Alternative Island 3

In reply to the solar tunnel idea,

It is, seemingly, more efficient energy-wise and space-wise to place
the tunnels at the cylinder axis.

Reasons being:
1.) Access for sunlight is at the stationary center of rotation,
meaning that few of the mirrors that concentrate the light must
rotate with the colony. A "reflector constellation" of numerous
mirrors could float nearby, and concentrate sunlight on the center of
an endcap, wheter this is the one farther fom the sun for a sun-
tracking colony, or either of the endcaps of a vertical colony. They
could be retargeted away from the colony during the night cycle, and
repurposed to energy generation. Alternatively, if multiple
cylinders inhabit the same planar space on different time-zones, the
mirrors could retarget to whichever needed the light at that time.
They could float independently, or be mounted to each other on a
light framework, like a spider web.

2.) If the tunnels are constructed of aluminum reflectors, fresnel
lenses, and vaccuum they will weigh almost nothing (relative to the
whole colony). Close to the center, they require almost no support,
as they brace off each other and a solid girder down the center. If
three are equally spaced around the center, the mutual forces will
balance.

3.) At the center, the sunlight only passes through half the
thickness of atmosphere, and so can cover twice as much area.

4.) Any light from one tunnel that falls on the other tunnels is
wasted, reflected back towards the sun through the reflector system.
Central sunlines are back-to-back, so they do not collect any light
except that which reflects off the surface below. (Now that I think
of it, that might eliminate some small part of the waste heat, if
unused sunlight that bounces off the ground is reflected back out the
way it came. Hmm.)

As to the engineering:

The internal reflectors and lenses would be arranged so that the disk
of the sun will appear to be overhead, at any point on the surface.
The relatively small area of the sunline will eliminate the view of
the stars, but an interesting night view can be provided. City
lights from the opposite side of the colony will be a diffuse,
twinkling source of illumination, and additional lights mounted on
the hub complex could provide more. An artificial moon, consisting
of a heavily filtered sun, could also be introduced. Just have the
sun fade through a natural succession of sunset colors to black, and
come back a moment later as a dim, white spot.

People wishing to stargaze would need to go to a dedicated area with
a mirror view of space (which could operate 24 hours), or presumably
the telecommunication network would carry multiple live views from
the outside of the colony. All in all, a convenient view of the real
stars overhead is a small sacrifice, though we dreamy-eyed space-
theory types would prioritize it more highly than most "everyday"
people.
(Watching the entire starfield rotate every 120 seconds would make me
ill, anyway.)

These are just my notes, for what they're worth. Glad for feedback.
-Al