Thin mirrors have become popular in the last ten years because they're lighter in weight and less expensive. In a Dobsonian telescope, the altazimuth motion reduces the stress on the mirror and allows the use of a sling-type mirror mount. Annealing is the process of cooling the glass during manufacture at a rate that allows the entire blank to solidify at a uniform rate so that internal stresses are minimized. This directly affects the ability of the polisher to make extremely accurate surfaces. There are three grades of annealing - commercial, fine, and precision.
Precision has the least internal stress.
This type of lens combination is more difficult to make due to the especially critical nature of the alignment and matching of optical elements. Doublets, Triplets, etc. The problem with a simple lens is that when a ray of light passes through it, the component colors come to a focus at slightly different points. With a very long focal length, the problem isn't as severe, but it becomes much more acute with shorter focal ratios. The problem with fluorite is that it's a softer glass that can be damaged easily, so such a lens is always protected from abrasion and exposure to the weather by being mounted as an internal element within the telescope system.