One Kilometer Radio Aperture (OKRA)

The One Kilometer Radio Aperture (OKRA) is a radio telescope collaboration between the colonies of Sirona and Grannus. Each colony houses near-identical dishes built into natural basins in their respective landscapes. The general design for OKRA sees a fixed, filled-aperture 1,000 m (3281 ft) diameter dish constructed into these natural depressions.
The OKRA telescopes have mixed passive-active surfaces constructed of metal panels. These panels can either remain in the full-sized, fixed spherical dish arrangement or automatically tilt into smaller parabolic sub-sections, helping change the focus to a different area of the sky. For both OKRA telescopes, the cabin containing the secondary mirror and feed antenna is suspended on a collection of 96 cables above the dish and can move automatically to steer the observations to a different direction. Each telescope observes at wavelengths of 5 cm to 10 m.
The proposal for OKRA was received and approved by the governing bodies of both involved colonies in 2469 with construction beginning later that year. First light for both telescopes fell in early-2473 and the telescopes were deemed fully operational by 2475 after two additional years of testing.
== History ==
=== OKRA-Sirona ===
=== OKRA-Grannus ===
== Overview ==
Both OKRA telescopes share the same overall design. Each has a spherical reflecting surface one kilometer in diameter located in a natural sinkhole in the landscape, serving to focus incoming radio waves onto a secondary mirror and eventual receiving antenna in a feed cabin suspended above it. Each reflector is made of perforated aluminum panels supported by a mesh of carbon nanotube cables hanging from the rim.
The surfaces are made of 71,200 panels, about 5 meters on a side, forming a geodesic dome. 35,600 winches are located underneath the surface, pulling on the joints between panels to both combat sag and allow active deflection of the surface.
In the passive configuration, the dish of an OKRA telescope remains stationary, while the cabin is moved to a proper focal point of the telescope to aim at a desired target. While this option affords the utilization of the entire collecting area, the spherical mirror shape creates a focus along a line rather than at one point, introducing spherical aberration. As such, a complex series of line feeds are needed for observations using the entire surface, with each line feed covering a narrow frequency band. As only a limited number of these feeds could be used at any one time, the telescope's flexibility was limited.
To combat this, the OKRA telescopes also include an active surface configuration. While this mode only allows a maximum diameter of 750 m, this smaller surface can be held in the correct parabolic shape to avoid the spherical aberration of the passive mode. Additionally, the telescope can more easily point at different areas of the sky by moving where on the surface the smaller section is being illuminated. To further increase zenith angle flexibility, the section of the telescope being used in the active configuration can be further reduced to a diameter as low as 150 m.
Each OKRA telescope has a working frequency range of 30 MHz to 6.0 GHz, with the upper limit set by the precision with which the primary can approximate a parabola. While there could be slight improvements made, the size of the triangular segments puts a limit on the shortest wavelength which can be received. To cover this frequency range, each feed cabin has a system of 20 receivers.
=== OKRA-Sirona ===
=== OKRA-Grannus ===
== Mission ==
=== Earth Survey Catalog ===
=== 2551 Radio Bounce ===
== List of Appearances ==