From the sibling comment seems like I had the right idea. The main restriction I was thinking of is what technology was used at the time. The SR 71 used film fo

From the sibling comment seems like I had the right idea. The main restriction I was thinking of is what technology was used at the time. The SR 71 used film for imaging, and the CCD array was developed later on and probably wouldn't have enough sensitivity even if there was an early prototype. You'd also need an InGaS(?) detector for SWIR and I'm not sure when they first came out. For this application you want as sensitive as you can get, since you want to detect minute differences in background intensity. That points to a big ole photodiode. Hence why I suggested that they use an off axis sensor to do online dark subtraction. Locally if there was no star, the diodes should give you the same intensity. If a star enters the field of view of one, but not the other, then you have a detection. It's more complicated than that apparently, but the idea is sound. From the patent it actually looks like they use some kind of optical chopper and a single diode. A chopper is a disc with slits cut out, in the fold of view. If there's a star that doesn't fill the field of view, the chopper will give you a periodic signal as it rotates. If the sky is empty then you'll get roughly the same signal all the way round. (from a quick skim)