3 
total stereoscopic coverage is steadily gaining signifi 
cance since the value of conference studies of the area 
of a single model or a sequence of adjoining models on 
the mapping table is fully appreciated by engineers re 
presenting the various disciplines of earth sciences 
when called upon to arrive by direct stereoscopic inspec 
tion at the most feasible solution to the engineering 
task for which the map is to be produced. 
Two new components had to be developed in order 
to add the 120° field capability to the Balplex system. 
A Super Wide Angle Projection Lens 
The optical veteran of mapping projection lenses, 
the two element "Hypergon" had to be abandoned, although 
a field expansion to the 180° limit has been proven pos 
sible. Its nearest relative, the 4-element Topogon and 
Metrogon offered a slightly better approach but fell 
short with regard to light distribution and field curva 
ture. However, another nearly symmetrical lens of the 
quadruplet category was derived which, when manufactured 
to extraordinary tight tolerances, met the requirements 
of 120° coverage, high resolution and near zero distor 
tion at a given finite magnification ratio. Its essential 
design parameters are: 
Focal length: 
Relative aperture: 
Magnification: 
Radial and tangential distortion: -10 micron 
Axial resolution: 110 lines/mm 
29.3mm 
f/11 
400 
31.80 
= 12.8