Most HyperStar telescopes will not need an equatorial wedge.  Read on to find out why you might need one with your setup, why you don't need one with HyperStar, and why that is an impressive feat!

Currently, the most popular telescopes -- especially for CCD imaging -- are the goto Schmidt-Cassegrain telescopes.  These telescopes offer a convenient, versatile, portable package at a reasonable price.  Part of their convenience comes from the fact that they are on alt-az mounts, meaning they are supported on vertical fork arms.  This places the eyepiece in a convenient position at all times.  But it means the telescope suffers from field rotation.

Field rotation arises from the fact that everything in the sky appear to move along an arc.  When an alt-az telescope tracks it does so by moving in small steps in both the vertical (altitude) and horizontal (azimuth) axes.  This motion causes an object in the field of view to rotate as the telescope crosses the sky.  This rotation can be eliminated by mounting the telescope equatorially.  An equatorially-mounted telescope has its fork arms pointed toward Polaris, the North Star, and thus rotates about the same axis as Earth.  Now it can follow the objects in the sky along the same arc and the object does not appear to rotate.

The effects of field rotation can show up even in exposures of just a few minutes.  HyperStar eliminates the need for long exposures, so field rotation is not an issue.  This is why HyperStar scopes do not necessarily need wedges.  For non-HyperStar configurations, an equatorial wedge will be essential.

Above:  An example of field rotation in an image taken without a wedge.  Notice the stars at the top are not nearly as trailed, and that unlike tracking errors, field rotation elongates the stars in different directions in different parts of the image.  Still impressive, though, is that this image was taken in a suburban location in less than 3 minutes!  (Image by Tad Denton.)

An equatorial wedge is already a part of older non-goto fork-mounted Schmidt-Cassegrains.  A telescope on a German equatorial mount is also free from field rotation.

For much more information on accessories for CCD imaging, visit the Equipment Basics section.

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