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SPHERE/ZIMPOL high resolution polarimetric imager
摘要： Context. The SPHERE “planet finder” is an extreme adaptive optics (AO) instrument for high resolution and high contrast observations at the Very Large Telescope (VLT). We describe the Zurich Imaging Polarimeter (ZIMPOL), the visual focal plane subsystem of SPHERE, which pushes the limits of current AO systems to shorter wavelengths, higher spatial resolution, and much improved polarimetric performance. Aims. We present a detailed characterization of SPHERE/ZIMPOL which should be useful for an optimal planning of observations and for improving the data reduction and calibration. We aim to provide new benchmarks for the performance of high contrast instruments, in particular for polarimetric differential imaging. Methods. We have analyzed SPHERE/ZIMPOL point spread functions (PSFs) and measure the normalized peak surface brightness, the encircled energy, and the full width half maximum (FWHM) for different wavelengths, atmospheric conditions, star brightness, and instrument modes. Coronagraphic images are described and the peak flux attenuation and the off-axis flux transmission are determined. Simultaneous images of the coronagraphic focal plane and the pupil plane are analyzed and the suppression of the diffraction rings by the pupil stop is investigated. We compared the performance at small separation for different coronagraphs with tests for the binary α Hyi with a separation of 92 mas and a contrast of Δm ≈ 6m. For the polarimetric mode we made the instrument calibrations using zero polarization and high polarization standard stars and here we give a recipe for the absolute calibration of polarimetric data. The data show small (<1 mas) but disturbing differential polarimetric beam shifts, which can be explained as Goos-H?nchen shifts from the inclined mirrors, and we discuss how to correct this effect. The polarimetric sensitivity is investigated with non-coronagraphic and deep, coronagraphic observations of the dust scattering around the symbiotic Mira variable R Aqr. Results. SPHERE/ZIMPOL reaches routinely an angular resolution (FWHM) of 22?28 mas, and a normalized peak surface brightness of SB0 ? mstar ≈ ?6.5m arcsec?2 for the V-, R- and I-band. The AO performance is worse for mediocre (?1.0″) seeing conditions, faint stars mR ? 9m, or in the presence of the “low wind” effect (telescope seeing). The coronagraphs are effective in attenuating the PSF peak by factors of >100, and the suppression of the diffracted light improves the contrast performance by a factor of approximately two in the separation range 0.06″?0.20″. The polarimetric sensitivity is Δp < 0.01% and the polarization zero point can be calibrated to better than Δp ≈ 0.1%. The contrast limits for differential polarimetric imaging for the 400 s I-band data of R Aqr at a separation of ρ = 0.86″ are for the surface brightness contrast SBpol(ρ)?mstar ≈ 8m arcsec?2 and for the point source contrast mpol(ρ)?mstar ≈ 15m and much lower limits are achievable with deeper observations. Conclusions. SPHERE/ZIMPOL achieves imaging performances in the visual range with unprecedented characteristics, in particular very high spatial resolution and very high polarimetric contrast. This instrument opens up many new research opportunities for the detailed investigation of circumstellar dust, in scattered and therefore polarized light, for the investigation of faint companions, and for the mapping of circumstellar Hα emission.
关键词： circumstellar matter,instrumentation: adaptive optics,planetary systems,instrumentation: polarimeters,instrumentation: high angular resolution,instrumentation: detectors
更新于2025-09-23 15:23:52
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WIRC+Pol: A Low-resolution Near-infrared Spectropolarimeter
摘要： WIRC+Pol is a newly commissioned low-resolution (R～100), near-infrared (J and H bands) spectropolarimetry mode of the Wide-?eld InfraRed Camera (WIRC) on the 200 inch Hale Telescope at Palomar Observatory. The instrument utilizes a novel polarimeter design based on a quarter-wave plate and a polarization grating (PG), which provides full linear polarization measurements (Stokes I, Q, and U) in one exposure. The PG also has high transmission across the J and H bands. The instrument is situated at the prime focus of an equatorially mounted telescope. As a result, the system only has one re?ection in the light path providing minimal telescope induced polarization. A data reduction pipeline has been developed for WIRC+Pol to produce linear polarization measurements from observations. WIRC+Pol has been on-sky since 2017 February. Results from the ?rst year commissioning data show that the instrument has a high dispersion ef?ciency as expected from the polarization grating. We demonstrate the polarimetric stability of the instrument with rms variation at 0.2% level over 30 minutes for a bright standard star (J=8.7). While the spectral extraction is photon noise limited, polarization calibration between sources remain limited by systematics, likely related to gravity dependent pointing effects. We discuss instrumental systematics we have uncovered in the data, their potential causes, along with calibrations that are necessary to eliminate them. We describe a modulator upgrade that will eliminate the slowly varying systematics and provide polarimetric accuracy better than 0.1%.
关键词： methods: observational,instrumentation: polarimeters,methods: data analysis,techniques: polarimetric
更新于2025-09-23 15:22:29
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New constraints on the dust surrounding HR 4796A
摘要： Context.HR 4796A is surrounded by a well-structured and very bright circumstellar disc shaped like an annulus with many interesting features: very sharp inner and outer edges, brightness asymmetries, centre o?set, and suspected distortions in the ring. Aims. We aim to constrain the properties of the dust surrounding the star HR 4796A, in particular the grain size and composition. We also want to con?rm and re?ne the morphological parameters derived from previous scattered light observations, and reveal the dust spatial extent in regions unexplored so far due to their proximity to the star. Methods. We have obtained new images in polarised light of the binary system HR 4796A and B in the Ks and L(cid:3) band with the NaCo instrument at the Very Large Telescope (VLT). In addition, we revisit two archival data sets obtained in the L(cid:3) band with that same instrument and at 2.2 μm with the NICMOS instrument on the Hubble Space Telescope. We analyse these observations with simulations using the radiative transfer code MCFOST to investigate the dust properties. We explore a grid of models with various dust compositions and sizes in a Bayesian approach. Results. We detect the disc in polarised light in the Ks band and reveal for the ?rst time the innermost regions down to 0.3(cid:3)(cid:3) along the semi-minor axis. We measure a polarised fraction of 29% ± 8% in the two disc ansae, with a maximum occurring more than 13? westwards from the ansae. A very pronounced brightness asymmetry between the north-west and south-east side is detected. This contradicts the asymmetry previously reported in all images of the disc in unpolarised light at wavelengths smaller than or equal to 2.2 μm and is inconsistent with the predicted scattered light from spherical grains using the Mie theory. Our modelling suggests the north-west side is most likely inclined towards the Earth, contrary to previous conclusions. It shows that the dust is composed of porous aggregates larger than 1 μm.
关键词： scattering,stars: individual: HR 4796A,planetary systems,instrumentation: polarimeters,instrumentation: high angular resolution
更新于2025-09-23 15:21:21
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The EBEX Balloon-borne Experiment—Detectors and Readout
摘要： EBEX was a long-duration balloon-borne experiment to measure the polarization of the cosmic microwave background. The experiment had three frequency bands centered at 150, 250, and 410 GHz and was the first to use a kilopixel array of transition edge sensor bolometers aboard a balloon platform. We describe the design and characterization of the array and the readout system. From the lowest to highest frequency, the median measured detectors' average thermal conductances were 39, 53, and 63 pW/K, the medians of transition temperatures were 0.45, 0.48, and 0.47 K, and the medians of normal resistances were 1.9, 1.5, and 1.4 Ω; we also give the measured distributions. With the exception of the thermal conductance at 150 GHz, all measured values are within 30% of their design. We measure median low-loop-gain time constants τ0 = 88, 46, and 57 ms. Two measurements of bolometer absorption efficiency gave results consistent within 10% and showing high (～0.9) efficiency at 150 GHz and medium (～0.35 and ～0.25) efficiency at the two higher bands. We measure a median total optical power absorbed of 3.6, 5.3, and 5.0 pW. EBEX pioneered the use of the digital version of the frequency domain multiplexing system. We multiplexed the bias and readout of 16 bolometers onto two wires. The median per-detector noise-equivalent temperatures are 400, 920, and 14,500 mK. We compare these values to our preflight predictions and to a previous balloon payload. We discuss the sources of excess noise and the path for a future payload to make full use of the balloon environment.
关键词： cosmology: observations,instrumentation: polarimeters,cosmic background radiation,balloons,instrumentation: detectors
更新于2025-09-23 15:21:01
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The EBEX Balloon-borne Experiment—Optics, Receiver, and Polarimetry
摘要： The E and B Experiment (EBEX) was a long-duration balloon-borne cosmic microwave background (CMB) polarimeter that flew over Antarctica in 2012. We describe the experiment’s optical system, receiver, and polarimetric approach and report on their in-flight performance. EBEX had three frequency bands centered on 150, 250, and 410 GHz. To make efficient use of limited mass and space, we designed a 115 cm2 sr high-throughput optical system that had two ambient temperature mirrors and four antireflection-coated polyethylene lenses per focal plane. All frequency bands shared the same optical train. Polarimetry was achieved with a continuously rotating achromatic half-wave plate (AHWP) that was levitated with a superconducting magnetic bearing (SMB). This is the first use of an SMB in astrophysics. Rotation stability was 0.45% over a period of 10 hr, and angular position accuracy was 0°.01. The measured modulation efficiency was above 90% for all bands. To our knowledge the 109% fractional bandwidth of the AHWP was the broadest implemented to date. The receiver, composed of one lens and the AHWP at a temperature of 4 K, the polarizing grid and other lenses at 1 K, and the two focal planes at 0.25 K, performed according to specifications, giving focal plane temperature stability with a fluctuation power spectrum that had a 1/f knee at 2 mHz. EBEX was the first balloon-borne instrument to implement technologies characteristic of modern CMB polarimeters, including high-throughput optical systems, and large arrays of transition edge sensor bolometric detectors with multiplexed readouts.
关键词： cosmology: observations,instrumentation: polarimeters,cosmic background radiation,polarization,balloons
更新于2025-09-23 15:21:01
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ASTROPOP: the ASTROnomical POlarimetry and Photometry Pipeline
摘要： We developed a new pure-Python pipeline to reduce photometric and polarimetric data: ASTROPOP. It has been designed and optimized to work fully automated with the IAGPOL polarimeter of Pico dos Dias Observatory (OPD; Brazil) and can reduce photometry and polarimetry data from other instruments, especially from SPARC4, a multi-channel polarimeter that has been developed for OPD. We present the results produced by this new code, and compare them with those obtained from PCCDPACK, a traditionally used IRAF package developed for IAGPOL. We also propose to use this code for automatic photometric reduction for the new ROBO40 telescope, also installed at OPD. ASTROPOP is fully open source and distributed under the BSD-3 clause license.
关键词： instrumentation: polarimeters,methods: observational,techniques: polarimetric,techniques: photometric,techniques: image processing
更新于2025-09-23 15:19:57
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Feasibility of spectro-polarimetric characterization of exoplanetary atmospheres with direct observing instruments
摘要： Context. Spectro-polarimetry of reflected light from exoplanets is anticipated to be a powerful method for probing atmospheric composition and structure. Aims. We aim to evaluate the feasibility of the search for a spectro-polarimetric feature of water vapor using a high-contrast polarimetric instrument on a 30–40 m-class ground-based telescope. Methods. Three types of errors are considered: (a) errors from the difference between efficiencies for two orthogonally polarized states; (b) errors caused by speckle noises; and (c) errors caused by photon noise from scattered starlight. Using the analytically derived error formulas, we estimate the number of planets for which feasible spectro-polarimetric detection of water vapor is possible. Results. Our calculations show that effective spectro-polarimetric searches for water vapor are possible for 5 to 14 known planets. Spectro-polarimetric characterization of exoplanetary atmospheres is feasible with an extremely large telescope and a direct observing spectro-polarimeter.
关键词： instrumentation: polarimeters,techniques: polarimetric,planetary systems
更新于2025-09-19 17:15:36
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Characterization of Cameras for the COSMO K-coronagraph
摘要： Digital image sensors are ubiquitous in astronomical instrumentation and it is well known that they suffer from issues that must be corrected for data to be scienti?cally useful. I present discussion on errors resulting from digitization and characterization of nonlinearity and ADC errors of the PhotonFocus MV-D1024E cameras selected for the K-coronagraph of the Coronal Solar Magnetism Observatory. I derive an analytic expression for quantization errors. The MV-D1024E camera has adequate bit depth for which quantization error is not an issue. I show that this is not the case for all cameras, particularly those with deep wells and low read noise. The impact of nonlinearity and ADC errors on science observations of the K-coronagraph is analyzed using a simpli?ed telescope model. Errors caused by the camera ADCs result in systematic errors in the measurement of the polarimetric signal of several times 10?9?Be, which is about an order of magnitude above the desired sensitivity. I demonstrate a method for post-facto data correction using a lookup table and derive parameters from camera characterization measurements that were made with a lab setup. Nonlinearity is traditionally addressed with a global correction. I show through analysis of calibration data that for the MV-D1024E this correction leaves residual systematic errors after dark and gain correction of up to 1% of the signal. I demonstrate that a pixel-wise correction of nonlinearity reduces the errors to below 0.1%. These corrections are necessary for the K-coronagraph data products to meet the science requirements. They have been implemented in the instrument data acquisition system and data reduction pipeline. While no other instruments besides the K-coronagraph or cameras besides the MV-D1024E are discussed here, the results are illustrative for all instruments and cameras.
关键词： instrumentation: detectors – instrumentation: polarimeters
更新于2025-09-04 15:30:14
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Instrumental polarisation at the Nasmyth focus of the E-ELT
摘要： The ～39-m European Extremely Large Telescope (E-ELT) will be the largest telescope ever built. This makes it particularly suitable for sensitive polarimetric observations, as polarimetry is a photon-starved technique. However, the telescope mirrors may severely limit the polarimetric accuracy of instruments on the Nasmyth platforms by creating instrumental polarisation and/or modifying the polarisation signal of the object. In this paper we characterise the polarisation e?ects of the two currently considered designs for the E-ELT Nasmyth ports as well as the e?ect of ageing of the mirrors. By means of the Mueller matrix formalism, we compute the response matrices of each mirror arrangement for a range of zenith angles and wavelengths. We then present two techniques to correct for these e?ects that require the addition of a modulating device at the “polarisation-free” intermediate focus that acts either as a switch or as a part of a two-stage modulator. We ?nd that the values of instrumental polarisation, Stokes transmission reduction and cross-talk vary signi?cantly with wavelength, and with pointing, for the lateral Nasmyth case, often exceeding the accuracy requirements for proposed polarimetric instruments. Realistic ageing e?ects of the mirrors after perfect calibration of these e?ects may cause polarimetric errors beyond the requirements. We show that the modulation approach with a polarimetric element located in the intermediate focus reduces the instrumental polarisation e?ects down to tolerable values, or even removes them altogether. The E-ELT will be suitable for sensitive and accurate polarimetry, provided frequent calibrations are carried out, or a dedicated polarimetric element is installed at the intermediate focus.
关键词： instrumentation: polarimeters,techniques: polarimetric,telescopes,polarization,methods: analytical
更新于2025-09-04 15:30:14