The Oldest Trick In The Book!
An external occulter mission is designed to provide coronagraphic-like capability
by blocking the light of bright stars and other targets from
reaching imaging detectors. This can be accomplished by
blocking most of the star light from reaching a space telescope
and by reducing the intensity of the diffraction rings
surrounding the stellar image.
The occulter, whose screen may take one or more of a myriad number of shapes, will act as
an opaque apodizing screen, redistributing the energy
over the telescope aperture. There are many sources that
describe diffraction phenomena related to this basic problem
(Born and Wolf 1980, Hecht and Zajac 1987, etc ...)
Only a brief description of the problem is given here.
Diffraction is used to describe the phenomenon of light bending
around the edges of an object and converging on the opposite
side. This phenomenon is due to the wave like
nature of light.
If we consider a square occulter 45 m on a side at a distance of
16,000 km from the telescope, it has a width of 0.58 arcsec. A
pattern of light from a blocked star will be visible within the
shadow of the occulter. The diffraction pattern within the shadow
area will be surrounded by a series of bright and dark (null)
fringes with bright diffraction spikes due to straight edges of
the occulter.
To the left is the PSF (point spread function) of a star as
seen through a conventional telescope. The PSF is the resultant image of
the light from a very distant, point-like source of light--such as a star.
The four spikes leading off the star are produced by diffraction of the
starlight around support arms (spider) for the secondary mirror in
this newtonian/reflector design. The spider in Newtonians gets in the
way of part of the light and produces these features in the image plane.
The role of the occulter is to:
- suppress the central
intensity,
- suppress the light in the 'wings' of the point spread
function,
- alter the shape of the diffraction pattern to something more
favorable.
The physical size of the occulter and its distance from the telescope
are the deciding factors in the resulting on-axis intensity of light and
the average intensity over the aperture. The larger the physical size
of the occulter (occulter subtends larger angle), the greater the reduction
in the on-axis and average intensities of the light in the image plane.
But, a larger subtended angle defeats the goal of looking for
planets near their parent stars. Therefore, placing the occulter further
away (occulter subtends smaller angle) is required.
