When Koenig announces his plans for the survival ship, he informs the others that he'd left one Eagle intact (the rest had the nuclear generators removed to power the forcefield), and they act surprised. However, earlier, they had all watched Alan use it to test the forcefield.
Although the script has the actors comparing the look of Bergman's forcefield to fish scales, the special effects shots more closely resemble the top half of a peeled green tangerine.
The first indication they are approaching a "black sun" is when an asteroid is pulled away from a collision and then destroyed. Why would putting a shield over a tiny portion of the surface area of the moon protect the vast remainder of the moon from the same fate? Assuming that the crew of moon base Alpha were at least marginally knowledgeable of science, this plan would not have comforted anyone.