Definitions and principles
Why
Resolution
Radio telescopes need to be bigger for same resolution
Young slits and fringe visibility
Point source through 2 slits separation d
Gives fringes, separation lam/d
With larger source the fringes become incoherent, and destructively interfere as the intensities add
With smaller slit separation, fringe spacing increases, and fringes become visible again.
No fringes if source size bigger than lam/d
Plots
Fringe visibility of interferometer give info about ft of sky brightness distribution
Long baselines give info on small scale structure but are insensitive to large scale structure (fringes wash out)
Short baselines record info about large scale structure but are insensitive to small scale structure (resolution limit)
Combining signals
Non-photon limited
electronic
relatively straighforward
can clone and combine signals "correlation" (multiplication and delay)
phase and time delay
Photon limited
use classical Michelson/Fizeau arrangements
delay lines for manipulation
cannot clone photons
Maths
for multiplying interferometer
Series of fast fringes whose amplitude is the ft of the source brightness distribution
may need to get rid of fringes before integrating (fringe stopping)
R(u,v) has amplitude and phase
2 waves
One delayed wrt other
phase delay=maths
Coherent light
beam coherant with constance phase relation
implies
Monochromatic source
small source
if big source, different parts will emit incoherently
Not exactly same as radio
Maths
Visibilty decreases if
non-monochromatic, decreases away from centre
source is extended
Can determine sources structure by varying d
how quasars investigated and size determined