Attempts to construct large parabolic reflectors for microwaves have met with the following difficulty. For normal operation of radio telescopes, the form of the reflecting surface must not deviate from the theoretical by more than 0.1λ; therefore, the larger its size the higher the requirements for relative accuracy (i.e., the ratio of the largest permissible deviation to the cross-section of the reflector). So far, a relative accuracy better than 10–4 has not been attained in any of the existing radio telescopes, and there are no grounds for supposing that it can be increased considerably.

Since the deviation from the theoretical form of the surface should not be more than 0.1λ, then for a relative accuracy of 10–4, the diameter of the reflector D cannot be much larger than 1000λ, and the beamwidth cannot be less than 3 minutes of arc (the beam angle is equal to about λ/D at half-power width).