Avoidance of internal reflections is critical to the design of
submillimeter-wave telescopes. Any receiver or detector placed at the
focus of the instrument will necessarily launch into the telescope some
amount of submillimeter-wave power in the band of interest. If there is a
reflection in the system, for example at the secondary mirror some 6 meters distant,
then a resonant cavity is formed whose modes are spaced at roughly 25 MHz intervals,
and spurious features at those intervals will appear in the spectrometer whenever
there is a shift in power level or small change in the cavity. Submillimeter-wave
spectrometers cover more than 1 GHz of bandwidth, and a 25 MHz spurious
feature can masquerade as a typical astronomical or atmospheric line with a
doppler broadening of 15 km/s.
A 10 meter telescope operating at 200 microns wavelength has a
diffraction-limited beam, and diffraction effects will tend to
dominate the classical optical aberrations such as coma and astigmatism.
Moving outward from the center of the sagittal image surface, aberrational blur is
significant only when it becomes comparable to diffraction blur.
Dragone has shown that if the offset angles in an offset
Gregorian antenna are chosen correctly, then aberrations and
cross-polarization effects in an offset
antenna are the same as those in a conventional on-axis antenna with the same diameter
and focal length. The beam efficiency and sidelobe levels in the off-axis antenna
are better than those in the on-axis antenna, because in the on-axis design there
will be diffraction, reflection, and blockage from the secondary mirror and its supports.
An off-axis Gregorian telescope with correctly chosen offset angles
will always be optically superior to a similar on-axis configuration.
The cost of the primary mirror assembly is increased by ~20% compared to an
on-axis design, but this additional cost is partially recovered in the lack of
design constraints on the secondary mirror supports. Further, homologous design
in the primary mirror is significantly improved by moving the gravitational
load of the secondary mirror supports away from the primary mirror.
(This can, of course, be done with an on-axis design as well.)
An offset telescope allows for large prime-focus instrument packages of the type
used successfully on PYTHON and other CMBR telescopes; these instruments cannot
be used with an on-axis design.
