scholarly journals Calibration of the instrumental polarization effects of SCExAO-CHARIS’ spectropolarimetric mode

2020 ◽  
Author(s):  
Rob G. van Holstein ◽  
Steven Bos ◽  
Jasper Ruigrok ◽  
Julien Lozi ◽  
Olivier Guyon ◽  
...  
Keyword(s):  
Polarization Effects ◽  
Instrumental Polarization

scholarly journals The Radio Lobes of Virgo A

1996 ◽  
Vol 175 ◽  
pp. 343-344
Author(s):  
H. Rottmann ◽  
K.-H. Mack ◽  
U. Klein ◽  
R. Wielebinski ◽  
N. Kassim ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Faraday Rotation ◽  
Linear Polarization ◽  
Dynamic Range ◽  
Low Frequency ◽  
Spectral Indices ◽  
Frequency Data ◽  
Polarization Effects ◽  
Instrumental Polarization

In the framework of our multi-frequency study of Virgo A we have performed observations of Vir A at 10.55 GHz with the Effelsberg 100-m telescope. Using our improved CLEAN procedure for single dish data we have increased the dynamic range to some 40 dB.By applying our newly developed polarization CLEANing technique we are able to diminish instrumental polarization effects. Since Faraday rotation is negligible at λ2.8 cm the measured linear polarization is a direct trace of the projected magnetic field in Vir A. In combination with low-frequency data obtained with the VLA it is possible to determine parameters like spectral indices, break frequencies, and spectral ages.

scholarly journals Polarimetric imaging mode of VLT/SPHERE/IRDIS

2020 ◽  
Vol 633 ◽  
pp. A64 ◽  
Author(s):  
R. G. van Holstein ◽  
J. H. Girard ◽  
J. de Boer ◽  
F. Snik ◽  
J. Milli ◽  
...  
Keyword(s):  
Linear Polarization ◽  
Data Reduction ◽  
Near Infrared ◽  
Circumstellar Disks ◽  
Correction Method ◽  
Polarization Effects ◽  
Very Large Telescope ◽  
Instrumental Polarization ◽  
Imaging Polarimetry ◽  
Data Reduction Method

Context. Circumstellar disks and self-luminous giant exoplanets or companion brown dwarfs can be characterized through direct-imaging polarimetry at near-infrared wavelengths. SPHERE/IRDIS at the Very Large Telescope has the capabilities to perform such measurements, but uncalibrated instrumental polarization effects limit the attainable polarimetric accuracy. Aims. We aim to characterize and correct the instrumental polarization effects of the complete optical system, that is, the telescope and SPHERE/IRDIS. Methods. We created a detailed Mueller matrix model in the broadband filters Y, J, H, and Ks and calibrated the model using measurements with SPHERE’s internal light source and observations of two unpolarized stars. We developed a data-reduction method that uses the model to correct for the instrumental polarization effects, and applied it to observations of the circumstellar disk of T Cha. Results. The instrumental polarization is almost exclusively produced by the telescope and SPHERE’s first mirror and varies with telescope altitude angle. The crosstalk primarily originates from the image derotator (K-mirror). At some orientations, the derotator causes severe loss of signal (> 90% loss in the H- and Ks-band) and strongly offsets the angle of linear polarization. With our correction method we reach, in all filters, a total polarimetric accuracy of ≲0.1% in the degree of linear polarization and an accuracy of a few degrees in angle of linear polarization. Conclusions. The correction method enables us to accurately measure the polarized intensity and angle of linear polarization of circumstellar disks, and is a vital tool for detecting spatially unresolved (inner) disks and measuring the polarization of substellar companions. We have incorporated the correction method in a highly-automated end-to-end data-reduction pipeline called IRDAP, which we made publicly available online.

Preliminary estimate of instrumental polarization effects in the LST-3 Large Solar Telescope

2020 ◽  
Vol 87 (12) ◽  
pp. 705
Author(s):  
D. E. Kukushkin ◽  
A. R. Belan ◽  
A. V. Bakholdin ◽  
D. Yu. Kolobov ◽  
S. A. Chuprakov ◽  
...  
Keyword(s):  
Solar Telescope ◽  
Preliminary Estimate ◽  
Polarization Effects ◽  
Instrumental Polarization

scholarly journals VLA Images of Two Extended Radio Galaxies

2002 ◽  
Vol 199 ◽  
pp. 193-194
Author(s):  
W. Junor ◽  
F. Mantovani ◽  
R. Morganti ◽  
L. Padrielli
Keyword(s):  
Radio Source ◽  
Low Frequency ◽  
Detection Limits ◽  
Radio Galaxies ◽  
Radio Structure ◽  
Polarization Effects ◽  
Compact Component ◽  
Instrumental Polarization ◽  
Extended Radio

There is some evidence from earlier studies that the two sources 0235 — 197 and 1203 + 043 exhibit low frequency (< 1 GHz) variability. This work shows that both sources have linear polarizations, if any, below the detection limits at 320 MHz, so we cannot explain the variability as being due to instrumental polarization effects as has been suggested for 3C159. Refractive scintillation may be the cause of the variability in 0235—197. The radio source 1203+043 lacks any bright compact component thereby ruling out a refractive scintillation mechanism for its variability. Consequently, it is possible that claims of variability in this source are spurious. However, the 320 MHz VLA observations show that 1203+043 has an ‘X'-shaped radio structure.

scholarly journals Polarimetric imaging mode of VLT/SPHERE/IRDIS

2020 ◽  
Vol 633 ◽  
pp. A63 ◽  
Author(s):  
J. de Boer ◽  
M. Langlois ◽  
R. G. van Holstein ◽  
J. H. Girard ◽  
D. Mouillet ◽  
...  
Keyword(s):  
Linear Polarization ◽  
Data Reduction ◽  
Correction Method ◽  
High Contrast ◽  
Polarization Effects ◽  
Instrumental Polarization ◽  
Polarimetric Imaging ◽  
Imaging Mode ◽  
Instrument Configuration

Context. Polarimetric imaging is one of the most effective techniques for high-contrast imaging and for the characterization of protoplanetary disks, and it has the potential of becoming instrumental in the characterization of exoplanets. The Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument installed on the Very Large Telescope (VLT) contains the InfraRed Dual-band Imager and Spectrograph (IRDIS) with a dual-beam polarimetric imaging (DPI) mode, which offers the capability of obtaining linear polarization images at high contrast and resolution. Aims. We aim to provide an overview of the polarimetric imaging mode of VLT/SPHERE/IRDIS and study its optical design to improve observing strategies and data reduction. Methods. For H-band observations of TW Hydrae, we compared two data reduction methods that correct for instrumental polarization effects in different ways: a minimization of the “noise” image (Uϕ), and a correction method based on a polarimetric model that we have developed, as presented in Paper II of this study. Results. We use observations of TW Hydrae to illustrate the data reduction. In the images of the protoplanetary disk around this star, we detect variability in the polarized intensity and angle of linear polarization that depend on the pointing-dependent instrument configuration. We explain these variations as instrumental polarization effects and correct for these effects using our model-based correction method. Conclusions. The polarimetric imaging mode of IRDIS has proven to be a very successful and productive high-contrast polarimetric imaging system. However, the instrument performance is strongly dependent on the specific instrument configuration. We suggest adjustments to future observing strategies to optimize polarimetric efficiency in field-tracking mode by avoiding unfavorable derotator angles. We recommend reducing on-sky data with the pipeline called IRDAP, which includes the model-based correction method (described in Paper II) to optimally account for the remaining telescope and instrumental polarization effects and to retrieve the true polarization state of the incident light.

Enhanced polarization effects in chiral composites

1994 ◽  
Vol 4 (12) ◽  
pp. 2601-2607 ◽  
Author(s):  
A. H. Sihvola
Keyword(s):  
Polarization Effects
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