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.
1st
2nd
Image
Plane
Image
Plane
Image Stop
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.
Diffracted light
from stop
Essentially, the edges of the stop have become secondary light
sources, due to diffraction. This light might only be a fraction of a
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ECEN 4616/5616
1/25/2013
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):
Lyot Stop:
(at image of
entrance pupil)
Light from dim object
is passed to image
plane
Diffracted light
from stop
Diffracted light from
bright object is stopped
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.
nd
2. The 2 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….
pg. 2