(omitting atmospheric absorption)

0.3-2 |m

0.3 photon/s/m2

6-20 |m

10 photons/s/m2

Let us now discuss the different direct-detection techniques, together with the associated instrumentation.

2.3.2 Coronagraphic Methods and Adaptive Optics A Historical Retrospect: The Lyot Coronagraph

The coronagraph was invented by Bernard Lyot at the beginning of the 1930s to observe the solar corona at times other than eclipses (Lyot, 1931; 1939). The principal approach - which was, nevertheless, empirical - that guided its inventor was to achieve the maximum reduction in the light scattered and diffracted by the optics. The principle of this instrument is shown in Fig. 2.16.

Lyot's trick was to re-image the primary image plane, and to place a diaphragm at the associated pupil plane to eliminate the effects of diffraction caused by the mask located as the primary image focal plane. This method, in conjunction with the use of extremely high-quality optics (with homogenous glass and exceptionally high quality polishing) enabled light scattered within the instrument to be reduced

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