JCMT Pointing Model - Parameter 7

The amount by which the telescope beam sags due to flexure of the secondary mirror support structure.

In the following diagram :

We see how flexure deflects the beam towards the horizon. When the antenna is not pointing towards the zenith the ideal telescope would point at A, whereas flexure of the secondary under gravity deflects the beam instead to B. The deflection theta is assumed finite even when the antenna is horizontal, so that the functional form of the deflection is easiest expressed in terms of cos(elevation).

This assumes that at least the structure behaves regularly throughout the elevation range, but, as mentioned by Pat Wallace in correspondence regarding refraction :

"Is it possible you're seeing non-Hooke's-law dish flexure? Most telescopes I have looked at have had significant tanZ-ish flexure, which I suspect arises from things changing from compression to tension as the ZD increases."