Hi Terry,

The highest accuracy is with electrolytic tilt sensors.

I have used sensors from two companies, Fredericks and Spectron. If you
need absolutely the best, Fredericks makes the best. You can achieve 2 or
3 arc-seconds over a commercial temperature range. Spectron's best is not
quite so good but considerably cheaper.

Fredericks also makes a cute little 2-axis sensor that is fairly
accurate, I think 30 arc-seconds or so, and it is also fairly linear so
you can use it to measure angle, not just a null.

If you are buying in small quantities, Fredericks might also be easier to
deal with.

Cheerful regards,

Bob

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