Telescopes and Images.

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Presentation transcript:

Telescopes and Images

The Electromagnetic Spectrum Objects can look different at different wavelengths (colours vs shades of grey) Wavelength Resolution = --------------------- Diameter => Want to make images at all wavelengths ..... with the same level of detail.

Detectors at multiple wavelengths GBT SWIFT WMAP SPITZER Parkes ARECIBO Longer Wavelengths (50m - 0.5m) Cannot build larger dishes !! HUBBLE

Building a really large detector Artificially create a large “dish” using many smaller ones... Very Large Array ... this is called ‘ Aperture Synthesis ‘ ... used in many Remote Sensing applications. But ... this large “dish” is not a real reflecting surface..... So how do you make it behave like one ? ... imitate the Physics of a lens.

Measure interference fringes Young's Double-Slit Experiment Distance between slits controls the wavelength of interference fringes One dish == One slit => Each pair of antennas measures a different 2D fringe.

Image Formation Build an image by combining all measured fringes. 2D Fourier transform : Image = sum of cosine 'fringes'. => Each pair of antennas measures a different 2D fringe.

Measuring each 'fringe' x x x x x Each Pair of Antennas : => Measures parameters of one 2D “fringe” Amplitude, Phase : From Complex Visibility ( : output of correlator) Orientation, Wavelength : From Array Geometry τ 〈 𝐸 𝑖 𝐸 𝑗 ∗ 〉 𝐸 𝑖 𝐸 𝑗 Correlator Amp x x x x x Amp Lag -> Frequency : FFT 〈 𝐸 𝑖 𝐸 𝑗 ∗ 〉 Data Processing Analog and Digital Electronics Disk Integrator

Data Processing (1) Editing : (2) Calibration : 1GHz 1.5GHz 2GHz Frequency VLA L-Band (1.4GHz) Spectrum GPS satellites (1) Editing : Some data are corrupted by man-made signals => Need to identify and remove bad data (2) Calibration : Fourier Optics applies only under ideal conditions => Need to model instrumental / atmospheric effects and apply corrections. (3) Image Reconstruction : Only some Fourier terms are measured. => Need to estimate the others... Before After Uses concepts from Numerical Analysis, Optimization Techniques, Computational Physics, Fourier Transforms, Fourier Optics, Statistics, Digital Filters....

Some radio telescopes of the world..... WSRT VLA CARMA GMRT LOFAR LWA You are here ATCA MeerKAT ASKAP ALMA MWA PAPER SKA (to be built)