Starlight from Vega

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Coherence and statistical optics · Coherence and statistical optics

What an Instrument Can See, Ch Spatial coherence and the van Cittert-Zernike theorem

Problem

Starlight from Vega, angular diameter

3.3\,\mathrm{mas}

, is collected by a telescope of aperture diameter

D = 200\,\mathrm{mm}

through a filter of centre wavelength

550\,\mathrm{nm}

and bandwidth

20\,\mathrm{nm}

. Take

1\,\mathrm{mas} = 4.8481\times10^{-9}\,\mathrm{rad}

. ((a)) Estimate the transverse coherence radius (first zero of

|\mu_{12}|

) of the starlight in the telescope's aperture plane, and compare it with

D

to say whether the field is coherent across the aperture. ((b)) Estimate the coherence length of the filtered light and check whether a path difference of

5\,\mu\mathrm{m}

inside the telescope's optics satisfies the quasi-monochromatic condition.

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