The technical difficulties involved in the design, construction, and operational use of such large aperture radio telescopes as the GBT, designed for use at very short wavelengths (i.e., high frequencies), come primarily from the need to provide and maintain a highly accurate parabolic dish surface. An important specification associated with this need that relates to all radio telescopes is a telescopes "RMS". Expressed in millimeters, a telescope's Root-Mean-Square (RMS) value is a scientific measurement indicating sensitivity. It defines what frequencies a telescope dish should be able to work at while showing how close the dish is to being a perfect parabola. The RMS values expected with the GBT, during various phases from construction through operational use, are shown in Table 1. To maximize performance and achieve astronomical work at frequencies as high as 35 GHz or better, techniques to establish an accurate dish surface are needed. This is where the Holography Receiver comes in to play. The word holography, known mostly by its association with the term hologram, is a name originally coined by a Hungarian physicist, Dennis Gabor, who in 1947 developed the first theories concerning holography. Techniques of holography will be used to measure and map the surface of the GBT's primary reflector (the big dish), thus prompting surface adjustments to obtain a "perfect parabola". The Holography Receiver, an instrument designed to measure a telescope's surface and detect areas of deformation under various elevations and gravitational loads, will be instrumental in ensuring the GBT's active surface is set and adjusted to perform as efficiently as possible.
Table 1