype-level. During 2019 we will build and test an engineering model of the detector subsystem. Following the testing of the engineering detector subsystem, we will build a flight detector unit and integrate it with a 6U CubeSat bus during late 2019 and 2020, in preparation for an on-orbit demonstration in 2021.
The 2007 National Research Council Decadal Survey for Earth Science identified needed measurements to improve understanding of the Earth’s climate system, recommending acquiring Earth spectral radiances with an unprecedented 0.2% absolute radiometric accuracy to track long-term climate change and to improve climate models and predictions. Current space-based imagers have radiometric uncertainties of ~2% or higher limited by the high degradation uncertainties of onboard solar diffusers or calibration lamps or by vicarious ground scenes viewed through the Earth’s atmosphere. The HyperSpectral Imager for Climate Science (HySICS) is a spatial/spectral imaging spectrometer with an emphasis on radiometric accuracy for such long-term climate studies based on Earth-reflected visible and near-infrared radiances. The HySICS’s accuracy is provided by direct views of the Sun, which is more stable and better characterized than traditional flight calibration sources. Two high-altitude balloon flights provided by NASA's Wallops Flight Facility and NASA’s Columbia Scientific Balloon Facility are intended to demonstrate the instrument’s 10× improvement in radiometric accuracy over existing instruments. We present the results of the first of these flights, during which measurements of the Sun, Earth, and lunar crescent were acquired from 37 km altitude. Covering the entire 350-2300 nm spectral region needed for shortwave Earth remote sensing with the HySICS’s single, flight-heritage detector array promises mass, cost, and size advantages for eventual space- and air-borne missions. A 6 nm spectral resolution with a 0.5 km spatial resolution from low Earth orbit helps in determinations of atmospheric composition, land usage, vegetation, and ocean color.
We demonstrate a visible and near-infrared prototype pushbroom hyperspectral imager for Earth climate studies that is
capable of using direct solar viewing for on-orbit cross calibration and degradation tracking. Direct calibration to solar
spectral irradiances allow the Earth-viewing instrument to achieve required climate-driven absolute radiometric
accuracies of <0.2% (1σ). A solar calibration requires viewing scenes having radiances 105 higher than typical Earth
scenes. To facilitate this calibration, the instrument features an attenuation system that uses an optimized combination of
different precision aperture sizes, neutral density filters, and variable integration timing for Earth and solar viewing. The
optical system consists of a three-mirror anastigmat telescope and an Offner spectrometer. The as-built system has a
12.2° cross track field of view with 3 arcmin spatial resolution and covers a 350-1050 nm spectral range with 10 nm
resolution. A polarization compensated configuration using the Offner in an out of plane alignment is demonstrated as a
viable approach to minimizing polarization sensitivity. The mechanical design takes advantage of relaxed tolerances in
the optical design by using rigid, non-adjustable diamond-turned tabs for optical mount locating surfaces. We show that
this approach achieves the required optical performance. A prototype spaceflight unit is also demonstrated to prove the
applicability of these solar cross calibration methods to on-orbit environments. This unit is evaluated for optical
performance prior to and after GEVS shake, thermal vacuum, and lifecycle tests.
The total solar irradiance (TSI) climate data record includes overlapping measurements from 10 spaceborne radiometers.
The continuity of this climate data record is essential for detecting potential long-term solar fluctuations, as offsets
between different instruments generally exceed the stated instrument uncertainties. The risk of loss of continuity in this
nearly 30-year record drives the need for future instruments with <0.01% uncertainty on a absolute scale. No facility
currently exists to calibrate a TSI instrument end-to-end for irradiance at solar power levels to these needed accuracy
levels. The new TSI Radiometer Facility (TRF) is intended to provide such calibrations. Based on a cryogenic
radiometer with a uniform input light source of solar irradiance power levels, the TRF allows direct comparisons
between a TSI instrument and a reference cryogenic radiometer viewing the same light beam in a common vacuum
system. We describe here the details of this facility designed to achieve 0.01% absolute accuracy.
Aperture area knowledge is a primary calibration in radiometric instruments. Corrections for edge effects, particularly
diffraction and scatter, must also be taken into account for high accuracy measurements. The Total Irradiance Monitor
(TIM) is a total solar irradiance radiometer on NASA's SORCE mission launched in 2003 and on the NASA/Glory
mission launching in 2008. In order to measure irradiance, the TIM instrument measures the total optical power that
passes through circular diamond-turned precision apertures. The geometric areas of the 8-mm diameter apertures are
measured to approximately 25 parts per million (ppm) at the National Institute of Standards and Technology . Due to
scatter and diffraction, not all light that passes through the geometric area of an aperture will enter the radiometer cavity
of the instrument, and corrections must be made for these edge effects. Diffraction effects are generally well understood
and are calculated from the instrument geometry. Scatter, on the other hand, is dependent on the microscopic edge
quality of each individual aperture, and so must be measured. This paper describes the measurement of aperture edge
diffraction and scatter for the precision apertures on NASA's Glory/TIM instrument.
The Total Irradiance Monitor (TIM) is a total solar irradiance radiometer on NASA's SORCE mission launched in 2003 and on the NASA/Glory mission launching in 2008. The primary sensors in TIM must absorb energy with accurately calibrated efficiency across the entire solar spectrum. To achieve high efficiency and good thermal conduction, the four sensors in each instrument are hollow conical silver cavities with a cylindrical entrance extension and a diffuse black nickel phosphorous (NiP) interior that converts absorbed incident radiation to thermal energy. A stable resistive heater wire embedded in the cone along with thermistors mounted on the cavity exterior are used in a temperature-sensing servo loop to measure the spectrally-integrated incident solar radiation. Characterization of the absorptance properties of the cavities across the solar spectrum is a dominant driver of instrument accuracy, and a dedicated facility has been developed to acquire these calibrations with uncertainties of approximately 50 ppm (0.005%). This paper describes the absorptance calibration facility, presents the preliminary cavity reflectance results for the Glory mission's TIM instrument, and details the uncertainty budget for measuring these cavity reflectances.
The solar Total Irradiance Monitor (TIM) on NASA's SORCE mission began taking data in early 2003. This instrument continues the 25-year record of space-borne, total solar irradiance (TSI) measurements, with improved precision from its new technologies and calibration methods. We present an overview of the TIM instrument, including the design features enabling its high precision, and we present preliminary on-orbit TSI data.
I describe a liquid crystal intensity modulator designed to achieve <10 parts per million (ppm) modulation to simulate a planetary transit like those required for ground testing of NASA's Kepler mission. The design uses a nematic liquid crystal as a variable retarder aligned between two linear polarizers, with the retardance values and the alignment chosen to provide low sensitivity of transmitted intensity to input liquid crystal voltage variations. Modulator test results give intensity fluctuations of a few ppm from millivolt modulations about the input 8 V baseline voltage.
The Total Irradiance Monitor (TIM), to be launched in 2002 on the NASA Earth Observing System (EOS) SOlar Radiation and Climate Experiment (SORCE), will stare at the Sun for five years, and measure the absolute total solar irradiance (TSI). The TIM is an active cavity radiometer with a relative standard uncertainty 100 ppm and a fractional stability of ? 10 ppm/year. The estimated uncertainties are “type B” determined from the parametric uncertainties in a model of the instrument; and the dominant uncertainty will be in the effective aperture area. To obtain such low uncertainty, we: 1. Use metallic NiP as the cavity (diffuse) black. 2. Retrieve the irradiance in the frequency domain. 3. Use phase sensitive detection. 4. Use four separate, duty-cycled cavities. 5. Measure the aperture transmission integral over area. 6. Use diamond thermal/electrical nodes. 7. Use 400 seconds for each completely independent data point for low noise. 8. Use a pulse-width-modulated “standard digital watt” as the onboard standard. 9. Take advantage of the 1 ppm noise level to discover systematic effects. 10. Measure IR shutter radiation from in-flight measurements of dark space. We compare with other TSI measurements on orbit, and as separate shuttle experiments.
The NASA Earth Observing Systems’ (EOS) SOlar Radiation and Climate Experiment (SORCE) mission consists of four instruments aboard a small satellite to measure the total solar irradiance (TSI) and solar spectral irradiance from 1 to 2000 nm. Solar irradiance, being the dominant energy source in the Earth's atmosphere, establishes much of the atmosphere's chemistry and dynamics. The SORCE measurements will therefore provide the requisite understanding of one of the primary climate system variables for the NASA EOS program. The SORCE primary science data product will be the TSI and solar spectral irradiance on a 6 hour cadence for a period of 5 years or more. The SORCE science team will study how much the solar irradiance varies, how the solar variability affects the Earth’s atmosphere, and how the magnetic structures on the Sun change its irradiance. The SORCE instruments are the Total Irradiance Monitor (TIM), the Spectral Irradiance Monitor (SIM), the SOLar STellar Irradiance Comparison Experiment (SOLSTICE), and the XUV Photometer System (XPS). The TIM is an active cavity radiometer similar in design to previous cavity radiometers, such as the VIRGO, ACRIM, and ERBS instruments, but with significant improvements in sensor and electrical design. TIM will provide a measurement of TSI directly traceable to SI units with an absolute accuracy of 0.01% and relative accuracy of 0.001% per year. The SIM is a Fery prism spectrometer with an Electrical Substitution Radiometer (ESR) as the reference detector and Si and InGaAs photodiodes as the working detectors. SIM will measure the solar spectral irradiance from 200 nm to 2000 nm with a spectral resolution varying from 0.5 nm to 34 nm, an absolute accuracy of 0.03%, and a relative accuracy of 0.006% per year. The SOLSTICE is an improved version of the UARS SOLSTICE instrument, both being ultraviolet (UV) grating spectrometers with photomultiplier tube detectors. SOLSTICE will measure the solar spectral irradiance from 115 nm to 320 nm with a spectral resolution of 0.1-0.2 nm, an absolute accuracy of 5%, and a relative accuracy of 0.5% per year. The XPS is a set of soft x-ray (XUV) photometers, consisting of Si photodiodes with thin-film filters to select moderate spectral bands. XPS will measure the solar spectral irradiance in the XUV (1-31 nm) and at Lyman-? (121.6 nm) with bandwidths of about 5 nm, an absolute accuracy of 20%, and a relative accuracy of 4% per year. Orbital Sciences Corporation is providing the SORCE satellite, a version of their GALEX spacecraft bus tailored for the SORCE mission. The SORCE satellite is a 3-axis stabilized satellite for pointing the instruments towards the Sun for the primary solar measurements as well as for pointing towards stars for the SOLSTICE in-flight calibrations. The SORCE spacecraft is scheduled for a launch on a Pegasus XL in July 2002 into an orbit with a 645 km altitude and 40° inclination.
We describe the features of the optical system for Terrestrial Planet Finder, a space-based, cryogenic interferometer for direct detection of Earth-type planets around nearby stars. Destructive interference in a stellar interferometer suppresses stellar glare by a factor of several thousand or more, and phase chopping distinguishes planet light from symmetric backgrounds. The mid-IR is favorable for detecting planetary emission relative to that from the star, and this spectral region also offers important molecular signatures indicative of key atmospheric gases.
The cholesteric liquid crystal (LC) has a chiral structure defined by pitch length. This unique structure reflects one handedness of circularly polarized light and is used as a circular polarizer. This polarizer can be actively switched between a state that is transparent to light of all polarizations and another state that in only transmissive to one handedness of circularly polarized light. The dual- frequency material is used to reduce the relaxation time. The dual-frequency circular polarizer can be switched from the transparent homeotropic texture by a low frequency driving signal, and switched to the reflecting planar texture by a high frequency signal. This polarizer functions as a switchable shutter for which we have characterized switching process, relaxation time, polarization purity, and extinction ratio.
A nulling interferometer for direct detection and spectral studies of the light from extra-solar planets would face daunting technical challenges. We outline a candidate optical architecture, discussing the major challenges in handling the starlight and controlling the optics to produce a deep on-axis null with high transmission a fraction of an arcsecond away.
Several proposed spacecraft missions require positional knowledge of their optical elements to very high precision. This knowledge can be provided by a metrology system based on a laser interferometer incorporating the spacecraft optics. We present results from fabrication and testing of a lab-based frequency-modulated (FM) Michelson interferometer intended to maintain length stability to a few picometers. The instrument can be used to make precise relative distance measurements or it can be used to characterize orientation and polarization effects of system components commonly used in metrology gauges. External frequency modulation of a frequency-stabilized laser source and phase-sensitive detection are used to detect changes in the arm length difference of the interferometer. Arm length adjustments are made via a closed loop feedback system. A second system having a shared beampath with the primary system monitors the performance of the primary system. Preliminary data, operating in an ambient lab environment, demonstrate control to roughly 20 picometers rms for measurement times around 100 seconds.
Several proposed space-based interferometry missions require positional knowledge of their optical elements to very high precision. To achieve the desired stellar position measurement precision, the internal optical path difference of the stellar interferometer must be measured to within 10 picometers. This knowledge can be provided by a metrology system based on a laser interferometer incorporating the spacecraft optics. We present results from fabrication and testing of a lab-based frequency-modulated (FM) Michelson interferometer intended to maintain length stability to a few picometers. The instrument can be used to make precise relative distance measurements or it can be used to characterize orientation and polarization effects of system components commonly used in metrology gauges. External frequency modulation of a frequency- stabilized laser source and phase-sensitive detection are used to detect changes in the arm length difference of the interferometer. Arm length adjustments are made via a closed loop feedback system. A second system having a shared beampath with the primary system monitors the performance of the primary system. Preliminary data, operating in an ambient lab environment, demonstrate control to roughly 6 picometers rms for measurement times around 10 seconds.
We discuss the design of tunable birefringent filters utilizing liquid crystals. The performance of several assembled filters is presented. We have incorporated nematic liquid crystals in Lyot and Solc filter designs to a low continuous spectral tunability. Liquid crystals can be manufactured as electrically variable retarders in the visible and near infrared spectral regions, and can provide filter tuning times as short as 20 msec. The performance characteristics and design enhancements of assembled tunable filters for diverse applications are discussed. These include: solar imaging in the near infrared with a narrow band filter; imaging fluorescence microscopy in the visible with a fast tuning filter; and airborne remote sensing over the 400-2500 nm spectral range with wide field of view filter.s Design considerations for improving speed, field of view, transmission and contrast of liquid crystal tunable filters are discussed.
Nematic liquid crystals are used in a variety of applications including polarization interference filters, Fabry-Perot etalons, spatial light modulators, and polarimeters. Due to substantial interest in utilizing these devices in space, we have performed numerous tests to indicate the potential suitability of nematic liquid crystal components in a space environment. We report on liquid crystal survivability under extreme vacuum and temperature conditions, as well as their long term functionality after exposure to gamma, ultraviolet, and free electron radiation. The only damage occurred with UV exposure. No damage was observed due to a gamma radiation dose of 2.3 krad or from electron beam dose of 2 Mrad. We report on continued tests designed to determine damage thresholds of liquid crystals to radiation.
We discuss tunable birefringent filters utilizing liquid crystals. We have incorporated nematic liquid crystals, manufactured as electrically variable, true zero-order retarders, in Lyot and Solc filter designs to allow continuous spectral tunability. The performance characteristics and design enhancements of several assembled tunable filters are discussed. These range from narrow band (0.13 nm) filters for solar imaging to wide field of view filters for airborne remote sensing spanning the spectral range 400 - 2500 nm. Design considerations for improving field of view, speed, and transmission of liquid crystal tunable filters are discussed.
The design of tunable birefringent filters utilizing liquid crystal retarders is discussed. Liquid crystals, which can be manufactured as electrically-variable, true zero-order retarders, can be used in most fixed birefringent filter designs to allow spectral tunability. The wavelength range for liquid crystal retarders includes the entire visible spectrum and most of the near infrared. Typical response times for current devices are approximately 10 ms. Two common filter designs that lend themselves well to liquid crystal variable retarder technology are Lyot and Solc filters. Each of these filters can be tuned to any wavelength within the transmission range of the optical components by incorporating with each fixed birefringent element a liquid crystal retarder capable of varying retardance through one full wavelength. The characteristics of an assembled liquid crystal tunable filter are presented, and design considerations for tunable birefringent filters are discussed.
We present data on two recently available uniaxial crystalline materials, cerium fluoride (CeF3) and lanthanum fluoride (LaF3). The characteristics presented include birefringence, transmission, temperature sensitivity, and field of view for wavelengths from the ultraviolet into the near infrared. Possible applications in polarization dependent devices are discussed. The optical properties of CeF3 and LaF3 are compared with commonly used crystalline materials.