I missed some questions... happy to answer them.
Chogy i have a few questions if your kind enough to address them
Your F-15 cockpit, exactly what altitude does it pressurize to ? And does it hold that even at 60-65,000 feet or does it change ? Also does your oxygen mask come into play at all for higher altitudes or is it more for safety/communication. (You have to keep it on at all times right ?)
Each jet has a different pressurization schedule. Commercial aircraft will have a cabin altitude of maybe 8,000 feet (2400 meters) while at cruise. Fighters are a bit different. Cabin altitude is obviously lower than the outside air, but it goes up as you climb. Most of the time it is less than 10,000 feet, but if you get very high, it can be > 10,000 feet. The oxygen mask is worn for two reasons - it allows you to communicate, and in case of battle damage (and lost pressurization) it immediately delivers oxygen, because around 40,000', your time of consciousness is only about 8 seconds.
We were not allowed to exceed 50,000' in peace-time, because if you lose pressure, it can kill you. You get the bends, bubbles in your blood and tissue. At 65,000'+, without at least a partial pressure suit, death is almost guaranteed, which is why you see SR-71 and U-2 guys in space suits. In war, you do what you have to, and simply hope the pressurization works properly.
On the HUD: The image is focused at infinity. It is really almost like magic, and hard to describe. Basically, if you focus your eyes on the bracket that holds the HUD combining glass, the green imagery is totally blurred. If you look through the glass, and let your eyes relax on a distant object, the green symbology snaps into focus. And as you move your head, the green remains overlaid on objects at infinity. So if you have a target designator box around an aircraft way out there, the box stays around him.
It's really a very cool technology, and I'm not sure how they do it, but it works very well. In every way, it is as if the HUD imagery is simply floating in the air in front of your face.