Photometric Standards Research Articles

Optical Wedge Effects in Instruments and Standards for Molecular Absorption Spectrophotometry

The optical wedge of some older, individual solid NIST absorbance filter standards has been found to cause bias in the indicated absorbance readings of certain instruments. In a collimated-beam spectrophotometer, the sample beam is deflected by about half of the wedge angle in the sample. For inverted geometry designs, small deflections can give rise to large changes in the field of view of the entrance slit of the spectrometer downstream from the sample. Beam deflection is also found to induce small apparent wavelength shifts in data taken through a wedged sample, resulting in spectral artifacts derived from spectral features of the system or of an absorbing sample. These spectral artifacts turn out to be robust, and simple cell-reversal difference spectra can provide useful diagnostic indicators of optical wedge. Solid photometric standards are found to be reliable, if manufactured to wedge angles of less than 0.1 mrad. Despite a wedge tolerance of 0.9 m rad in the sealed cuvettes of a wavelength standard, wavelength shifts are shown to be negligible when compared to the stated uncertainties. For normal use, accuracy may be achieved by utilizing the same cell in the same orientation for both background and sample spectra or by hand selection of cells.

Asteroids as Far‐Infrared Photometric Standards

With the successful launch of the Infrared Space Observatory (ISO) in 1995 November, it has been necessary to establish a set of celestial standards for the photometric calibration of the instruments. In support of the imaging photopolarimeter ISOPHOT, a sample of 10 asteroids, which cover the flux range between 5 Jy (bright standard stars) and 1000 Jy (Uranus and Neptune) at wavelengths between 50 and 200 mm, has been investigated. Targeted ground-based mid-infrared and submillimeter observations, as well as spaceborne Infrared Astronomical Satellite (IRAS) and ISO measurements have been used to test the reliability and accuracy of several thermal flux models. A new approach for the thermophysical description of minor planets (J. S. V. Lagerros, AA A&A, 315, 625 [1996b]), including an ellipsoidal shape, thermal behavior, and surface roughness effects, led to the best agreement between observed and predicted brightness. The necessary input parameters for an optimized far-infrared application have been derived from the observational results. The uncertainties of the absolute calibration depend on the individual object and the wavelength interval. In the far-infrared ISOPHOT wavelength range (50–200 mm), an accuracy between 5% and 20% can be achieved. First efforts to use the asteroids as photometric standards in the far-infrared (ISO) and submillimeter (Heinrich-Hertz-Telescope and James Clerk Maxwell Telescope) have begun already. Future application for airborne (SOFIA) and spaceborne (FIRST and Space Infrared Telescope Facility [SIRTF]) experiments can be expected.

Luminous and photon standards by trap detectors

This paper presents the realization of photometric and photon standards with the same detector. To this aim, a measurement system has been set up at the Istituto Elettrotecnico Nazionale (IEN) based on trap detectors able to measure both luminous and photon flux of a light source. To validate the experimental results, the Planck's constant is obtained from the measured data. This paper highlights some of the problems encountered and discusses the relationship among lumen and mole of photons for electromagnetic radiation in the visible range.

Improved Photometric Standards and Calibration Procedures at NIST.

NIST has recently established a detector-based luminous intensity unit (candela, cd), which is derived from the NIST absolute cryogenic radiometer. Subsequently, the luminous flux unit (lumen, lm) and the luminance unit (cd/m2) have been established based on the detector-based candela, and now all the NIST photometric units are tied to the cryogenic radiometer. The illuminance unit is realized and maintained on five standard photometers. The large dynamic range of the standard photometers eliminates the need for maintaining many working standard lamps of various wattages. The luminous intensities of lamps are determined from the illuminances measured with these photometers and the distances measured with a linear encoder system. Transfer photometers and illuminance meters are calibrated by direct comparison with the standard photometers with no distance measurements involved. The luminous flux unit is realized using an absolute integrating sphere method newly developed at NIST. The luminance unit is realized on an integrating sphere source, which is used for calibration of other luminance sources and luminance meters. These detector-based methods have made it possible to reduce the uncertainties of photometric calibrations and to provide more varieties of photometric calibration services at NIST.

SECONDARY PHOTOMETRIC STANDARDS FOR NORTHERN CATACLYSMIC VARIABLES AND RELATED OBJECTS

ABSTRACT An average of 30 B,V secondary standards per field have been established for 76 variable stars. These are mostly cataclysmic variables of various subclasses but also include a variety of other kinds of variable stars. The standards are within a few arc minutes of the variable and are therefore well suited for CCD photometry. The standards have brightnesses of V ∼ 12-17 and have a typical accuracy per standard of ± 0.01 mag.

Spectrophotometry of Flux Calibration Stars for Hubble Space Telescope

view Abstract Citations (30) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectrophotometry of Flux Calibration Stars for Hubble Space Telescope Stone, Remington P. S. Abstract Fluxes are presented for 103 stars which were observed for Hubble Space Telescope (HST) calibration. Perhaps the most important aspect of this work is confirmation of a revised zero point for optical flux calibration of HST as described by Colina and Bohlin from synthetic photometry. Two cautions are offered for users of the Oke and Gunn spectrophotometric system, and simple corrections are suggested. The present stars are of interest because of their accessibility and their importance in a variety of standard systems. Included are UV and optical spectrophotometric flux standards, UBVRI and IR photometric standards, and many stars from the Equatorial Selected Areas for which UBVRI colors exist as well. It is hoped that the present data will be of use to those relating results between systems. The spectra generally cover the range λλ4040-8800, with a nominal resolution of about 5.5 Å pixel-1. The data are referred to the absolute calibration of α Lyrae recommended by Mégessier. Tabular magnitude values for Hayes and Oke-Guim wavelengths are presented. The same tables as well as mean magnitudes within 25 Å bins every 25 Å in ASCII and PostScript format, and flux-calibrated spectra, may be obtained from the author. Publication: The Astrophysical Journal Supplement Series Pub Date: November 1996 DOI: 10.1086/192369 Bibcode: 1996ApJS..107..423S Keywords: TECHNIQUES: PHOTOMETRIC full text sources ADS | data products SIMBAD (106)

Temperature and density in the middle corona through the activity cycle determined from white light observations

Abstract We investigated the solar cycle variations of the physical characteristics of coronal plasmas in equatorial regions and polar holes in the range from 1.2 to 2.5R [odot] from the Sun's center. We use coronal white light data obtained from total solar eclipses since the 1950s. To eliminate the effects of inhomogeneous data, all observational results for each eclipse were reduced to a common system in which the F-corona model of Koutchmy and Lamy (1985) was used as the photometric standard. Then the K-coronal brightness was determined by subtracting the F corona brightness from the total K + F emission. Values of temperature and density were determined by comparing the observed brightness distributions with distributions calculated assuming a hydrostatic and isothermal corona. We find that in equatorial regions the temperature in the middle corona is constant and equal to 1.4 × 106 K, but n 0, the parameter of the hydrostatic distribution, gradually increases from 2 × 108 cm−3 at solar minimum to 4 × 108 cm−3 at maximum. The specific value of n 0 depends on the level of activity of the underlying solar regions and could be connected with the presence of coronal streamers. In polar regions, n 0 does not depend on the phase of the cycle and is apparently determined by the properties of the associated hoie; those values vary in the range from 0.65 to 1.4 × 108 cm−3. The temperature in polar regions increases from 0.9 × 106 K at minimum to 1.4 × 106 K at maximum. Above 2.5R [odot], a deviation from the hydrostatic density distribution is found, which is most likely connected with the gas-dynamical coronal expansion developing at these heights. The emission measure and mass of the entire corona are evaluated.

SECONDARY PHOTOMETRIC STANDARDS IN SELECTED NORTHERN DWARF NOVA FIELDS

ABSTRACT We have compiled a list of secondary photometric standards for 88 northern dwarf nova fields. An average of 6 stars per field were calibrated through broadband BVR filters. The secondary standards range in brightness from V ~ 11 to V ~ 17, have a B-V color range of ~0.1 to ~2.2 and lie within ~3' of the dwarf nova. Typical 1-alpha errors are ± ~0.05 mag.

Characterization of modified FEL quartz-halogen lamps for photometric standards

The stability of luminous intensity and colour temperature of modified FEL-type, 1000W quartz-halogen lamps has been tested at various colour temperatures to investigate the suitability of these lamps for use as photometric transfer standards. During 60 h of operation at 2856 K, after seasoning for 100 h, the changes of luminous intensity of these lamps were 0,2% to 0,6%, and the changes of colour temperature were less than 2 K. The changes in relative spectral power distributions (normalized at 550 nm) over the 60 h of operation were within 0,5% in the visible region. The FEL-type lamps operated at 2856 K showed satisfactory stability within the limited duration of these tests. Characteristics of the lamps have also been investigated for operation at 2000 K to 3100 K, for reversed polarity, and for long-term storage.

The Tautenburg Schmidt Telescope and Galactic Proper Motion Studies

AbstractThe combination of Tautenburg plates and automatic measuring machines provides a powerful tool to obtain photometry and proper motions of a great number of stars for statistical investigations of our Galaxy. Photographic photometry with an accuracy of about 0.07 mag can be obtained provided two plates of the same colour and a sufficient number of photometric standards are available. With two plate pairs and a 20 years baseline, a proper motion accuracy better than 4 mas/year can be achieved for stars over a wide range of magnitudes. Outside the Galactic plane proper motions are determined with respect to hundreds of background galaxies.

SECONDARY PHOTOMETRIC STANDARD STARS FOR THE THUAN-GUNN AND THE JOHNSON-KRON-COUSINS SYSTEMS

ABSTRACT CCD photometry is presented for 119 stars observed in the Thuan-Gunn photometric system (the passbands vgr). The sample includes 82 stars for which magnitudes in this system have not previously been published. Photometry in the Johnson-Kron-Cousins photometric system (BV or UBV) has been obtained for a sub-sample of 85 of the stars. Of those stars which are not primary standard stars for the Thuan-Gunn system 70 observed in Gunn r, 48 observed in Gunn g, and 37 observed in Gunn v have sufficient repeat measurements that they may be used as secondary standard stars for CCD photometry. The typical uncertainties for the magnitudes of these stars of 0.006m.

CCD photometry of the old open cluster M67

view Abstract Citations (326) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CCD Photometry of the Old Open Cluster Montgomery, Kent A. ; Marschall, Laurence A. ; Janes, Kenneth A. Abstract We present a CCD photometric survey of the central one-half degree of the old open cluster, M67, in U, B, V, and I colors to magnitude V = 20. Extensive comparison of our photometry with other published datasets shows excellent agreement, indicating that CCD photometry is capable of producing a uniform set of measurements consistent with the photometric system defined primarily by the Landolt standard sequence. The color-magnitude diagram of the cluster shows a well-defined main sequence extending at least to the limit of the photometry at M(V) = 10.55 and a substantial binary sequence. At least 38 percent of cluster stars are binaries. The current generation of theoretical isochrones cannot be fit to the observed sequences within the observational errors. We find a tendency for more massive members of the cluster to be more centrally concentrated, along with a turnover in the cluster luminosity function at low masses, which may be due to dynamical relaxation of the cluster. To the limit of our photometry, we find a mass of the cluster of 724 solar masses. In addition, we present a sample of stars of well-determined standard magnitudes that are suitable as photometric standards for further studies of this cluster and for general calibration of UBVI photometry using CCDs. Publication: The Astronomical Journal Pub Date: July 1993 DOI: 10.1086/116628 Bibcode: 1993AJ....106..181M Keywords: Astronomical Photometry; Charge Coupled Devices; Color-Magnitude Diagram; Open Clusters; Sky Surveys (Astronomy); Ubv Spectra; Interstellar Extinction; Main Sequence Stars; Solar Oscillations; Stellar Oscillations; Astrophysics; OPEN CLUSTERS AND ASSOCIATIONS: INDIVIDUAL: M67 full text sources ADS | data products SIMBAD (1441) CDS (1)

Network and Traceability of the Radiometric and Photometric Standards at the PTB

The system of radiometric and photometric standards at the PTB is presented, emphasizing traceability, cross-linking, and future developments. Traceability from the national standards to the respective reference standards of calibration laboratories of the German Calibration Service (DKD) is also included. The future realization of the detector-based spectral irradiance scale resulting in a reduced uncertainty is described.

Infrared Photometric Systems, Standards and Variability

AbstractThe accuracy with which infrared photometry can be carried out is currently limited by poor definition of the instrumental system and by the accuracy of the available standard stars. We present new colour transformations between the CIT J, H, K photometric system and systems currently in use at Cerro Tololo, Mauna Kea and Kitt Peak. The precision of the J, H, K data for some of the stars observed by Elias et al. is improved and the system extended to fainter stars suitable for use with larger telescopes and/or infrared arrays. Evidence of infrared variability has been detected for one M dwarf star in our programme.

VRI photometry of M67 for CCD standardization at 2.3m VBT

We present the results from CCD photometry in theV, R andI bands, of the ‘Dipper Asterism’ region of the open cluster M67 based on observations carried out at the prime focus of the 2.3 m Vainu Bappu Telescope of the Vainu Bappu Observatory, Kavalur. The CCD parameters like the system gain and the readout noise are measured using several flatfield frames taken through the standardI filter. The CCD chip is calibrated using the photometric standards in the field and linear colour transformation relations are derived. Also a few newVRI photometric measurements are reported for the members of the cluster.

A Monte Carlo approach to the analysis of the dust tail of comet P/Halley

A quantitative analysis of three wide-field photographs of Comet P/Halley obtained between 3 and 6 May 1986 has been carried out. The digitally processed maps were used as input for the Monte Carlo approach to interpreting cometary dust tails. The developed model considers the conditions of the ejection of the dust particles from the collision zone of the cometary coma, the time-averaged dust size distribution, and the dependency between the dust production rate and the heliocentric distance. The lack of any photometric standardization has not allowed an absolute value of the dust production rate to be obtained. The use of panchromatic plates without filters may cause ion plasma interference and therefore a certain biasing of the results. The adjustment procedure gives the model parameters in general agreement with the results of the investigations by Fulle et al. (1989, Astron. Astrophys. 201, 362–372) and by Cremonese and Fulle (1989, Icarus 80, 267–279), and the results of in situ experiments. The maximum value of the dust ejection velocity is 0.46 (±0.23) km sec −1 and is achieved generally after the perihelion passage. The dust ejection velocity dependency on the particle radius is very weak with a power index of −0.25 (±0.10). The dust number production rate varies with heliocentric distance as r −3.8(±0.7) and attains its maximum value for the true anomaly 8°. Power indices of the dust size distribution function are −3.22 (±0.36) and −3.71 (±0.22) for particles of radius less than and greater than 7.3 μm, respectively. These values are in good agreement with the time-averaged size distribution obtained for P/Halley by Fulle et al.

IMPROVED CCD STANDARD FIELDS

ABSTRACT We have combined multiple nights of CCD observations of NGC 7790, NGC 7006 and NGC 4147 in order to establish a deep sequence of photometric standards whose errors have been objectively derived through statistical comparisons. In addition to extending these previously studied fields to a fainter limit, we have increased the range in observed color, making the new mean sequences more suitable for deriving photometric transformation coefficients.

Silicon photodiode self-calibration using white light for photometric standards: theoretical analysis

A method is proposed in which the responsivity (A/Im) for the photometric quantity of a silicon photodiode, which is combined with a V(lambda) filter of a known spectral transmittance, can be directly self-calibrated by measuring the surface reflectance and the recombination losses of the photodiode for white light. The derived equation for this method assumes approximations that can cause systematic errors. A computer simulation was made to quantify these errors, and an accuracy of better than 0.2% was predicted. Although experimental validation is necessary, this method is promising as a new, practical method for high-accuracy photometric standards.

白色光を用いたシリコンホトダイオードの自己校正法の理論的検討

Photometric standards can be realized using silicon photodiodes whose absolute spectral responsivity is self-calibrated. However the conventional method required laborious measurements at many wavelengths over the visible region. To simplify such calibration procedures, a new method is investigated in which the responsivity [A/1m] for the photometric quantity of a silicon photodiode, which is combined with a V (λE)-correction filter having a known spectral transmittance, can be self-calibrated by measuring the surface reflectance and the recombination losses of the photodiode for white light. The derived equation for this method requires that the spectral responsivity of the reference detector for surface reflectance measurements be proportional to wavelength. This requirement is approximately fulfilled by using high quality silicon photodiodes, but this approximation causes a systematic error. To clarify this error, a computer analysis has been made using measured spectral data of commercially available silicon photodiodes both for the self-calibration sample and for the reference detector, and of V (λE)-correction filters. The analysis revealed that the systematic error was less than 0.2%, and that this method was promising for simple and accurate realization of the photometric scale.

Comparison of VNIIOFI and CSIRO Photometric and Radiometric Quantities

Soviet and Australian photometric and radiometric standards have been intercompared. Luminous intensity and luminous flux values were compared using lamps, and illuminance, irradiance and spectral responsivity using detectors. Agreement at the 1% level or better was found for these values as determined by the two national laboratories.