NavList:

Dennis Peterson, you asked:
"When is silvering so old that it needs to be replace?"

If there are patches missing or tarnished, it doesn't really matter. Since the observer's eye is focused at infinity when looking through a sextant while the mirrors are just a few inches away, even large missing patches are only a minor inconvenience. They may look bad, but they don't necessarily affect performance. The only case I had where the silvering failed was a modern sextant. The silvering actually bubbled and peeled away. That mirror became useless as soon as the silvering began to lift away from the glass since it was no longer a flat surface.

-FER

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