The RoboPol sample of optical polarimetric standards

Blinov, D.; Maharana, S.; Bouzelou, F.; Casadio, C.; Gjerløw, E.; Jormanainen, J.; Kiehlmann, S.; Kypriotakis, J. A.; Liodakis, I.; Mandarakas, N.; Markopoulioti, L.; Panopoulou, G. V.; Pelgrims, V.; Pouliasi, A.; Romanopoulos, S.; Skalidis, R.; Anche, R. M.; Angelakis, E.; Antoniadis, J.; Medhi, B. J.; Hovatta, T.; Kus, A.; Kylafis, N.; Mahabal, A.; Myserlis, I.; Paleologou, E.; Papadakis, I.; Pavlidou, V.; Papamastorakis, I.; Pearson, T. J.; Potter, S. B.; Ramaprakash, A. N.; Readhead, A. C. S.; Reig, P.; Słowikowska, A.; Tassis, K.; Zensus, J. A.

doi:10.1051/0004-6361/202346778

Cited by 8 publications

(4 citation statements)

References 33 publications

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“…In the K band, Wilking et al (1980) found p(K ) = 0.0133 ± 0.0005, and Bailey & Hough (1982) found p(K ) = 0.0113 ± 0.0003. Blinov et al (2023) report substantial variations of U/I and Q/I over 2400 days of observations.…”

Section: » mentioning

confidence: 91%

Sensitivity of Polarization to Grain Shape. I. Convex Shapes

Draine

2024

ApJ

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Aligned interstellar grains produce polarized extinction (observed at wavelengths from the far-ultraviolet to the mid-infrared) and polarized thermal emission (observed at far-infrared and submm wavelengths). The grains must be quite nonspherical, but the actual shapes are unknown. The relative efficacy for aligned grains to produce polarization at optical versus infrared wavelengths depends on particle shape. The discrete dipole approximation is used to calculate polarization cross sections for 20 different convex shapes, for wavelengths from 0.1 to 100 μm, and grain sizes a eff from 0.05 to 0.3 μm. Spheroids, cylinders, square prisms, and triaxial ellipsoids are considered. Minimum aspect ratios required by the observed starlight polarization are determined. Some shapes can also be ruled out because they provide too little or too much polarization at far-infrared and submm wavelengths. The ratio of 10 μm polarization to integrated optical polarization is almost independent of grain shape, varying by only ±8% among the viable convex shapes; thus, at least for convex grains, uncertainties in grain shape cannot account for the discrepancy between predicted and observed 10 μm polarization toward Cyg OB2-12.

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Section: » mentioning

confidence: 91%

Sensitivity of Polarization to Grain Shape. I. Convex Shapes

Draine

2024

ApJ

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“…The observations were conducted in Sloan Digital Sky Survey (SDSS)-r ′ and Johnsons-Cousins R bands. Observations of both bands were considered together, as we verified with standard stars measurements that these filters are essentially equivalent (Blinov et al 2023). The selection of these filters was also informed by the fact that the upcoming PASIPHAE survey will be operating in SDSS-r ′ .…”

Section: Data Aquisition and Reduction: Northmentioning

confidence: 99%

“…The data reduction and calibration were performed with the standard RoboPol pipeline, as outlined in King et al (2014); Panopoulou et al (2015); Blinov et al (2021). Uncertainties of the systematic error are on the order of ∼0.1% in the mask of the instrument (Blinov et al 2023). We note that these uncertainties concern measurements of individual targets.…”

Section: Data Aquisition and Reduction: Northmentioning

confidence: 99%

“…To date, the development of optical polarimetry instrumentation has primarily relied on the creation of instruments tailored for single-source observations or with limited field-of-view (FoV) capabilities. The calibration process for these instruments typically involves utilizing polarimetric standard stars (e.g., Blinov et al 2023). When there is a need for calibrating the entire FoV, calibration methods often employ techniques such as rastering, where a star of known polarization is placed in different positions of the CCD to assess the instrument-induced polarization variations accross the field (e.g., King et al 2014).…”

Section: Introductionmentioning

confidence: 99%

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Zero-polarization candidate regions for the calibration of wide-field optical polarimeters

Mandarakas,

Panopoulou,

Pelgrims

et al. 2024

A&A

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The calibration of optical polarimeters relies on the use of stars with negligible polarization (i.e., unpolarized standard stars) for determining the instrumental polarization zero point. For wide-field polarimeters, calibration is often done by imaging the same star over multiple positions in the field of view (FoV), which is a time-consuming process. A more effective technique is to target fields containing multiple standard stars. While this method has been used for fields with highly polarized stars, there are no such sky regions with well measured unpolarized standard stars. We aim to identify sky regions with tens of stars exhibiting negligible polarization that are suitable for a zero-point calibration of wide-field polarimeters. We selected stars in regions with extremely low reddening, located at high Galactic latitudes. We targeted four $ 40'$ fields in the northern and eight in the southern equatorial hemispheres. Observations were carried out at the Skinakas Observatory and the South African Astronomical Observatory. We found two fields in the north and seven in the south characterized by a mean polarization lower than $p<0.1<!PCT!>$. At least 9 out of the 12 fields can be used for a zero-point calibration of wide-field polarimeters.

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Bright-Moon sky as a wide-field linear Polarimetric flat source for calibration

Maharana,

Kiehlmann,

Blinov

et al. 2023

A&A

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Context. Next-generation wide-field optical polarimeters such as Wide-Area Linear Optical Polarimeters (WALOPs) have a field of view (FoV) of tens of arcminutes. Wide-field polarimetric flat sources are essential to the efficient and accurate calibration of these instruments. However, no established wide-field polarimetric standard or flat sources exist at present. Aims. This study tests the feasibility of using the polarized sky patches of the size of around 10 × 10 arcminutes2, at a distance of up to 20° from the Moon, on bright-Moon nights as a wide-field linear polarimetric flat source. Methods. We observed 19 patches of the sky adjacent to the bright-Moon with the RoboPol instrument in the SDSS-r broadband filter. These patches were observed on five nights within two days of the full-Moon across two RoboPol observing seasons. Results. We find that for 18 of the 19 patches, the uniformity in the measured normalized Stokes parameters q and u is within 0.2%, with 12 patches exhibiting uniformity within 0.07% or better for both q and u simultaneously, making them reliable and stable wide-field linear polarization flats. Conclusions. We demonstrate that the sky on bright-Moon nights is an excellent wide-field linear polarization flat source. Various combinations of the normalized Stokes parameters q and u can be obtained by choosing suitable locations of the sky patch with respect to the Moon.

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The RoboPol sample of optical polarimetric standards

Cited by 8 publications

References 33 publications

Sensitivity of Polarization to Grain Shape. I. Convex Shapes

Sensitivity of Polarization to Grain Shape. I. Convex Shapes

Zero-polarization candidate regions for the calibration of wide-field optical polarimeters

Bright-Moon sky as a wide-field linear Polarimetric flat source for calibration

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