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1.
Six different calibrations of the polarimetric albedo scale of asteroids have been published so far. Each of them contains its particular random and systematic errors and yields its values of geometric albedo. On the one hand, this complicates their analysis and comparison; on the other hand, it becomes more and more difficult to decide which of the proposed calibrations should be used. Moreover, in recent years, new databases on the albedo of asteroids obtained from the radiometric surveys of the sky with the orbital space facilities (the InfraRed Astronomical Satellite (IRAS), the Japanese astronomical satellite AKARI (which means “light”), the Wide-field Infrared Survey Explorer (WISE), and the Near-Earth Object Wide-field Survey Explorer (NEOWISE)) have appeared; and the database on the diameters and albedos of asteroids obtained from their occultations of stars has substantially increased. Here, we critically review the currently available calibrations and propose a new generalized calibration derived from the interrelations between the slope h and the albedo and between Pmin and the albedo. This calibration is based on all of the available series of the asteroid albedos and the most complete data on the polarization parameters of asteroids. The generalized calibration yields the values of the polarimetric albedo of asteroids in the system unified with the radiometric albedos and the albedos obtained from occultations of stars by asteroids. This, in turn, removes the difficulties in their comparison, joint analysis, etc.  相似文献   

2.
During the Near-Earth Asteroid Rendezvous (NEAR) spacecraft's investigation of asteroid 433 Eros, inflight calibration measurements from the multispectral imager (MSI) have provided refined knowledge of the camera's radiometric performance, pointing, and light-scattering characteristics. Measurements while at Eros corroborate most earlier calibration results, although there appears to be a small, gradual change in instrument dark current and flat field due to effects of aging in the space environment. The most pronounced change in instrument behavior, however, is a dramatic increase in scattered light due to contaminants accumulated on the optics during unscheduled fuel usage in December 1998. Procedures to accurately quantify and to remediate the scattered light are described in a companion paper (Li et al. 2002, Icarus155, 00-00). Acquisition of Eros measurements has clarified the relative, filter-to-filter, radiometric performance of the MSI. Absolute radiometric calibration appears very well constrained from flight measurements, with an accuracy of ∼5%. Pointing relative to the spacecraft coordinate system can be determined from the temperature of the spacecraft deck with an accuracy of ∼1 pixel.  相似文献   

3.
Photometric Stability of the Lunar Surface   总被引：1，自引：0，他引：1  
Hugh H. Kieffer 《Icarus》1997,130(2):323-327
The rate at which cratering events currently occur on the Moon is considered in light of their influence on the use of the Moon as a radiometric standard. The radiometric effect of small impact events is determined empirically from the study of Clementine images. Events that would change the integral brightness of the moon by 1% are expected once per 1.4 Gyr. Events that cause a 1% shift in one pixel for low Earth-orbiting instruments with a 1-km nadir field of view are expected approximately once each 43 Myr. Events discernible at 1% radiometric resolution with a 5 arc-sec telescope resolution correspond to crater diameters of approximately 210 m and are expected once every 200 years. These rates are uncertain by a factor of two. For a fixed illumination and observation geometry, the Moon can be considered photometrically stable to 1 × 10−8per annum for irradiance, and 1 × 10−7per annum for radiance at a resolution common for spacecraft imaging instruments, exceeding reasonable instrument goals by six orders of magnitude.  相似文献   

4.
David Morrison 《Icarus》1977,31(2):185-220
The radiometric method of determining diameters of asteroids is reviewed, and a synthesis of radiometric and polarimetric measurements of the diameters and geometric albedos of a total of 187 asteroids is presented. All asteroids with diameters greater than 250 km are identified, and statistical studies can be carried out of the size distributions of different albedo classes down to 80-km diameter for the entire main asteroid belt (2.0–3.5 AU). The distribution of albedos is strongly bimodal, with mean albedos for the C and S groups of 0.035 and 0.15, respectively. The C asteroids outnumber the S at all sizes and all values of semi-major axis, increasing from a little over half the population inside 2.5 AU to more than 95% beyond 3.0 AU; for all objects with D > 70 km, the ratio C/(C+S) is 0.88 ± 0.04. More than half of all asteroids in this size range have a > 3.0 AU. The M asteroids constitute about 5% of the population for a < 3.0 AU, but no members of of this class have been identified in the outer belt. There are no significant differences between the distributions of C, S, and M asteroids for the largest asteroids (D > 200 km) and for those of intermediate size (200–270 km). The total mass in the belt, down to 70-km size, but excluding Ceres, is about 2 × 1024 g. Evidence is presented that several large asteroids rotate in a prograde sense, and that a real difference existsbetween the bulk densities of Ceres and Vesta.  相似文献   

5.
COR1 is an internally occulted Lyot coronagraph, part of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument suite aboard the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft. Because the front objective lens is subjected to a full solar flux, the images are dominated by instrumental scattered light which has to be removed to uncover the underlying K corona data. We describe a procedure for removing the instrumental background from COR1 images. F coronal emission is subtracted at the same time. The resulting images are compared with simultaneous data from the Mauna Loa Solar Observatory Mk4 coronagraph. We find that the background subtraction technique is successful in coronal streamers, while the baseline emission in coronal holes (i.e. between plumes) is suppressed. This is an expected behavior of the background subtraction technique. The COR1 radiometric calibration is found to be either 10 – 15% lower, or 5 – 10% higher than that of the Mk4, depending on what value is used for the Mk4 plate scale, while an earlier study found the COR1 radiometric response to be ∼ 20% higher than that of the Large Angle Spectroscopic Coronagraph (LASCO) C2 telescope. Thus, the COR1 calibration is solidly within the range of other operating coronagraphs. The background levels in both COR1 telescopes have been quite steady in time, with the exception of a single contamination event on 30 January 2009. Barring too many additional events of this kind, there is every reason to believe that both COR1 telescopes will maintain usable levels of scattered light for the remainder of the STEREO mission.  相似文献   

6.
We have constructed a new high resolution solar reference spectrum in the spectral range between 250 and 550 nm. The primary use of this spectrum is for the calibration of the Dutch – Finnish Ozone Monitoring Instrument (OMI), but other applications are mentioned. The incentive for deriving a new high resolution solar reference spectrum is that available spectra do not meet our requirements on radiometric accuracy or spectral resolution. In this paper we explain the steps involved in constructing the new spectrum, based on available low and high resolution spectra and discuss the main sources of uncertainty. We compare the result with solar measurements obtained with the OMI as well as with other UV-VIS space-borne spectrometers with a similar spectral resolution. We obtain excellent agreement with the OMI measurements, which indicates that both the newly derived solar reference spectrum and our characterization of the OMI instrument are well understood. We also find good agreement with previously published low resolution spectra. The absolute intensity scale, wavelength calibration and representation of the strength of the Fraunhofer lines have been investigated and optimized to obtain the resulting high resolution solar reference spectrum.  相似文献   

7.
Temporal variations in the visible/near-infrared reflectance spectra of the radiometric calibration targets on the Mars Pathfinder (MPF) lander obtained by the Imager for Mars Pathfinder (IMP) camera reveal the effects of aeolian dust deposition at the MPF site throughout the mission. Sky brightness models in combination with two-layer radiative transfer models were used with these data to track changes in dust opacity on the radiometric calibration targets (RCTs) to constrain the dust deposition rate and the spectral properties of the deposited dust. Two-layer models were run assuming both linear and nonlinear dust accumulation rates, and suggest that RCT dust optical depth at the end of the 83-sol mission was 0.08 to 0.16, or on the order of 5- to 10-μm thickness for plausible values for dust porosity and grain size. These values correspond to dust fall rates of about 20-45 μm per Earth year, consistent with previous studies of dust deposition on Mars. The single scattering albedos of the dust derived from the models fall between those previously determined for atmospheric dust and bright soils. Comparisons of relative reflectance spectra calibrated using observed RCT radiances from late in the mission versus using radiances from modeled (dust-free) RCTs also reveal distinct spectral differences consistent with dust on the RCTs. Temporal variations in RCT dust opacity are not clearly linked to known passages of vortices at the MPF site, but overall suggest that deposition of dust onto the targets by local dust devils may be favored over erosion. Analyses of temporal changes in visible/near-infrared spectra will provide valuable information for future missions by constraining how dust deposition affects landed spacecraft operability (e.g., solar power availability), instrument calibration, and interpretations of surface mineralogy and composition.  相似文献   

8.
The absolute chronology of Mars is poorly known and, as a consequence, a key science aim is to perform accurate radiometric dating of martian geological materials. The scientific benefits of in situ radiometric dating are significant and arguably of most importance is the calibration of the martian cratering rate, similar to what has been achieved for the Moon, to reduce the large uncertainties on absolute boundary ages of martian epochs. The Beagle 2 Mars lander was capable of performing radiometric date measurements of rocks using the analyses from two instruments in its payload: (i) the X-ray Spectrometer (XRS) and (ii) the Gas Analysis Package (GAP). We have investigated the feasibility of in situ radiometric dating using the K-Ar technique employing flight-like versions of Beagle 2 instrumentation. The K-Ar ages of six terrestrial basalts were measured and compared to the ‘control’ Ar-Ar radiometric ages in the range 171-1141 Ma. The K content of each basalt was measured by the flight spare XRS and the 40Ar content using a laboratory analogue of the GAP. The K-Ar ages of five basalts broadly agreed with their corresponding Ar-Ar ages. For one final basalt, the 40Ar content was below the detection limit and so an age could not be derived. The precision of the K-Ar ages was ∼30% on average. The conclusions from this study are that careful attention must be paid to improving the analytical performance of the instruments, in particular the accuracy and detection limits. The accuracy of the K and Ar measurements are the biggest source of uncertainty in the derived K-Ar age. Having investigated the technique using flight-type planetary instrumentation, we conclude that come of the principle challenges of conducting accurate in situ radiometric dating on Mars using instruments of these types include determining the sample mass, ensuring all the argon is liberated from the sample given the maximum achievable temperature of the mass spectrometer ovens, and argon loss and non-radiogenic argon in the analysed samples.  相似文献   

9.
The thermal flux in the 20-μm Q filter band of comet P/Schwassmann-Wachmann 1 was measured some 77 days after the most recent previous eruption, and when the visual magnitude was about 17.6. Considerations of the eruptive history of the comet, and in particular its tendency to fade to a minimum threshold brightness level between eruptions, suggest that the measurement refers to an essentially bare nucleus. Using the standard photometric/radiometric technique for calculating the diameters and geometric albedos of asteroids and planetary satellites, and the assumption that the bare nucleus is measured, we find that the diameter of the nucleus is 40±5 km, and the geometric albedo is 0.13±0.04.  相似文献   

10.
A long-term project of polarimetric observations of minor planets has been carried out since 1995 at the 2.15-m telescope of the El Leoncito Observatory (San Juan, Argentina) using the Torino photopolarimeter. We present here an updated summary of the results, including many measurements obtained during the most recent observing runs. The main purpose of the observations has been to obtain albedo estimates and compare them with previous IRAS radiometric determinations, mainly for objects having small (<50 km) IRAS diameters. Another field of investigation is the measurement of the degree of linear polarization at very small phase angles (less than 1°). The latter observations can be a useful input for modern theoretical models of light scattering from asteroid surfaces. In general, we have obtained a wealth of new results, including objects exhibiting a peculiar polarimetric behavior.  相似文献   

11.
We report 10 micrometers infrared photometry for 22 Aten, Apollo, and Amor asteroids. Thermal models are used to derive the corresponding radiometric albedos and diameters. Several of these asteroids appear to have surfaces of relatively high thermal inertia due to the exposure of bare rock or a coarse regolith. The Apollo asteroid 3103, 1982 BB, is recognized as class E. The Jupiter-crossing Amor asteroid 3552, 1983 SA, is confirmed as class D, but low albedos remain rare for near-Earth asteroids.  相似文献   

12.
J.L. Elliot  J. Veverka  J. Goguen 《Icarus》1975,26(4):387-407
The diameters of Tethys, Dione, Rhea, Titan and Iapetus were determined from observations of their March 30, 1974, lunar occultations, made with the Mauna Kea 224 and 61 cm telescopes. Light curves were obtained simultaneously in four colors, and the difference between the time of occultation at the two telescopes provided a direct measurement of the slope of the lunar limb, found to be small in all cases. The satellite diameters were determined by least-squares fits of model occultation light curves to the data. In these fits the diameter and degree of limb darkening of the satellite are correlated variables, requiring the limb darkening to be specified before the diameter can be determined, or vice versa. However, for Titan the signal-to-noise ratio is sufficiently high to allow some assessment of the amount of limb darkening, which was found to be substantial. Titan's diameter must be at least 5800 km, much larger than the currently accepted value of 5000 km, making it the largest satellite in the solar system. This larger diameter implies a low mean density. For the other four satellites arguments are presented in favor of accepting the occultation diameters corresponding to limb darkened disks. Except for Titan, the lunar occultation diameters generally agree with previous diskmeter and radiometric determinations.  相似文献   

13.
Radarclinometry, the invention of which has been previously reported, is a technique for deriving a topographic map from a single radar image by using the dependence upon terrain-surface orientation of the integrated signal of an individual image pixel. The radiometric calibration required for precise operation and testing does not yet exist, but the imminence of important applications justifies parallel, rather than serial, development of radarclinometry and radiometrically calibrated radar. The present investigation reports three developmental advances: (1) The solid angle of integration of back-scattered specific intensity constituting a pixel signal is more accurately accounted for in its dependence on surface orientation than in previous work. (2) The local curvature hypothesis, which removes the requirement of a ground-truth profile as a boundary condition and enables the formulation of the theory in terms of a line integral, has been expanded to include the three possibilities of Local Cylindricity, Local Biaxial Ellipsoidal Hyperbolicity, and Least-Squares Local Sphericity. (3) The theory is integrated in the cross-ground-range direction, which is ill-conditioned compared to the ground-range direction, whereas the original formulation was based on enforced isotropy in the two-dimensional power spectrum of the topography. It was found necessary to prohibit the hypothesis of Local Biaxial Ellipsoidal Hyperbolicity in the cross-range stepping, for reasons not completely clear. Variation in the proportioning between curvature assumptions had produced topographic maps that are in good mutual agreement but not realistic in appearance. They are severely banded parallel to the ground-range direction, most especially at small radar zenith angles. Numerical experimentation with the falsification of topography through incorrect decalibration as performed on a Gaussian hill suggests that the banding and its exaggeration at high radar incidence angles could easily be due to our lack of radiometric calibration.  相似文献   

14.
The MARS-3 third-generation matrix radiometric system has been developed, manufactured, and is currently being used in observations within the framework of the “Cosmological Gene” program. The system is based on new hardware components and consists of 16 independent radiometers (32 horns with a step of 20 mm). Each pair of horns is connected to the input of an amplifier unit via a square-loop modulator. The parameters of each radiometer are: central frequency, 30.0 GHz; bandwidth, 5GHz; average noise temperature of the system, 250 K; and a sensitivity of about 5 mK for τ = RC = 1 s.  相似文献   

15.
The NASA Solar Dynamics Observatory (SDO), scheduled for launch in early 2010, incorporates a suite of instruments including the Extreme Ultraviolet Variability Experiment (EVE). EVE has multiple instruments including the Multiple Extreme ultraviolet Grating Spectrographs (MEGS) A, B, and P instruments, the Solar Aspect Monitor (SAM), and the Extreme ultraviolet SpectroPhotometer (ESP). The radiometric calibration of EVE, necessary to convert the instrument counts to physical units, was performed at the National Institute of Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF III) located in Gaithersburg, Maryland. This paper presents the results and derived accuracy of this radiometric calibration for the MEGS A, B, P, and SAM instruments, while the calibration of the ESP instrument is addressed by Didkovsky et?al. (Solar Phys., 2010, doi: 10.1007/s11207-009-9485-8 ). In addition, solar measurements that were taken on 14 April 2008, during the NASA 36.240 sounding-rocket flight, are shown for the prototype EVE instruments.  相似文献   

16.
The Visual and Infrared Mapping Spectrometer (VIMS) observed the Galilean satellites during the Cassini spacecraft's 2000/2001 flyby of Jupiter, providing compositional and thermal information about their surfaces. The Cassini spacecraft approached the jovian system no closer than about 126 Jupiter radii, about 9 million kilometers, at a phase angle of <90°, resulting in only sub-pixel observations by VIMS of the Galilean satellites. Nevertheless, most of the spectral features discovered by the Near Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft during more than four years of observations have been identified in the VIMS data analyzed so far, including a possible 13C absorption. In addition, VIMS made observations in the visible part of the spectrum and at several new phase angles for all the Galilean satellites and the calculated phase functions are presented. In the process of analyzing these data, the VIMS radiometric and spectral calibrations were better determined in preparation for entry into the Saturn system. Treatment of these data is presented as an example of the VIMS data reduction, calibration and analysis process and a detailed explanation is given of the calibration process applied to the Jupiter data.  相似文献   

17.
We present a flux calibration scheme for the PACS chopped point-source photometry observing mode based on the photometry of five stellar standard sources. This mode was used for science observations only early in the mission. Later, it was only used for pointing and flux calibration measurements. Its calibration turns this type of observation into fully validated data products in the Herschel Science Archive. Systematic differences in calibration with regard to the principal photometer observation mode, the scan map, are derived and amount to 5 ? 6 %. An empirical method to calibrate out an apparent response drift during the first 300 Operational Days is presented. The relative photometric calibration accuracy (repeatability) is as good as 1 % in the blue and green band and up to 5 % in the red band. Like for the scan map mode, inconsistencies among the stellar calibration models become visible and amount to 2 % for the five standard stars used. The absolute calibration accuracy is therefore mainly limited by the model uncertainty, which is 5 % for all three bands.  相似文献   

18.
Olav L. Hansen 《Icarus》1977,31(4):456-482
A new radiometric model for disk-integrated photometry of asteroids is presented. With empirical support from photometry of Mercury and the Moon, the model assumes that observed sunward beaming of the infrared emission is due to craters. In contrast to earlier theoretical studies of the lunar emission, the observable flux ratio between a cratered sphere and a smooth sphere is calculated for large ranges in wavelength, temperature, and phase angle. Revised diameters and albedos based on the crater model are given for 84 asteroids. The revised values are in good agreement with Morrison's (1977) radiometric results. It is shown that the systematic discrepancy between radiometric and polarimetric albedos (Zellner and Gradie, 1976) is probably a double-valued function of albedo. Some typical geometric albedos from this paper, Morrison (1977), and Zellner and Gradie (1976), respectively, are: Ceres (0.050 ± 0.005, 0.053 ± 0.004, 0.068), Vesta (0.235 ± 0.032, 0235 ± 0.11, 0.271), mean C type (0.031 ± 0.009, 0.035 ± 0.009, 0.061 ± 0.005), mean S type (0.117 ± 0.030, 0.136 ± 0.032, 0.181 ± 0.23), and mean M type (0.105 ± 0.037, 0.115 ± 0.033, 0.157 ± 0.079). Areas of disagreement between radiometry and polarimetry are underscored, and research to resolve them is suggested.  相似文献   

19.
Quantitative solar spectroscopy must be based on calibrated instrumentation. The basic requirement of a calibration, i.e., a comparison between the instrument under test and a primary laboratory standard through appropriate procedures, will be briefly reviewed, and the application to modern space instruments will be illustrated. Quantitative measurements of spectral radiances with high spectral and spatial resolutions as well as spectral irradiances yield detailed information on temperatures, electron densities, bulk and turbulent motions, element abundances of plasma structures in various regions of the solar atmosphere – from the photosphere to the outer corona and the solar wind. The particular requirements for helioseismology and magnetic‐field observations will not be covered in any depth in this review. Calibration by a laboratory standard is necessary, but not sufficient, because an adequate radiometric stability can only be achieved together with a stringent cleanliness concept that rules out a contamination of the optical system and the detectors as much as possible. In addition, there is a need for calibration monitoring through inter‐calibration and other means (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

20.
Measurements of bright stars passing through the fields of view of the inner coronagraphs (COR1) on board the Solar Terrestrial Relations Observatory (STEREO) are used to monitor changes in the radiometric calibration over the course of the mission. Annual decline rates are found to be \(0.648 \pm 0.066\)%/year for COR1-A on STEREO Ahead and \(0.258 \pm 0.060\)%/year for COR1-B on STEREO Behind. These rates are consistent with decline rates found for other space-based coronagraphs in similar radiation environments. The theorized cause for the decline in sensitivity is darkening of the lenses and other optical elements due to exposure to high-energy solar particles and photons, although other causes are also possible. The total decline in the COR-B sensitivity when contact with Behind was lost on 1 October 2014 was 1.7%, while COR1-A was down by 4.4%. As of 1 November 2017, the COR1-A decline is estimated to be 6.4%. The SECCHI calibration routines will be updated to take these COR1 decline rates into account.  相似文献   

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