Instrumental Effects and their Calibration · Calibrators in radio astronomy • Astronomical calibrators — to set flux density scales, feed polarisation angle, slow-varying gains…

Instrumental Effects and their Calibration

wasim raja

Why calibrate your instrument?

Why calibrate your instrument?

Radio telescopes measure Electric Fields

Animation Courtesy: www.best animations.com

Radio telescopes measure Electric Fields

Animation Courtesy: www.best animations.com

Radio telescopes measure Electric Fields

Animation Courtesy: www.best animations.com

Radio telescopes measure Electric Fields

Animation Courtesy: www.best animations.com

Radio telescopes measure Electric Fields

Animation Courtesy: www.best animations.com

What needs calibration?

Detector Orientation or Feed Position Angle

Feed Types

Leakages

Gains or Amplification factors

Instruments are generally moody

But often in a predictable way

• Slow variation in Time • Smooth changes across Frequency • Different response in different look-

directions. • System restarts/Power cycles can

be tricky

System variations in time and frequency

Calibration: Inadequate Cadence Higher Cadence

Calibration: Inadequate Cadence Higher Cadence

Wide-field artefacts

Wide-field artefacts

Wide-field artefacts

Need for Standards

Need for Standards

Calibrators in radio astronomy

• Astronomical calibrators — to set flux density scales, feed polarisation angle, slow-varying gains… — Time Overhead!

• Synthetic calibrators — to correct time-dependent effects, bandpass calibration — Calibration of Calibrator?

• Certain a priori information: Self-calibration

Challenges

Challenges

Challenges