A dual-channel telescope allows for a
wide-field telescope design with a good,
narrow field channel of fewer surfaces for
shorter-wavelength or planet-finding
applications. The design starts with a
Korsch three-mirror-anastigmat (TMA)
telescope that meets the mission criteria
for image quality over a wide field of view.
The internal image at the Cassegrain
focus is typically blurry due to the aberration
balancing among the three mirrors.
The Cassegrain focus is then re-optimized
on the axis of the system where the narrow
field channel instrument is picked off
by bending the primary mirror. This now
makes the wide-field channel blurry (i.e.,
the TMA image), and it must be re-optimized
while holding the fore-optics fixed.
This leaves the tertiary mirror as a variable,
as well as a fold mirror strategically
placed at the image of the primary mirror
(i.e., exit pupil of the telescope). This fold
mirror can then be used to compensate
for the departure in primary mirror figure
used to optimize the narrow field channel.
As such, only an aspheric term is
needed for this final optimization on this
“corrector” fold mirror. This work was done by Joseph M. Howard
and David Content of Goddard Space Flight
Center. For further information, contact the
Goddard Innovative Partnerships Office at
(301) 286-5810. GSC-15574-1
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