A detailed design for the optical configuration of the ALMA receivers is presented. Individual frequency bands are implemented as self-contained cartridges holding two orthogonally polarized channels. The cartridges are arranged on concentric circles round the center of a 970 mm diameter dewar located on the telescope axis. The beams from them illuminate the secondary mirror through windows on the top of the dewar, either directly or via reflective optics. By having all the beams separate in the focal plane, all bands view the sky simultaneously and selection of the observing band simply requires re-pointing the antenna.
Where possible all the optical elements are integral with the cartridge. For the lowest frequency bands, the optics are too large to go on the cartridge and are located on the top of the dewar. There are no optical elements inside the dewar that are not attached to a cartridge. Since some of the cartridges are far off the telescope axis, mirrors are used to bring the beam closer to the center to reduce aberrations, polarization distortion, and vignetting by the hole in the primary. Provision is made for a mirror to bring the beam of the water vapor radiometer for atmospheric phase correction to the center of the focal plane so it is close to all observing beams.
Several measures are taken to ensure low optical losses: the number of elements is minimized; reflective optics are used where possible; large beam clearances are maintained; and accurate fabrication and alignment tolerances specified. A major driver was to generate minimal cross-polarization, and this was realized by minimizing angles of incidence on offset reflectors, and balancing cross-polarization between consecutive mirrors.
Detailed calculations of the performance, including losses, noise, and polarization have been carried out and are tabulated. There are also estimates of the cryogenic loading. The principal uncertainties are the optimum designs for the vacuum windows and infrared filters.
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Download a ps version of ALMA Memo 362.
Last modified: March 27, 2001
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