Daily Image
23-07-2014
The Standalone Selfcal Tool (SST)
| Submitter: | Nicolas Vilchez |
| Description: | This image(*) of a LOFAR HBA observation centrered on J1441+1331 was generated by the Standalone Selfcal Tool (SST) that is part of the LOFAR imaging pipeline. It is one of those tools that does not presume to reach the noise, but achieves a considerable improvement, efficiently and robustly. Self-calibration solves for instrumental errors by comparing the observed data with values that are predicted by a model of the observed field. It is an iterative process, in which the sky model is updated in each cycle, until some convergence criterion is met. In addition, the SST also determines what is the best resolution available in the data, and it will start at 15 times the estimated value. At each cycle (the number of cycles is an input parameter defined by the user), the resolution is improved and finally converges, cycle by cycle, to the best resolution. Each cycle contains 3 steps: The SST generally improves the image quality by a factor 6 to 10. The final noise is typically 5-6 times the thermal noise. For this image, we used the SST on 14 groups of 20 subbands each. The resulting 14 images were then combined into one continuum image with a total bandwidth of 280 HBA subbands, or 56 MHz (115 MHz to 171 MHz). The final spatial resolution is 5 arcsec. The final noise is 0.5mJy while the thermal noise (confusing noise) is 0.2 mJy. The target object (J1441+1331) is at the center of the image, and the FOV is 4°x1.6°. In future improvements, we plan to implement directional phase calibration, and to take taking account of ionospheric effects, especially for long baselines. (*) An entry into the recent ASTRON astronomical poster competition that was held to select impressive wall decorations for the new astronomy wing. |
| Copyright: | A.shulevski, R.Morganti, N.Vilchez |
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