JCMT Pointing Model - Parameter 3

The amount by which the main drives are non-perpendicular

In the following diagram :

Z is the true zenith, and OZ' is perpendicular to the elevation axis.
ZOZ' = theta .
A star at S is located in the ideal (Z) system by rotating a point P, with true elevation E , through azimuth a . In the system tilted by theta , the equivalent point is P' :

Then in the triangle PAP', PA = E , P'A = E' , PAP' = theta , APP' = 90. So
sin(PP')/sin(theta) = sin(P'A)/sin(APP') , i.e.
PP' = theta*sin(E') , for small theta.

This is equivalent to a change in azimuth of PP'/cos(E') = theta*tan(E'), but the distance on the sky (daz) is still theta*sin(E').

Also, in the triangle APP', cos(E') = cos(PP')*cos(E), so E' = E, i.e. del = 0

Further notes