Instrumental Polarization Research Articles

Instrumental polarization concerning magnetographic measurements

To eliminate the influence of the instrumental polarization from magnetographic measurements an approximate method is presented, which enables to evaluate with a sufficient high accuracy and practicability the magnetic vector by deriving at first the longitudinal field component and the miscentering effect.

Limits to the 2,695 MHz Circular Polarization of the September Cyg X-3 Flare

THE decay of the first September Cyg X-3 radio outburst was observed at 2,695 MHz (λ 11 cm) with the resurfaced 300-foot meridian transit telescope of the National Radio Astronomy Observatory (operated by Associated Universities Inc. under contract with the National Science Foundation). An on-axis feed system provided both left and right-handed circular polarizations. An instrumental polarization of 1.2±0.5% (left-handed) was determined by observing fourteen extra-galactic sources at declinations between +39° and +42° and assuming these to have zero average circular polarization. Table 1 lists the total flux density and circular polarization measurements; column 3 gives values of the ratio of the Stokes parameters illustration together with their standard errors. Limits to the circular polarization of illustration (right-handed) may be inferred. Open image in new window Open image in new window

Optical Polarization of X Per

IT has been suggested that the variable star X Per may be the X-ray source 2ASE 0352 + 30 (refs. 1 and 2, and R. J. Brucato and J. Kristian, personal communication). Nikulin et al.3 report strong and variable circular polarization of Sco X-1 and HD 226868 (the possible optical counterpart of Cyg X-1), so the polarization of X Per (and its variation) is of interest. One of us (J. T., unpublished thesis) has recently developed a wideband polarization modulator of the Dollfus type4,5. A prototype using plastic retarders was used on a 30-cm Cassegrain reflector to observe the polarization of nearby bright stars, and of ζ Per and X Per. The observations on bright stars and on ζ Per show that instrumental polarization (linear or circular) is certainly less than 0.02% of the total intensity, even in the presence of several % of total intensity in polarization forms other than that being measured. Our bandwidth of approximately 1000 A is centred between 5000 A and 5500 A.

Precise Determinations of the Magnetic Vector in Sunspots

The reality of some physical effects suggested for sunspot magnetic fields is still doubtful because of the low, or unknown, precision of the observations from which the effects are deduced. For the determination of magnetic field configurations Treanor's method, involving micrometer position measurements, is considerably more sensitive and more powerful than the usual methods depending upon line intensity measurements, but its full potential accuracy has not hitherto been achieved. Several modifications have now been made in the techniques of Treanor's method in order to obtain greater precision. The chief change is to convert the polarization-state analyser into a simple spectroscope slit device: this entails an extra mirror reflection before the analysis is made and the corrections for instrumental polarization have accordingly been reconsidered. The theory has now been extended to include the dependence of the corrections upon absorption line strength. The revised procedure has been used for magnetic field determinations across a medium sized spot and the consistency of the results for Fe I lines indicates that the direction of the magnetic vector is now obtained with a mean precision of 2–3°. The spot magnetic field, when the observations have this degree of certainty, is free from the anomalous effects suggested by earlier work.

Magnetic Field Structure around the Crab Pulsar

LINEAR polarization observations of the Crab nebula at optical wavelengths indicate that the magnetic field is uniform in direction over a large region (about 0.5 pc) around the pulsar. Observations were made with a completely automatic sky compensated polarimeter-cum-photometer with an S20 photomultiplier and a GG13 filter. A ‘Fabri-Tek’ computer in the pulse counting mode was used to store data for each degree of rotation of the Polaroid filter during each observation and a digital magnetic tape recorder was used for permanent data storage. A ‘CDC 6400’ computer was then used for direct reduction of the magnetic tapes. A complete description of the instrument and data system will be given in a later publication. Observations were made on November 4 and 5, 1969, and on February 1 and 2, 1970, with the 84 inch telescope at Kitt Peak. Three standard stars, HI 29, HDE 253683 and HDE 251204 (ref. 1), were used to calibrate the observations and an unpolarized star was used to evaluate the instrumental polarization. All angles reported in this paper are measured from north towards east.

The Polarization of Variable Radio Sources at 8 GHz. I. The Observations

view Abstract Citations (71) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Polarization of Variable Radio Sources at 8 GHz. I. The Observations Aller, Hugh D. Abstract A program to monitor the linear polarization of variable radio sources at 8 GHz has been carried out at the University of Michigan since mid-1965. H ii regions and extended nonthermal sources were observed frequently in order to determine the instrumental polarization and to check the stability of the equipment. Five sources exhibited significant changes in polarization: 3C 120, 3C 273, 3C 279, 3C 345, and 3C 454.3; three sources, 3C 84, NRAO 150, and 3C 446 probably have variable polarizations. Changes in both the degree of polarization and the position angle of the polarization have been observed in these sources with time scales as short as 1 week. The most rapid variations in polarization usually occurred when the total flux density was increasing or had reached a maximum value. The amplitudes of the variations in polarized flux density were as great as 10 percent of the amplitudes of the variations in total flux density. Two sources, 3C 279 and 3C 454.3, exhibited large changes in polarized flux density during periods when the total flux densities were not changing significantly. Publication: The Astrophysical Journal Pub Date: July 1970 DOI: 10.1086/150508 Bibcode: 1970ApJ...161....1A full text sources ADS | data products NED (39) SIMBAD (1) Related Materials (1) Part 2: 1970ApJ...161...19A

Variations of Linear Polarization in Extragalactic Radio Sources

SINCE mid-1965 the linear polarizations of the stronger extragalactic variable radio sources have been monitored at 8 GHz using the University of Michigan 85-foot paraboloid equipped with a rotating, linearly polarized feed horn. Strong thermal and non-thermal sources were measured each observing day to determine the stability of the instrumental polarization. Five sources, 3C 120, 3C 273, 30 279, 3C 345 and 3C 454.3, have exhibited significant changes in polarization, and three sources, 3C 84, NRAO 150 and 3C 446, are suspected of having variable polarizations. Preliminary observational results for several of these sources have been reported previously1–2.

Elimination of instrumental polarization from polarization measurements

A method of measuring the polarization of light free from instrumental effects is described. The effect of imperfect polarizing filters is discussed and an equation is derived with which a measured polarization factor may be corrected.

Elimination of Polarization Corrections from Optical Excitation-Function Measurements

Woolsey and McConkey have recently described a method by which the effects of instrumental polarization may be eliminated from optical excitation functions measured perpendicular to the exciting beam. It is shown that their method must be the same as that of Burrows and Dunn. For cases where there is strong instrumental polarization, a superior method is described. Two further methods of eliminating instrumental-polarization effects are suggested, in which the excitation is observed at other angles to the exciting beam. A method of measuring the polarization of impact radiation, free from the effects of instrumental polarization, is described and a typical result is presented.

Elimination of instrument polarization in spectropolarimetric luminescence studies

A method of measuring small degrees of polarization of luminescence emission and excitation in the presence of wavelength-dependent instrument polarization is outlined. The method requires the measurement of radiation intensities with the crystal positioned parallel and perpendicular to a reference direction in a plane perpendicular to the optical axis of the system. The accuracy of the method is considered quantitatively and computed curves are presented to show the range of validity.

Photographic Measures of the Polarization of Starlight in h and Persei

A new photographic method is described for measuring the polarization of starlight using a polaroid filter which allows exposures to be taken at three position angles and simultaneously records brightness differences not due to polarization effects through three control areas. Overlapping of star images limits the use of the method to areas of sky where the star density is approximately |$\leqslant\,6\,\times\,{10}^{3}$| stars per square degree, on average to V ~16 at the galactic equator. About 50 000 individual measures of 527 stars to V ~13 in hand χ Persei were made on 16 fine grain Schmidt plates with a semi-automatic digital iris photometer. The computer programmes used to reduce the data are described. Using the Stokes parameters, comparison with published photoelectric measures of 54 stars gives an r.m.s. deviation of |$\pm\,{0}^\text{m}. 029$| per star. For 53 stars, the mean photoelectric value is |$p\,=\,{0}^\text{m}.083$| at 115 ° and the corresponding photographic mean is |$p\,=\,{0}^\text{m}.080$| at 114 ° , suggesting that any instrumental polarization is less than the accuracy achieved, which is confirmed by some photographic measures at the north galactic pole. Stars with large proper motions have a much smaller amount of polarization at 112 ° , and stars fainter than V = 12 have a rather larger amount of polarization at 114 ° Three new variable stars have been found from the magnitude measures.

The Color Dependence of Interstellar Polarization

Measurements of the Interstellar Polarization of Starlight at different wavelengths have been reported by several observers in references 1 to 8. With a few exceptions the wavelength range of the measurements is still relatively small, and up to now only a few dozens of stars have been measured with high precision. There is still a remarkable discrepancy between the results of different observers, as can be seen in figure 1. The scattering of the individual values is significantly larger than the probable error given by the respective authors. One of the most serious error sources seems to be the instrumental polarization which itself is highly color dependent. (See ref. 9.)Nevertheless, even if a much greater individual error is assumed, a remarkable conclusion can be drawn: the wavelength dependence of polarization is not the same for all stars.

The Wavelength Dependence of Polarization

Preliminary results are described of a program of polarimetry and photometry. Astronomical telescopes and a differential polarimeter are used with calibration by a Lyot depolarizer. The wide-band filters have effective wavelengths between 3250 Å and 9900 Å. The problems of instrumental polarization and depolarization are discussed; most of the instrumental effects can be avoided by proper aluminization of the telescope mirrors. The multiple molecular scattering of the sunlit sky and of the poles of Jupiter is closely represented by the Rayleigh-Chandrasekhar theory; the optical depth at the Jovian poles is 0.4–0.8. The same particle sizes that explain the interstellar reddening, 0.05–0.3 μ, are found from the interstellar polarization. Strong polarization-wavelength dependence is found for Venus and for lunar regions, but the detailed explanations require extension of the wavelength range. The range of wavelengths will be extended by using high-altitude balloons and spacecraft.

The Spatial Analysis of Magnetic Fields in Sunspots

A review of existing methods of determining the strength and direction of magnetic fields in sunspots, as a function of position and depth within the spot shows that these have given fragmentary and somewhat discordant results. In particular, little is known of the direction of the magnetic vector. A new method of determining this quantity is described. By means of a Babinet compensator, polarization fringe systems are produced in the components of a normal Zeeman triplet. From the phase difference of the σ -component fringes, the constants of the elliptical polarization at each point in the spot are determined, and the orientation of the lines of force deduced. The method minimizes the effect of instrumental polarization; the reduction is simple and fairly short. An experimental test on four sunspots has been carried out, using the Oxford 35 m solar tower and 12 m spectrograph with a Babcock grating. Measurements of field direction were made, using the triplet λ 6173 (Fe I ); the associated field strengths were determined from the same line and from λ 6149, a weak line of Fe II . The resulting field-strength variation is in agreement with Broxon's parabolic law, but in contrast with Joy and Nicholson's measures of direction, 40 per cent of the flux is found to be confined within a cone of unit solid angle. Comparisons of field-strength at λ 6173 (Fe I ) and λ 6149 (Fe II ) suggest that the gradient in depth does not exceed 1 oersted/km.

On The Reduction Of Polarization Measures Affected by Instrumental Polarization

The effect of oblique reflection on measures of polarization has been studied. Simple formulae are derived whereby observations of stars whose polarizations are known can be used to calibrate observations subject to oblique reflection. The method is illustrated by Mrs E. van P. Smith's observations. The utility of the quantities p cos 2 θ , p sin 2 θ for the discussion of internal accuracy and comparison of independent series is stressed.

Photoelectric observations of the electron solar corona.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photoelectric observations of the electron solar corona. Wlerick, Gerard ; Axtell, James ; Lee, Robert Abstract The High Altitude Observatory staff has recently installed in Boulder a new instrument designed for the detection without eclipse of the electron solar corona. This component of the corona, often called the K-corona, is strongly polarized, the direction of the polarization being practically radial. It was successfully detected without eclipse in 1930 by B. Lyot at Pic du Midi with a coronagraph and a visual polarimeter able to detect a percentage of polarization as low as 0.1 per cent. Lyot's observations went as far as 0.35 R0 from the limb. The new instrument, called the K-coronameter, has been designed to permit observations at a greater distance from the edge of the sun, of the order of 0.5 to i.o R0. It is a coronagraph supplemented by a sensitive photoelectric polarimeter. The analyzer of polarization consists essentially of an electrically birefringent crystal excited by an a.c. voltage, a polarizer and a Lallemand photomultiplier. The a.c. signal from the multiplier is fed to a narrow band amplifier with synchronous detection. The band width is of the order of a few cycles per second. To reduce as much as possible the instrumental polarization, the coronal observations are always made on the axis. Thus, when scanning around the limb, the axis of the instrument describes a cone. A complete scan, at one given distance from the limb, takes 4 minutes. During the scan, a half-wave plate rotating at the speed of I round/ 8 min. keeps the plane of polarization of the light entering the analyzer in a fixed direction. With a relatively clear sky, Brightness B5 i~-~ B0, it has been possible in Boulder to detect the main features of the corona streamers up to 0.5 R0 and in one case up to o.66 R0. There is generally a good correlation between the brightness of the green coronal emission line measured with a conventional coronograph. It is planned to move the instrument to Climax this summer, at elevation I I 000 feet where better sky conditions prevail, and to make routine observations on clear days during the International Geophysical Year. The principal development work on this instrument was supported by the U. S. National Committee for the I.G.Y. under a grant from the National Science Foundation; a portion was supported by Radio Corporation of America. High Altitude Observatory, Boulder, Colo. Publication: The Astronomical Journal Pub Date: 1957 DOI: 10.1086/107631 Bibcode: 1957AJ.....62R..39W full text sources ADS |

An experimental spaced-loop direction-finder for very high frequencies

A description is given of an experimental spaced-loop direction-finder for use with vertically polarized waves in the frequency range 30-100 Mc/s (10-3 m). The screened loops are 28 cm square and are mounted coaxially with a spacing of 1.57 m. The sensitivity of the system in conjunction with a good commercial receiver is such that a field strength of 20 μV/m is required for a silent swing of ± 5°. Means are provided for selecting the required minima of the polar diagram. Instrumental errors are liable to become excessive at the resonant frequencies of the aerial system or its screen, but may be minimized by careful construction and adjustment of the system. Reciprocal error can be reduced to less than ½ deg at all frequencies in the above range. No satisfactory method exists for measuring any small residual polarization errors of a spaced-loop system, because the curvature of the wavefront from a local transmitter introduces polarization errors which would not be present with a distant transmitter. Errors measured with a local test transmitter at an angle of elevation of 25° with a dipole aerial inclined at 60° to the vertical have been compared with calculated errors due to wavefront curvature alone. It is estimated that there were no instrumental polarization errors in excess of 2 deg in the frequency range 30-100 Mc/s under the above test conditions. A brief account is given of a more sensitive system with loops 92 cm square, designed for a small frequency range around 30 Mc/s (10 m). In the Appendix an analysis is made of the polarization errors due to the proximity of a local test transmitter.