Thanks, but no thanks, Bill D. One rare occassions if I'm lazy I'll use software somebody else wrote, but that's hazardous, because programs often don't things the way I like them done, and results are often misleading.
For one-shot through I use True Basic now, but for something like 3D raytracing through a system, (I do 225 rays at a time, through a system of as many as 60 optical surfaces), I wrote my own Fortran Program. I'll use it to check out the design of the transfer optics I'm working on now for a high resolution FTS spectrometer at Battelle Northwest labs.
The multiple reflection absorption cell will just be a copy of the NIST one I did, because it's zero astigmatism, and nobody's beaten it yet for long path length. Last info I need just arived by email, so it's off to my 3 concave mirror system Basic program to figure out the focal lengths I need, the to my ray transfer matrix program to find the diameters I need for the mirrors.