Beamshaping is necessary

A representation of beam divergence that is wide in "longitude"

If the divergence of the sunlight reaching the lamp is too wide in "longitude," then, short of beamshaping, there is nothing a rotationally symmetric lens can do to fix it. It is therefore important to accurately model the divergence of this light.

Relations for the 180° and the general locus near the beam down optics. R is the field radius and r is the radius of the 180-degree locus. Theta is the beam angular radius. 

At a radius r = Rsinθ, the converging sunlight fills a full 180° of longitude. At a larger radius x, the sunlight fills an angle 2φ, where φ = sin^-1(r/x).

For θ = 1.5°, r/R = 0.026. For R = 2600m, r = 68 m.

For φ = 10°, x/r = 5.8

For R = 2600m and x = 120m, φ = 35°; x = 190m, φ = 21°.

Since the width of the beam in the latitude dimension is not likely to be more than 4 degrees, the beam aspect ratio is 10 or greater. Thus beamshaping is necessary.