If a 70mm f=500mm objective is used, we can place a 40mm aperture f=-286mm lens at around 214mm behind the objective to get parallel ray.
The outcoming parallel ray will be around 40mm in width.
If my simple calculation works, that would be a good afocal lens system for the Solarmax 40.
PST lens to etalon = ~210mm
Aperture of the PST lens = 40mm
Aperture of the afocal lens = 20mm
40 / 20 = (210 + x) / x
~ x = 210mm
Therefore, focal length of the afocal lens = -210mm
And from the black box, I believe the focal length of the rear lens should be around 150mm, with the etalon to lens distance = 100mm + 50mm from the prism to the visual back, it should be slightly more due to the longer optical path of the prism, but the difference is not very much due to the small focusing prism.
[Additional Remarks on 29 March: I guess that I've enough reason to say that the design assumption for the above calculations were probably wrong]