1. Telescopes and Images

2. 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.

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

4. 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.

5. 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.

6. 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.

7. 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

8. Data Processing (1) Editing : (2) Calibration :
1GHz GHz GHz
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....

9. 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)