Absolute Radiometer Research Articles

Simultaneous measurement of the total solar irradiance and solar diameter by the PICARD mission

A mission dedicated to simultaneous measurements of the solar diameter, spectral, and total solar irradiance is presently in development for launch end of the year 2008 on board of a microsatellite under the responsibility of Centre National d’Etudes Spatiales. The payload will consist of an imaging telescope, three filter radiometers with in total twelve channels, and two independent absolute radiometers. The scientific aims are presented as well as the concepts and properties of the instrumentation. This mission is named PICARD after the pioneering work of Jean Picard (1620–1682) who precisely determined the solar diameter during the Maunder minimum.

Design and performance of sliced-aperture corrugated feed horn antennas

We report on the design of corrugated feed horn microwave antennas at 3.3, 5.6, 7.8, and 10.2 GHz for the absolute radiometer for cosmology, astrophysics, and diffuse emission experiment. These horns have low sidelobe symmetrical beams with 12° full width at half power, and three noteworthy features: a 30° slice at the aperture, a profiled rather than a linear taper, and a slowly varying groove depth along the length of the horn. The profiled taper and varying groove depth provided a narrow beam given the existing physical spatial constraints of the instrument in which the horns are used. The 30° slice was necessary for instrumental considerations and has a minimal effect on the symmetry of the beam. The slice reduces the effective aperture radius and overall length to that corresponding to an unsliced horn with an aperture at roughly the middle of the slice and does not introduce any undesirable effects.

The Present State and Future Prospects for Developing a System for Ensuring Common Standards of Measurement in the Area of Solar Radiation Radiometry

A system for ensuring common standards of measurements of energy illumination produced by solar radiation is considered. The results of many years of international comparisons of a standard absolute radiometer with the standard radiometers and pyrheliometers of other countries are presented.

ISS-SOLAR: Total (TSI) and spectral (SSI) irradiance measurements

The primary objective of the ISS-SOLAR mission on Columbus (to be launched in 2006) is the quasi-continuous measurement of the solar irradiance variability with highest possible accuracy. For this reason the total spectral range will be recorded simultaneously from 3000 to 17 nm by three sets of instruments: SOVIM is combining two types of absolute radiometers and three-channel filter radiometers. SOLSPEC is composed of three double monochromators using concave gratings, covering the wavelength range from 3000 to 180 nm. SOL-ACES has four grazing incidence planar grating spectrometers plus two three-signal ionization chambers (two signals from a two stage chamber plus a third signal from a silicon diode at the end of the chamber) with exchangeable band pass filters to determine the absolute fluxes from 220 to 17 nm repeatedly during the mission. For the TSI the relative standard uncertainty (RSU) to be achieved is of the order of 0.15% and for the SSI from 1% in the IR/Vis, 2% in the UV, 5% in the FUV up to 10% in the XUV spectral regions. The general requirements for the TSI and SSI measurements and their conceptual realization within this payload will be discussed with emphasis on instrumental realization and calibration aspects.

NIST VUV metrology programs to support space-based research

Vacuum ultraviolet (VUV) radiation, spanning the electromagnetic spectrum from about 2 nm (620 eV) to 200 nm (6.2 eV) has long been important in astronomy, solar physics, and Earth observing systems, among other applications. The National Institute of Standards and Technology (NIST) has several programs to serve the VUV user community, from the Synchrotron Ultraviolet Radiation Facility (SURF III) – a standard of irradiance from 2 to 400 nm – to measurement and calibration services for mirrors, photodiodes, and filters. We have recently reduced the uncertainty of our extreme ultraviolet (EUV) detector calibrations by implementing an absolute cryogenic radiometer on one of the SURF beamlines, and have effected several improvements to the EUV detector calibration beamline at SURF. We continue to investigate wide-bandgap semiconductors for use as solar-blind detector technologies, and have recently obtained quantum efficiency and uniformity data from 1 cm 2 active area GaN and SiC photodiodes.

A simple transfer-optics system for an extreme-ultraviolet synchrotron beamline

Radiometric experiments often require comparatively high intensities in the extreme ultraviolet, in the microwatt range. The monochromators that provide the high throughput needed for these experiments, though, do not always allow for end stations to be switched out easily. At the National Institute of Standards and Technology Synchrotron Ultraviolet Radiation Facility, the only beamline with sufficient extreme-ultraviolet power has a multi-ton endstation, which cannot be moved. We will describe a set of transfer optics that allow the photon beam to be collimated and deflected through a port on the downstream end of the large chamber. This allowed an absolute cryogenic radiometer to be attached, with the entrance cavity underfilled. We will describe ray-tracing results and offer preliminary results of the radiometer-based throughput of the system.

ACR II: Improved absolute cryogenic radiometer for low background infrared calibrations

A second-generation absolute cryogenic radiometer (ACR II) was developed for use at the Low Background Infrared calibration facility at the National Institute of Standards and Technology. The need for spectral calibrations of very sensitive [D* = 10(14) cm (Hz)1/2W(-1)] infrared detectors necessitated the use of a cryogenic infrared monochromator and a more sensitive radiometer. The improved low-power performance of the ACR II compared with the older absolute cryogenic radiometer (ACR) has also made it useful as the primary standard for the calibration of cryogenic blackbody sources that are used as low-power infrared sources. The responsivity of the new radiometer's receiver is 210 K/mW with a type A (random component) standard uncertainty of at most 7 pW when making power measurements of less than 10 nW. The original ACR has a responsivity of 29 K/mW and has a type A standard uncertainty of approximately 100 pW when making a similar low-noise-power measurement. Other properties of the radiometers are also described and compared.

Design and calibration of a cryogenic blackbody calibrator at centimeter wavelengths

We describe the design and calibration of an external cryogenic blackbody calibrator used for the first two flights of the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) instrument. The calibrator consists of a microwave absorber weakly coupled to a superfluid liquid helium bath. Half-wave corrugations viewed 30° off axis reduce the return loss below −35 dB within a compact footprint. Ruthenium oxide resistive thermometers embedded within the absorber monitor the temperature across the face of the calibrator. The thermal calibration transfers the calibration of a reference thermometer to the flight thermometers using the flight thermometer readout system. The calibrator thermometry is stable in time over four years, with statistical uncertainty in the temperature calibration of order 2 mK near 2.7 K, limited primarily by thermal fluctuations in the liquid helium bath. Observations of the superfluid transition demonstrate that the absolute temperature scale is accurate within 0.3 mK.

극저온 절대복사계에 소급한 칸델라 국가표준 실현

국제단위계 중의 하나인 칸델라 눈금을 두 개의 광도계를 사용하여 실현하였다. 광도계의 분광감응도는 광출력을 전기출력으로 치환하여 측정하는 극저온 절대복사계에 소급하여 측정하였다. 광도계 외부에 설치된 정밀 개구는 다이아몬드 회전가공기로 가공하였으며 그 면적은 불확도 0.05%(k = 1) 이내로 측정하였다. 이 광도계를 사용하여 광도 벤치 위에서 1 kW FEL 전구로 칸델라 단위를 실현할 때 불확도를 분광감응도, 감응도의 공간 균일도, 개구 면적, FEL 전구의 색온도와 위치 재현성을 고려하여 평가한 결과 0.25%(k = 1)를 얻었다. 실현한 눈금의 유효성을 확인하기 위하여 미국 NIST의 눈금과 비교한 결과 0.1% 이내에서 일치하였다. We realized the national standard of the candela, one of the SI units, by using two photometers with the spectral responsivity measured in reference to the absolute cryogenic radiometer. The external apertures of the photometers were fabricated using a diamond turning machine, and measured in terms of area with uncertainty of 0.05 %(k = 1). The candela is realized using a 1 kW FEL lamp and the characterized photometers on an optical bench. The uncertainty is budgeted to be 0.25 %(k = 1) considering the uncertainty of the spectral responsivity and the response uniformity of the detectors, the area of the external apertures, the color temperature of the lamp, and the positioning reproducibility of the photometers and the lamp. We verified the realized scale by comparing with the scale of National Institute of Standards and Technology, USA. They coincided with each other within 0.1%.

An Instrument to Measure the Temperature of the Cosmic Microwave Background Radiation at Centimeter Wavelengths

The Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) is a balloon-borne instrument to measure the temperature of the cosmic microwave background at centimeter wavelengths. ARCADE uses narrow-band cryogenic radiometers to compare the sky to an external full-aperture calibrator. To minimize potential sources of systematic error, ARCADE uses a novel open-aperture design which maintains the antennas and calibrator at temperatures near 3 K at the mouth of an open bucket Dewar, without windows or other warm objects between the antennas and the sky. We discuss the design and performance of the ARCADE instrument from its 2001 and 2003 flights.

Diffraction corrections in radiometry: spectral and total power and asymptotic properties

Wolf's result for integrated flux in the case of diffraction by a circular lens or aperture in the scalar, paraxial Fresnel approximation is considered anew. Compact integral formulas for pertinent infinite sums are derived, and the result's generalizations to extended sources and Planckian sources and asymptotic aspects at small wavelength and high temperature are all considered. Simplification of calculations for an actual absolute radiometer is demonstrated.

The Temperature of the Cosmic Microwave Background at 10 GHz

We report the results of an effort to measure the low-frequency portion of the spectrum of the cosmic microwave background (CMB) radiation, using a balloon-borne instrument called the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE). These measurements are to search for deviations from a thermal spectrum that are expected to exist in the CMB as a result of various processes in the early universe. The radiometric temperature was measured at 10 and 30 GHz using a cryogenic open-aperture instrument with no emissive windows. An external blackbody calibrator provides an in situ reference. Systematic errors were greatly reduced by using differential radiometers and cooling all critical components to physical temperatures approximating the antenna temperature of the sky. A linear model is used to compare the radiometer output to a set of thermometers on the instrument. The unmodeled residuals are less than 50 mK peak to peak with a weighted rms of 6 mK. Small corrections are made for the residual emission from the flight train, atmosphere, and foreground Galactic emission. The measured radiometric temperature of the CMB is 2.721 ± 0.010 K at 10 GHz and 2.694 ± 0.032 K at 30 GHz.

Measurements of fibre optic power using photodiodes with and without an integrating sphere

We have studied two kinds of reference detectors to be used for calibration of fibre optic power meters. The first type consists of an InGaAs photodiode mounted directly on an adapter using a standard FC connector. In the second type, a similar photodiode is fitted into a 5 cm diameter integrating sphere. The plain photodiodes are used for low power levels and the sphere detector for high power levels because of the low throughput (∼0.7%) of the sphere. The detectors, with their FC connectors removed, are calibrated using collimated laser beams, using as the reference a spectrally flat pyroelectric radiometer traceable to a cryogenic absolute radiometer. The spatial non-uniformities of the plain photodiodes are of the order of 0.3% throughout the active area of 5 mm diameter. The angular responsivity varies less than ±0.1% within the angle of ±8°, which covers 99% of the radiation emitted from the fibre end. The angular non-uniformity of the sphere detector is less than 0.1%. The results indicate that a plain photodiode can be used as a reference detector of fibre optic power. The calibration uncertainty at HUT is presently of the order of 1% (k = 2).

Free-Free Emission at Low Radio Frequencies

We discuss free-free radio emission from ionized gas in the intergalactic medium. Because the emissivity is proportional to the square of the electron density, the mean background is strongly sensitive to the spatial clumping of free electrons. Using several existing models for the clumping of ionized gas, we find that the expected free-free distortion to the cosmic microwave background blackbody spectrum is at a level detectable with upcoming experiments such as the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission experiment. However, the dominant contribution to the distortion comes from clumpy gas at z 3, and the integrated signal does not strongly constrain the epoch of reionization. In addition to the mean emission, we consider spatial fluctuations in the free-free background and the extent to which these anisotropies confuse the search for fluctuations in 21 cm line emission from neutral hydrogen during and prior to reionization. This background is smooth in frequency space and hence can be removed through frequency differencing, but only so long as the 21 cm signal and the free-free emission are uncorrelated. We show that because the free-free background is generated primarily at low redshifts, the cross-correlation between the two fields is smaller than a few percent. Thus, multifrequency cleaning should be an effective way to eliminate the free-free confusion.

Total solar irradiance observations from DIARAD/VIRGO

The Differential Absolute Radiometer is making measurements of the total solar irradiance as part of the Variability of Irradiance and Gravity Oscillations experiment on the Solar and Heliospheric Observatory. We present the measurements made during its 7.8 years of operation, from 1996 until the present (2003). The aging due to UV exposure of the continuously measuring left channel is determined by comparison with the backup right channel; the loss in sensitivity of the left channel is 0.5 W/m2 or 364 ppm over 7.8 years. A raise of the total solar irradiance from a level of 1365.5 W/m2 at the end of the solar minimum in 1996 towards a maximum level of 1368 W/m2 at the beginning of 2002 has been measured by DIARAD.

Reionization and structure formation with ARCADE

Structure formation in the early universe is expected to produce a cosmological background of free–free emission. The Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) will measure the spectrum of the cosmic microwave background at centimeter wavelengths to detect this signature of structure formation. I describe the ARCADE instrument and the cryogenic engineering required to achieve millikelvin accuracy at such long wavelengths.

Detecting Reflected Light from Close‐in Extrasolar Giant Planets with theKeplerPhotometer

NASA's Kepler mission promises to detect transiting Earth-sized planets in the habitable zones of solar-like stars. In addition, it will be poised to detect the reflected light component from close-in extrasolar giant planets (CEGPs) similar to 51 Peg b. Here we use the Dual Irradiance Absolute Radiometer aboard the Solar Heliospheric Observatory time series along with models for the reflected light signatures of CEGPs to evaluate Kepler's ability to detect such planets. We examine the detectability as a function of stellar brightness, stellar rotation period, planetary orbital inclination angle, and planetary orbital period and then estimate the total number of CEGPs that Kepler will detect over its four-year mission. The analysis shows that intrinsic stellar variability of solar-like stars is a major obstacle to detecting the reflected light from CEGPs. Monte Carlo trials are used to estimate the detection threshold required to limit the total number of expected false alarms to no more than one for a survey of 100,000 stellar light curves. Kepler will likely detect 100-760 51 Peg b-like planets by reflected light with orbital periods up to 7 days.

Primary realization of a spectral irradiance scale employing monochromator-based cryogenic radiometry between 200 nm and 20 µm

Here we report on an alternative approach for making spectral irradiance measurements of radiation sources traceable, employing our monochromator-based absolute cryogenic radiometer (ACR) facility to its full extent. The method makes use of the continuously tunable absolute radiant flux emerging from the ACR facility to characterize and calibrate the spectral irradiance responsivity of a second adjacent double-monochromator system. This system will be part of a new facility at NMi-VSL, called SIR, that will be used to measure spectral irradiance distributions of radiation sources from the ultraviolet to the far infrared. Following this traceability route, a fully characterized high-temperature Planckian radiator is not needed. Proof-of-principle measurements in the visible part of the spectrum show very encouraging results. Although the new facility is under construction, the theoretical background and a schematic description will be presented in this paper.

Long-term temporal stability of the National Institute of Standards and Technology spectral irradiance scale determined with absolute filter radiometers.

The temporal stability of the National Institute of Standards and Technology (NIST) spectral irradiance scale as measured with broadband filter radiometers calibrated for absolute spectral irradiance responsivity is described. The working standard free-electron laser (FEL) lamps and the check standard FEL lamps have been monitored with radiometers in the ultraviolet and the visible wavelength regions. The measurements made with these two radiometers reveal that the NIST spectral irradiance scale as compared with an absolute thermodynamic scale has not changed by more than 1.5% in the visible from 1993 to 1999. Similar measurements in the ultraviolet reveal that the corresponding change is less than 1.5% from 1995 to 1999. Furthermore, a check of the spectral irradiance scale by six different filter radiometers calibrated for absolute spectral irradiance responsivity based on the high-accuracy cryogenic radiometer shows that the agreement between the present scale and the detector-based scale is better than 1.3% throughout the visible to the near-infrared wavelength region. These results validate the assigned spectral irradiance of the widely disseminated NIST or NIST-traceable standard sources.

A low noise thermometer readout for ruthenium oxide resistors

The thermometer readout and thermal control system for the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) experiment is described, including the design, testing, and results from the first flight of ARCADE. The noise is equivalent to about 1 Ω or 0.15 mK in a second for the RuO2 resistive thermometers at 2.7 K. The average power dissipation in each thermometer is 1 nW. The control system can take full advantage of the thermometers to maintain stable temperatures. Systematic effects are still under investigation, but the measured precision and accuracy are sufficient to allow measurement of the cosmic background spectrum.