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A starting point might be to ask how flat does the mirror have to be. I asked the question about sextant mirrors on this list a couple of years or so ago and the consensus seemed to be that in a sextant it should be flat to within half a wave, though this raises a further question "Over what length?"

The cost of an optical flat goes up with the degree of flatness and of course with the area and the substrate. But does it need to be flat to 1/4 wave or still less 1/10th wave? Freiberger seem to think so. This is perhaps a matter for experiment.

I was given a large piece of reflective smoked glass of 40 percent transmissivity (if that's the right word)by a kind glazier about ten years ago and have been cutting pieces off it ever since whenever I need a front surface mirror. It's the sort of glass used on large windows to cut down heat loading from the sun.It's about 5 mm thick and flat to half a wavelength over the area of my 45 mm diameter optical flat (1/10th wave). One side is coated with a highly reflective and tough film. Presumably it is aluminised and overcoated.

I would suggest that as one is looking at a bundle of parallel rays of a diameter no greater than that of the aperture of the sextant telescope, this may well be adequate. Try your friendly local glazier, Paul. He might be as generous as mine and you can experiment. The glass doesn't have to be circular (though most glaziers can cut a circle for you). It's just that its easier to optically work a disc.

Bill Morris
Pukenui
New Zealand
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