Charleston Lake
Imaging the Night Sky from Ontario Canada
Ultima PEC / Mechanical

Celestron Ultima 8 PEC
The Ultima 8 PEC is highly regarded for its extremely robust and stable mount.  The drive utilizes a precision Byers wormgear set, machined by Edward Byers. I have immense respect for Edward R Byers and the quality work he produced. 
The main worm wheel is 7.5" diameter and 359 teeth. At Sidereal rate, the worm rotates 1 turn in 4 minutes.
Edward R Byers was a highly regarded machinist. His gear sets are often cited for being less than 20 arc-second accuracy!! How much is 20 arc-seconds? With the 7.5" main gear, in terms of machining tolerances, about 0.36 thousands of an inch or less... That is 0.00036". 
The Tangent controls bring the ability to train and playback speed variation to compensate for what little Periodic Error exists in the geartrain. (PEC = Periodic Error Correction) 
Approximately 12 of the 20 arcseconds of error is periodic error, which can be nulled out using PEC, leaving approximately 8 arcseconds of non-periodic error remaining. The 8 arcseconds of remaining error equates to 0.0022224 degrees. Realizing that there is some non periodic error existing in the small gearbox between the motor and the worm, there are ways to also reduce this further. It is always recommended to have the telescope slightly unbalanced in Right Ascension, so that any backlash on the drive is negated. This backlash typically is in the 3 arc-second range. 
The end result of approx 5 arc seconds of error, and with a CCD camera at typical prime focus, being able to image integrations up to 180 seconds duration at 600mm focal length without auto guiding, while still achieving circular stars. (This is of course, focal length dependant). Employing auto guiding to null out the remaining 5 arc seconds of error, allows integrations of 5-30 minutes, beyond which the CCD camera artifacts become critical path to success. A higher quality CCD camera would experience less amp glow etc, allowing longer integrations, but satellites and airplanes also have a nasty habit of sailing through the field if integrating longer than 30 minutes. 
Each of the 30 minute integrations are stacked in software to achieve longer total image integration time. The stacking process reduces noise, while adding signal, resulting in a final image of greater Signal to Noise ratio than each of the individual sub-frames. 
The strength and rigidity of the mount, coupled with the Byers precision gearset and Periodic Error Correction... 
After you have fought with any mount with inferior tracking while doing long exposure imaging, you will very quickly appreciate a quality mechanism such as this. The Ultima PEC mount has far better stability and accuracy than the majority of consumer grade mounts made to this day.  
In a Sky & Telescope review of 8" Schmidt-Cassegrains, it was noted, "the Ultima 8's drive error was the least I have ever seen in a mass-market telescope." Astronomy Magazine said of the drive, "a textbook straight line" and "the PEC is impressive. It worked better than advertised." 
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(Original Celestron Document)
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