The Coronagraph: a Significant Problem Solved with Graphical Ray Tracing (And Ingenuity)

ECEN 4616/5616 1/25/2013

The Coronagraph: A significant problem solved with graphical ray tracing (and ingenuity)

Problem:

How to image something dim that is close (in angular terms) to a much brighter object?

Examples:

·  The Sun’s corona, when the Sun is not eclipsed by the Moon.

·  A planet orbiting a star.

First Attempt:

Image the objects on a plane that contains an occulting disk to block the bright object, but let the dim object pass, then re-image the dim object.

Why it doesn’t work:

The system stop and entrance pupil (the first lens, in this example) is brightly and uniformly illuminated by the bright object. Hence, there is significant diffraction from the edges of the stop, which is spread over both image planes, and not removed by the Image Stop.

Essentially, the edges of the stop have become secondary light sources, due to diffraction. This light might only be a fraction of a percent of the bright object’s light, but that is enough to obscure an image of an object that is thousands (or millions) of times dimmer.

The Cure (The Lyot Stop):

What is happening here is that the entrance pupil (the first lens, in this case) is re-imaged at the Lyot Stop plane. Hence, the light that diffracts from the edges of the entrance pupil is re-imaged into a bright ring of light at the Lyot plane. The Lyot stop is just an annular opaque ring which blocks the (re-imaged) diffracted light.

This method can be carried out for multiple iterations:

0.  The bright object is imaged to a plane, where an opaque stop blocks it.

1.  The first brightly illuminated lens (aperture) diffracts light from near its edges, producing a secondary source of interfering light.

a.  The offending lens is re-imaged and an annular stop blocks the diffracted light.

2.  The 2nd lens (which re-images the first lens) may also be too brightly lit, and diffracts interfering light from its edge.

a.  The 2nd lens is re-imaged by a third lens and an annular stop blocks the edge-diffracted light.

And etc….

Eventually, scattered light from the surface and/or bulk of the lenses will dominate the stray light. There is no way to separate this light from the image light optically. (However, digitally….

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