QUESTION: I am using the Universal Plot and have NSDD as my Dependent Variable. When I press OK I get the error “Some of the operands cannot be computed!”. Why is this?

ANSWER: Instead of using NSDD as the Operand for the dependent variable, it must be set to Merit, and the Line should be set to NSDD:

This is because to use NSDD like many other Non Sequential operands, the detectors should be cleared, and a ray trace must be run. This can be done by using blank NSDD and NSTR operands prior to the NSDD operand you are trying to plot in the merit function: