Charleston Lake
Astronomical
Imaging the Night Sky from Ontario Canada
Ultima PEC Telescope
Celestron Ultima 8 PEC
The Ultima 8 PEC was made by Celestron, late 1980's into early 1990's. It combines an OTA of 8" aperture (203.2mm) /2000mm focal length, on a very robust cast aluminum fork mount.
Perhaps more important than the optics, is the mount and it's tracking ability. The Telescope mount must rotate at the Sidereal rate, to remain aimed at the same point in space, during the rotation of the earth. For CCD imaging, this is critical, or the stars will be oblate and details smeared.
The Ultima PEC was, bar none, the most robust, stable, fork mount SCT ever mass produced. It was specifically designed for imaging purposes, and it performs this task very well.
The base and fork arms are ribbed, sand cast aluminum, damping any vibrations. The polar axis shaft is integrally cast into the base, the mount rides on a large preloaded bearing. Further, they used a 7.5 inch wormgear set, made by Ed Byers, for the Right Ascension axis. Ed Byers was a craftsman machinist, and the tracking is typically very good on the Ultima PEC mounts.
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."
Autoguiding is used for the fine-speed adjustments during long exposure imaging, using the Hand Controller port.
The Ultima PEC is not computerized, it is a manually positioned telescope without GOTO ability. Slewing is done via the Hand Controller. It has Periodic Error Correction ability (PEC). The PEC period is 4 minutes, one rotation of the worm gear against the worm wheel. Once trained, the correction is repeatedly played back to offset the small periodic error that is inherent to any worm drive.
Tangent Instruments designed and built the control electronics for this telescope, on contract to Celestron.
The Ultima PEC was discontinued long ago, but these mounts remain excellent performers.
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(Original Celestron Document)
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