Click here to close this overlay, or press the "Escape" key on your keyboard. Hough 1 , P. Lucas 1 , J.
Click here to close this overlay, or press the "Escape" key on Best dating sites australia 2013 P-2000 Digital Polarimeter keyboard. Hough 1P. Lucas 1J. Bailey 2M. Tamura leute aus bielefeld kennenlernenE. Hirst 4D. Harrison 1and M.
The Astronomical Society of the Pacific. Berliner zeitung er sucht sie by 17 articles. Get permission to re-use this article. Hough et al PASP Buy this article in print. Sign up for new issue notifications. Although the polarimeter can be used for a wide range of astronomy, the very high sensitivity was singles kennenlernen app by the goal of detecting the polarization signature of unresolved extrasolar planets.
It is a differential technique that in principle is not affected by the Earth's atmosphere and is limited only by photon noise. However, in such systems the sensitivity is limited by the precision to which the two beams are calibrated.
By contrast, systems with fast modulators remove the need for high precision by separating the polarized flux from the unpolarized flux and measuring the former as a signal in the time domain. Such direct detections, particularly with color information, can provide information about the planet's albedo and radius and the nature of the scatterers in the planetary atmosphere.
To date, extrasolar planets have been detected mostly by indirect means, principally through radial velocity measurements of the central star or, far less frequently, through reductions in brightness of the star as a planet transits. Recently, Deming et al. To date, there are no detections of the reflected light from planets, and for the hot Jupiters with orbital radii less than 0.
Hence, to study their atmospheres, very high sensitivity observations have to be made in order to separate the reflected light from the direct stellar light. For the simple case of a Lambert sphere, Seager et al.
Using more detailed models, Seager et al. The rest of the paper is structured as follows. The polarimeter, known as PlanetPol, was designed for use on a range of telescopes, mounted at the unfolded Cassegrain so as to minimize telescope polarization. As with any polarimeter, it is important that the first optical element, apart from calibration polarizers, is the modulator, immediately followed by the analyzer.
Following the analyzer are wheels with color and neutral density ND filters. An electromechanical shutter not shown is located between the aperture wheel and the Wollaston prism. PlanetPol has two channels: In practice, singlebörse elsterwerda increased telescope polarization in the sky channel is not a problem, since the sky signals are much lower and hence the polarized flux contribution is very low.
Brief descriptions of PlanetPol are given in Hough et al. There are two identical channels, with only one shown here: The highest sensitivity polarization measurements, such as those pioneered by J. Kemp Kemp et al. Such devices are ideal as polarization modulators, as they operate at frequencies of tens of kHz, well above seeing or scintillation fluctuations produced by turbulence in the Earth's atmosphere.
Furthermore, they do not involve any rotating parts and so do not produce any periodic motion of the image on the detector, nor any periodic fluctuations in signal caused by dust on the modulator, both of which might lead to spurious polarization signals. Another distinct advantage of PEMs is that they can be used in fast beams, thus eliminating the need for any prior optics that could change the polarization state of the incident radiation. All observations reported here were Best dating sites australia 2013 P-2000 Digital Polarimeter at the WHT, where dynamic control maintains the position of the focal plane to better than 0.
Even a 10 mm change in focus leads to a change of only 0 in the angle to the normal of the extreme rays hitting the PEM, giving a change in retardance that is less than 1 part in 10 6.
This is at the bottom end of the frequency range of PEMs, and hence the physical size is relatively large. In addition, the reflection losses for the two beams are not equal, and therefore the two beams do not have the same intensity, even for an unpolarized incident beam.
Again, this is not an important effect Best dating sites australia 2013 P-2000 Digital Polarimeter PlanetPol. All rotations are measured using absolute encoders, with a precision of 0 Avalanche photodiodes APDs produce more noise than the dynode chain in photomultiplier tubes; however, they do Free japanese dating games in english Gabor TB higher quantum efficiency than photocathodes, they are able to handle Best dating sites australia 2013 P-2000 Digital Polarimeter high photon fluxes, and as they have less noise than the external amplifier of a photodiode, they are the best Best dating sites australia 2013 P-2000 Digital Polarimeter for the photon rates expected with PlanetPol.
The active area of the detector has a nominal diameter of 1. These are mounted in two wheels, with six positions for each wheel. The ND filters are from Omega Optical. The number and length of each sequence is chosen depending on the brightness of the source. The fractional polarizations shown are not calibrated.
The DC values, representing the total flux detected by all four detectors, are all output to a fifth file. All five files contain time stamp information at intervals of approximately 20 s. Note the "polarization offsets" see item 4 with the star channel outputs black and blue lines Best dating sites australia 2013 P-2000 Digital Polarimeter from zero.
The fractional polarization is the ratio of the modulated signals to the DC signals shown in the middle panel. The modulated flux from the sky channels green and red traces has been multiplied by 10 here but still appears weaker, and the sky DC flux is too low to measure.
The bottom panel displays the distribution of fractional polarization values about Best dating sites australia 2013 P-2000 Digital Polarimeter mean for the two star Best dating sites australia 2013 P-2000 Digital Polarimeter. This appears approximately Gaussian, with no outlying points. Note that polarization offsets in the sky channels are insignificant, since the sky DC level is usually too low to be measured.
The sky channels exist so that modulated flux from the sky can be measured and removed. We find that modulated sky signals make a significant contribution only in very bright conditions moonlit cirrus clouds or twilight when using a 5 '' aperture. The standard deviation of all data points is then calculated. For completeness, the full equation, e. When using the simple 0. The derivation of the full equation is given in Appendix A.
Most observations with PlanetPol have been made with a very broadband red filter covering the wavelength range from nm to the detector cutoff at about nm. With such a wide band, the exact effective passband and effective wavelength will depend on the color of the object being observed. To account for such effects, we have developed a detailed model for the passband response, which operates as follows. We start with a spectral energy distribution SED appropriate to the source being observed.
For this purpose, we typically use one of the Kurucz stellar atmosphere models. This SED can optionally be modified by applying wie flirtet ein mann per sms interstellar extinction correction using the empirical model of Cardelli et al.
The SED is then corrected for Best dating sites australia 2013 P-2000 Digital Polarimeter passage through the Earth's atmosphere by multiplying by an atmospheric transmission spectrum derived from a radiative transfer model for the Earth's atmosphere.
The Earth atmosphere spectrum is calculated at a resolution that berliner zeitung er sucht sie resolves the molecular absorption lines. This means that it can be straightforwardly adjusted for any air mass, something that is no longer possible when it is binned to a lower resolution. The polarization efficiency correction due to the variation of retardance of the PEM over the bandpass is given by.
The effects of changing the air mass and the water vapor content used in the atmospheric model have also been investigated but are found to be negligible for practical purposes. Changing air mass from 1 to 2 typically shifts the effective wavelength by less than a nanometer and does not change the polarization efficiency.
An example of the response curve model is shown in Figure 4. The following Best dating sites australia 2013 P-2000 Digital Polarimeter describe the performance of PlanetPol, including the measurement of telescope polarization, instrument polarization, and polarized standards from the literature, recorded at the WHT during three runs covering the period April to April.
These runs represent a total of 21 nights' allocated time, with science data being taken on 12 of them. All observations have been made with a 5 2 aperture, and the WHT autoguider was used for all observations. The TP was determined by observing bright nearby stars typically within 25 pc with the telescope derotator enabled, causing the TP to rotate while the interstellar polarization and instrument polarization are fixed. These equations are solved simultaneously for PPA maxand the q i and u i values for all the nearby bright stars.
Variance weighting of each datum is employed again to weight all detected photons equally. The uncertainties in the fitted parameters are not produced by the procedure, but these can be estimated by comparing the results when using only subsets of the data and by Monte Carlo simulations with artificial data. Monte Carlo simulations were christliche partnervermittlung sachsen with 10, artificially generated data sets with Gaussian errors, using the same sets of parallactic angles and random errors as were found in the real data sets.
The results show that the expected standard errors on the fitted parameters for our three runs at the WHT are 0. The fitting procedure does not introduce any systematic errors, at least for the data sets reported here, which sample the range of parallactic angles well.
This demonstrates that the fitting process does not add any noise to the derived values of q i and u i. The standard errors on P and PA max that were derived from these Monte Carlo tests agree well with those derived by comparing the results of fits to independent subsets of the real data.
This is borne out by a larger than expected scatter in the q i and u i values derived for individual stars by fitting subsets of the data. The value of R varies widely for individual stars, since the number of measurements is generally small, but there is no sign that a floor exists in the achievable precision for very bright stars or stars observed a large number of times.
Small values of R are found for both. Note that since IP is independent of parallactic angle, it does not contribute to the expected residuals to the fit. A typical fit gives a TP amplitude of In principle, the procedure would be most accurate if only one or two nearby stars were used, in order to minimize the number of free parameters.
However, in the first few PlanetPol runs, we have used four or five stars in order to 1 allow redundancy in case some did not have constant polarization, and 2 provide a range of spectral types A0 to M0 to determine whether TP changes due to the slight change in effective bandpass. Tests of the fitting procedure with different combinations of stars indicate that TP is independent of spectral type to within measurement error.
In addition, most stars even young debris disk systems appear to have constant polarization over at least the Best dating sites australia 2013 P-2000 Digital Polarimeter of an observing run.
The constant polarization of most stars can be attributed to extinction by aligned interstellar dust grains.
Science highlights with SPHERE
Unlike the five other Galactic redbacks discovered to date, PSR J− has not In , a redback system in the globular cluster (GC). Radio polarimetry as a probe of unresolved jets: the outburst of XTE J + the Library will make the material inaccessible and/or remove it from the website. Download date: 21 Sep Research, Curtin University, GPO Box U, Perth, WA , Australia choux P., Sood R., , A&A, , Spectro-Polarimetric High-contrast Exoplanet REsearch instrument “The polarimetric differential imaging mode of SPHERE works on this principle: the light.