Crossing the Saddle at Night

Leaving Hilo I turn towards the shortest path home. It is also my favorite path by far. Not for me the twisting turns, small towns, and driving rains of the Hamakua coast road. I turn towards Saddle Road, to the pass between the enormous volcanoes of Hawaii.

Saddle Road
Saddle Road seen as it was in 2007, before rebuilding
The road is smooth and fast now. The Saddle of legend and rental car prohibition is mostly gone, only fragments remain. While you can still drive bits of the old Saddle, they are no longer the main road, bypassed by the new highway.

Even before the road was re-built this was my favorite route to cross the island. The traffic is far heavier now, the new road no longer offers the challenges and dangers of the old road. Drivers no longer deterred by those dangers now use the new road to cross the island rather than driving around the northern belt road.

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Old School Drive Corrector

Telescope clock drives from the 1980’s or earlier often used AC synchronous motors. These commonly available AC motors are used to power timeclocks, record player turntables, and telescopes, anyplace a motor needed to run at a very accurate speed.

Sidereal Drive Platform
A Celestron telescope drive configured as a sidereal rate tracking camera platform

The speed of a synchronous motor is set by the frequency of the powerline, in North America and many other places this is 60Hz. As the frequency must be synchronized for every power station on the grid the frequency is quite accurate, a feature exploited by clockmakers and telescope builders. Once found everywhere these motors are less common, but are still around.

Drive Correctors

It was the common use of these motors in telescope drives that led to the invention of the drive corrector, a device that was once a required piece of kit for serious amateur astronomers. Drive correctors like this were needed when operating from a battery at some remote location, generating AC from a 12Vdc car battery.

You also needed a drive corrector for guiding while doing astrophotography. The corrector could speed up or slow down the telescope drive a bit to correct the telescope drive speed and stay on target, something not possible with the fixed 60Hz of the mains supply. Thus the term drive corrector.

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