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This is a used Celestron 20mm ERFLE 1.25" eyepiece in great condition. The body and optics are clean. Any dust you see in the photos is easily removable.
Heinrich Erfle invented this design during the First World War. The basic Erfle uses five elements in three groups and delivers a wide apparent field of view of about 60-degrees. Erfle variations are also made with six elements in three groups, as in the diagram below. Erfles have a nice, flat field from edge to edge, although sharpness and contrast is slightly inferior to the Orthoscopic and Plossl designs. They also feature long eye relief, especially important for eyeglass wearers.
This combination of features makes these oculars excellent for wide sky viewing of open clusters, star fields, etc. They are usually found in focal lengths in the 18mm to 32mm range.