One approach that comes to mind is to place two infinity objectives back-to-back, with an infinity section in the middle. For example, placing two 4X NA 0.10 objectives back-to-back will give you a 1:1 system with the same field and sensor sizes that the objectives would normally have, so roughly 6-10 mm diagonal.
Rik - please explain "with an infinity section in the middle" to me in greater detail. Thanks. What this is for in the concept of the optical design. Do you mean that there needs to be a gap between the two objectives which will need to be determined experimentally?
Here's a ray-trace of the concept, showing two objectives with 50mm FL and 5mm radius (NA 0.1), placed back-to-back with 40mm of separation and a 2.5mm radius Waterhouse stop to take the system down to NA 0.05.
My phrase "infinity section in the middle" refers to the area between the two lenses, where each point on the subject is represented by a bundle of parallel rays. Different points on the subject are represented by bundles of rays that are parallel within each bundle, but at different angles for different bundles. This is just like looking at the night sky, "infinity".
I've set up this model to use a 4 mm object radius, so 8 mm diagonal. Notice that there's no vignetting in the model, because the stop and the distance between lenses are sized so that any ray aimed to go through the stop will also be able to make it through the glass.
There's nothing very particular about distance between the objectives. The shorter it is, the more you're using the central area of each lens, which will probably minimize aberrations.
The system can also be made telecentric by extending it far enough that the stop is be placed at the rear focus distance for the front lens. That forces the edge of the field to use more peripheral sections of the lens, which probably cuts into the image quality. The system may also have to be stopped down farther to avoid vignetting.
In this second model, I've made the system telecentric, kept the 8mm diameter field, and reduced the NA to 0.03 so as to eliminate most but not all of the vignetting. Notice that the ray fan for the edge of the field is narrower than for the center (so, corner darkening), and also that the edge of the field uses only the edge of the lens.
I hope this helps.