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Unformatted text preview: Astronomy 423 at UNM Radio Astronomy Selfcalibration Greg Taylor University of New Mexico Spring 2011 G. Taylor, Astr 423 at UNM 2 Outline Why selfcalibrate? How to selfcalibrate What to watch out for Limitations of selfcalibration Practical examples of selfcalibration in action Demo of calibration in AIPS This lecture is complementary to Chapter 10 of ASP 180 and is based on a lecture by Tim Cornwell G. Taylor, Astr 423 at UNM 3 Selfcalibration of a VLA snapshot Original Image Final image Initial image G. Taylor, Astr 423 at UNM 4 Fundamental calibration equation V ij ( t ) = g i ( t ) g j * ( t ) V true ( t ) + ij ( t ) V ij ( t ) Visibility measured between antennas i and j g i ( t ) Complex gain of antenna i V true ( t ) True visibility ij ( t ) Additive noise Calibration equation G. Taylor, Astr 423 at UNM 5 Calibration equation becomes V ij ( t ) = g i ( t ) g j * ( t ) S + ij ( t ) S Strength of point source Calibration using a point source Solve for antenna gains via least squares algorithm Works well  lots of redundancy N1 baselines contribute to gain estimate for any given antenna G. Taylor, Astr 423 at UNM 6 The complex gains usually have been derived by means of observation of a calibration source before/after the target source Initial gain calibration is insufficient Gains were derived at a different time Troposphere and ionosphere are variable Electronics may be variable Gains were derived for a different direction Troposphere and ionosphere are not uniform Observation might have been scheduled poorly for the existing conditions Why is a priori calibration insufficient? G. Taylor, Astr 423 at UNM 7 What is the Troposphere doing? Neutral atmosphere contains water vapor Index of refraction differs from dry air Variety of moving spatial structures G. Taylor, Astr 423 at UNM 8 Dont need point source  can use model V ij ( t ) = g i ( t ) g j * ( t ) V ij model + ij ( t ) V ij model Model visibility Calibration using a model of a complex source Redundancy means that errors in the model average down Can smooth or interpolate gains if desired G. Taylor, Astr 423 at UNM 9 Made a fake point source by dividing by model visibilities X ij ( t ) = g i ( t ) g j * ( t ) + ' ij ( t ) X ij ( t ) = V ij ( t ) V ij model ' ij ( t ) Modified noise term Relationship to point source calibration G. Taylor, Astr 423 at UNM 10 Why does selfcalibration work?...
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This note was uploaded on 12/13/2011 for the course ASTRO 423 taught by Professor Gregtaylor during the Fall '11 term at New Mexico.
 Fall '11
 GregTaylor
 Astronomy

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