I donāt have a āwavienā for you, but if you find a spherical glass mirror with the right focal length, itās easy to drill out the center with a $10 diamond hole saw, preferably in a lathe.
Correct. But it doesnāt need to be exactly half a sphere to be called a spherical mirror. You can get a mirror based on a bigger sphere that is cut down in diameter, so the focal point is in front of the actual mirror edge. The focal point hasnāt moved, but parts of the mirror is gone. But if you want to be very specific, I guess the correct terminology for this is effective focal length, as it is listed here: https://www.edmundoptics.com/optics/optical-mirrors/focusing-concave-mirrors/concave-spherical-mirrors/
Pretty sure that any round mirror is called a spherical mirror, the one that is exactly half is called hemispherical, which means āhalf of a sphereā.
In principle yes, but the light source from a LEP is many times smaller than from a led, so 1) the optical quality of the collar must be so good that the image of the light source produced by it is as small as the light source itself (or else the collar just produces an extra halo around the hotspot without much added throw) and 2) the alignment of the collar in all 3 dimensions must be an order of magnitude more precise to get the image exactly over the source.