Oh, jeez, what an idiot I am. I overlooked the amazingly obvious fact that the focal lengths in my table were in INCHES. I somehow thought they were in FEET, but they weren’t.
As they say about scientific investigation in general, the easiest person to fool is yourself. I see that I did a pretty good job of doing just that. Thanks to those who pointed out my error, for example Mel Bartels. I guess that’s the great thing about these interwefts: you can get immediate feedback and have other, wiser souls point out your stupid mistakes in a matter of hours or minutes.
Here’s how the table should go:
As you can see, the distances are still pretty long, but not nearly as long as I claimed. For our fairly-standard 8-inch (20-cm) f/5 mirrors, the distance we would need for a good star test would be 73 feet (22 meters). We still can’t fit that inside the CCCC building in one straight line. However, I guess that if we get some good first-surface flat mirrors, we could arrange for 73 feet/22 meters if we reflect the pinhole light a few times. I also bet we won’t need that lens to reduce the size of the pinhole, which you can see here.
A 12.5″ f/6 mirror will need 119 feet, or about 3 times the width of the CCCC game room. My 16″ f/5 will need a distance of 287 feet or 87 meters. That’s a long way; 7 times the distance across our largest room! I guess we’ll need a bunch of decent small flats from Surplus Shed, as well as precision tip-tilt devices to line all those mirrors up properly.