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Target field acquisition, alignment, focus, and closed loop guiding are controlled by the use of the Field Orientation Probes (FOPS). Twelve such probes are mounted symmetrically about the focal plate. Each probe consists of a 7-fiber coherent array with 1 arc-second fibers.
A TV views the output of all twelve probes for the provision of guider signals and for the visual inspection of the field alignment and the telescope focus.
Since the guider must perform 3-axis guiding (altitude, azimuth, and field rotation), it is absolutely necessary to have at least 2 probes in use within the target field. However, we strongly recommend a minimum of 3 or 4 probes so that astrometry errors can also be somewhat compensated for.
Filters are available for use in the FOPS TV (BG-39, RG610, or clear glass) to optimize alignment with respect to the spectral bandpass being observed.