I thought up of this design idea after my still post.
If it was for purifying fresh water, I'd use a linear parabolic mirror and pipe for the boiling chamber. Some polished stainless steel is all that would be needed for he mirror. To keep from having to adjust the mirror's position all day, I'd orientate the pipe/mirror in alignment with the sun's path, with a slope from one end to the other. At the top end of the pipe I'd have a vertical output pipe, and the tube to the still's cooling coils would be attached to the top of it. At the bottom end of the heating pipe I'd have a source water pipe. That would be connected to a relatively large water container that has it's top close to the mid height of the vertical output pipe. As water is boiled off in the heating pipe the source jug will replenish it. As the system boils water, the water in the pipe will get loaded up with a brine that can't be boiled off like the salts, etc. Every so often, turn off the source pipe, and drain the water in the heating pipe. That means there needs to be a valve on the source water line, and a spigot at the bottom end of the heating pipe for draining it. It likely should be done every gallon produced or so. Also setup the system so it is easy to remove and descale the heating pipe and vertical output pipe. If the water level in the system is maintained high enough that the top level is in the vertical output pipe, little scaling will happen in the heating pipe.
The reason I orientated the pipe with the path of the sun is now the system only needs the mirror moved every few days instead of needing to track the sun all day. It does have the disadvantage of only producing for part of the day, but with long heating pipes with low slopes it can easily produce for the middle 8 hours of the day when the sun is highest.
The diameter of the heating pipe and vertical output pipe needs to be such that steam bubbles don't pump water so it likely should be at least 1.5" in diameter.
Because the volume in the pipes isn't that great, this still should be able to produce quite quickly after setup, and can produce on lightly cloudy days, and in the sunny periods between clouds. On the other hand the kettle of a conventional still could be heated by fire.