The lens group in this picture would have its own protective lens tubes like the one on a camera but one for each lens group. What would be an issue having a gap between the lens group and stray light if used at night?
contiguous
Related but NOT a duplicate: Can a camera be a mile long?
You need to differentiate the camera, on where the sensor or film resides and the optical elements. This can be either lenses or mirrors.
A mirror telescope is a good example of an open structure. This structure is only to hold the elements aligned.
But it still needs housing at the end, to make a labyrinth so no direct light enters the last element, where the sensor resides.
But the concept of a camera is, in fact, a housing itself. Remember that this comes from the "camera obscura". one dark place where only the light that you are interested in is projected in the surface.
Several telescope designs, some of which were used with a camera at the prime focus, were an open tube design. Such a design, under certain conditions, is favored because it is light weight. Naturally they are highly susceptible to stray light as well as differing densities of air interfering with the optical path.
Taking Rafael's answer to an astronomical extreme, in the case of gravitational lensing, where the lens element can be the sun, galaxy clusters, etc., there's no possibility of making the lens an enclosed tube. So no, a camera (more specifically, a camera's lens) does not have to have a contiguous housing.
