Contributions of the USNO to the Optical Reference Frame

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A good, working definition of what is required in a celestial reference frame is that it must provide observable fiducial points on the Celestial Sphere with internally consistent positions that are referred to coordinate axes of known direction. In reality, this statement gives the goals in the reverse order from that in which each must be achieved, the definition of the axes, or zero points of the system give orientation to the observationally defined set of primary objects whose coordinate relation to each other must give the frame rigidity. Finally, the primary objects are generally too sparse to define the frame within areas of less than tens of square degrees, and so additional objects must be related to the frame to increase the density. This last step is required to make the frame useful for most observational applications.