epi illumination

[microb]

I can understand this first set-up from Thor Labs:

Epi_General_D5-780.gif (22.7 KiB) Viewed 1329 times

But this next one, with the two achromatic planars on the left throws me as to the theory in its design:

Epi_General_D9-700.gif (32.01 KiB) Viewed 1329 times

[Scarodactyl]

It appears that they get the light collimated in that intermediate bit whwre the extra mirror is, maybe so you can also admit a laser or something like that.

[microb]

It appears that they get the light collimated in that intermediate bit whwre the extra mirror is, maybe so you can also admit a laser or something like that.

I'm curious about the two lenses on the left between the light source and the field stop that I'd like to understand. Why two? And how does that make the collimation better? If the one closest to the filed stop was a negative/concave lens, then the two lenses would be a typical beam expander but in the direction to narrow the beam. Here they are just bending it a bit like having two meniscus lens in an objective to better handle lateral distortions (https://youtu.be/DDoryfCXxPI?t=260). But this is a light source, not an image pathway.