Demotrip is a Cooke triplet used throughout OSLO as a demonstration lens. It is good for this purpose because it contains the degrees of freedom needed to illustrated several optical principles, uses vignetting as a design variable, and is relatively straightforward to understand. The Cooke triplet is pro ably the most common extended field lens ever designed. This lens has been used for many years to demonstrate OSLO. It is discussed throughout the manual in a wide variety of contexts. As a historical note, this lens was not designed using OSLO, but is rather an adaptation of a design done by R. Kingslake and included in his book "Lens Design Fundamentals", pp 286-295, (Academic Press 1978, ISBN 0-12-408650-1). As another historical note, this design form was actually invented by H. Dennis Taylor, who worked for Cooke. According to Kingslake, the lens was not manufactured by Cooke, but rather by Taylor-Hobson, another optical firm whose principals were not related to Taylor, despite the name. In any case, this form has been used in millions of low cost cameras, projectors, and various other instruments. The design here is typical and can be used as a starting system for adaptation. You can compare its performance to the dblgauss.len and petzval.len designs included in the OSLO demo library.