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You can trust Bill Morris that new mirrors are an inexpensive, effective solution. And there's something else to keep in mind: you may, at some point in the future, decide that you want to sell this instrument. If so, being able to offer the original mirrors, unmodified, might be critical to some buyers. Others might prefer a new set. Better to have both so that the octant maintains the highest possible value, as a historical artifact and as a practical instrument.

Dustin B mentioned those "mirror in a can" spray paints. They do produce excellent mirrored surfaces when applied to good, clean glass. Note that in the video (here's the link again), the creator basically makes a giant split-horizon glass, and it looks great. I haven't experimented myself, but I know artists who use this stuff. The result strikes me as a little "foggy" (not as good as in the video). I suppose the material that is sprayed-on contains small particles that reflect light diffusely in addition to the specular reflection from the "silver" coating. This fogginess would not necessarily be a serious problem for a sextant mirror, but it might be annoying under some circumstances. But yes, this stuff works! Can you buy a can and acquire plain glass pieces cut to the right size for less than two mirrored pieces cut to size? It's probably a fair match!

Frank Reed