Correcting direction-dependent gains in the deconvolution of radio interferometric images

By S. Bhatnagar, Tim J. Cornwell, K. Golap, J. Usón. 2008

183 citations recorded by the source corpus.

Astronomical imaging using aperture synthesis telescopes requires deconvolution of the point spread function as well as calibration of instrumental and atmospheric effects. In general, such effects are time-variable and vary across the field of view as well, resulting in direction-dependent (DD), time-varying gains. Most existing imaging and calibration algorithms assume that the corruptions are direction independent, preventing even moderate dynamic range full-beam, full-Stokes imaging. We present a general framework for imaging algorithms which incorporate DD errors. We describe as well an iterative deconvolution algorithm that corrects known DD errors due to the antenna power patterns (including errors due to the antenna polarization response) as well as pointing errors for high dynamic range full-beam polarimetric imaging. Using simulations we demonstrate that errors due to realistic primary beams as well as antenna pointing errors will limit the dynamic range of upcoming higher sensitivity instruments like the EVLA and ALMA and that our new algorithm can be used to correct for such errors. We show that the technique described here corrects for effects that can be described as approximate unitary operators in the interferometric measurement equation, such as those due to antenna pointing errors and non-azimuthally symmetric antenna power patterns. We have applied this algorithm to VLA 1.4 GHz observations of a field that contains two “4C” sources and have obtained Stokes I and V images with systematic errors that are one order of magnitude lower than those obtained with conventional imaging tools. Residual systematic errors that are seen at a level slightly above that of the thermal noise are likely due to selfcalibration instabilities that are triggered by a combination of unknown pointing errors and errors in our assumption of the shape of the primary beam of each antenna. We hope to present a more refined algorithm to deal with the fully general case in due course. Our simulations show that on data with no other calibration errors, the algorithm corrects pointing errors as well as errors due to known asymmetries in the antenna pattern.

Subfields: Astronomy and Astrophysics, Environmental Engineering

View Paper: https://www.semanticscholar.org/paper/593dbab61dac908f347118e58178eb250a4daac1

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Citations in the corpus

LOFAR: The LOw-Frequency ARray (2013, methodology): “This imager applies the w-projection algorithm (Cornwell et al. 2008) to remove the effects of noncoplanar baselines when imaging large fields and the new Aprojection algorithm (Bhatnagar et al. 2008) to take into account the varying primary beam during synthesis observations.”
Reionization and Cosmology with 21-cm Fluctuations (2009, methodology): “The interested reader is referred to the review by Rau et al. (2009), the VLA summer school proceedings by Taylor, Carilli & Perley (1999), and the papers by Bhatnagar et al. (2008), Cornwell, Golap & Bhatnagar (2008) and Morales & Matejek (2009).”
wsclean: an implementation of a fast, generic wide-field imager for radio astronomy (2014, methodology): “To reach high dynamic ranges, it can be necessary to deal with direction-dependent effects (DDEs).”
An optimized algorithm for multiscale wideband deconvolution of radio astronomical images (2017, background): “…hundreds of times, which is infeasible for large data sets (without significant parallelization and distribution efforts); ii) the so-called w-term (Perley 1999) or beam and ionospheric effects (Bhatnagar et al. 2008) that break the assumption of a spatially in- c© 2016 The Authors ar X iv :1 70 6.”
Revisiting the radio interferometer measurement equation. I. A full-sky Jones formalism (2011, methodology): “This is mathematically fully equivalent to the 2× 2 form introduced by Hamaker (2000) in the fourth paper, and has since been adopted by many authors (Noordam 1996; Thompson et al. 2001; Bhatnagar et al. 2008; Rau et al. 2009).”
The Murchison Widefield Array: The Square Kilometre Array Precursor at Low Radio Frequencies (2012, background): “…corrections at the sample level is possible if one incorporates primary beam information into the visibility gridding kernels, an approach that is being investigated by a number of groups (Myers et al. 2003; Bhatnagar et al. 2008; Morales & Matejek 2009; Smirnov 2011; Mitchell et al. 2012).”
New Limits on 21cm EoR From PAPER-32 Consistent with an X-Ray Heated IGM at z=7.7 (2013, background): “Each measured visibility in a wide-field interferometer represents the integral over a kernel in the uv-plane that reflects the primary beam of the elements (Bhatnagar et al. 2008; Morales & Matejek 2009) and the w component of the baseline (Cornwell et al. 2003).”
Ionospheric calibration of low frequency radio interferometric observations using the peeling scheme I. Method description and first results (2009, background): “…ionospheric phase errors after SPAM calibration, but also residual RFI, collective sidelobe noise from many non-deconvolved sources (too faint or outside the FoV) and variable source amplitude errors (e.g., due to pointing errors and non-circular antenna beam patterns; see Bhatnagar et al. 2008).”
FIRST SEASON MWA EOR POWER SPECTRUM RESULTS AT REDSHIFT 7 (2016, methodology): “…gridded using the primary beam as the gridding kernel, placing the data in the “holographic frame,” which has been shown to be the optimal weighting scheme for combining images in the sense that it preserves all information for parameter estimation (Morales & Matejek 2009; Bhatnagar et al. 2008).”
CASA, the Common Astronomy Software Applications for Radio Astronomy (2022, background): “A-Projection is an example of an advanced gridding algorithm in CASA, which can correct for complex widefield (Bhatnagar et al. 2008) and wideband (Bhatnagar et al. 2013) effects (see also Tasse et al. 2013).”