The point is it's not that inexpensive for the optics. I've worked in an interferometry lab where we used high-volume photographic lenses where we could, and

The point is it's not that inexpensive for the optics. I've worked in an interferometry lab where we used high-volume photographic lenses where we could, and yeah, they were probably an order of magnitude less expensive than something made for the purpose, and real exotic optics were more than two orders of magnitude more expensive. But talking apples to apples, a $12000 professional camera lens is closer to something like a $12000 research microscope than you think in terms of the build-or-buy decision. There's also a whole industry of telescopes for amateurs that are made in low volumes to much higher optical standards than photographic lenses that are mostly not that expensive. A top tier 6" refractor might be $15K and blow the camera lens away as a general purpose astronomical instrument, but it would not be nearly as fast, which is important for this application, and if you placed an order for 120 of them, you might get them in five years. Maybe. I'd guess that made-for-the-purpose tubes would come in within a factor of two or three of the off-the-shelf lens if you could find a supplier. They might even be cheaper. The project risk would be larger, and that might be determining also.