causes balancing issues in Schmidt-Cassegrains under 10 inches, refractors under 5 inches, and reflectors under 16 inches. Also, due to their large field stops, without larger secondary mirrors most reflectors and Schmidt-Cassegrains will have severe vignetting with these eyepieces. Makers of these eyepieces include Explore Scientific and Siebert Optics. Telescopes that can accept these eyepieces are made by Explore Scientific and Orion Telescopes and Binoculars. 4 in. (102 mm) – These eyepieces are rare and only commonly used in observatories. They are made by very few manufacturers, and demand for them is low. Microscope eyepieces Eyepieces for microscopes have barrel diameters measured in millimeters such as 23.2 mm and 30 mm. Eye relief The eye needs to be held at a certain distance behind the eye lens of an eyepiece to see images properly through it. This distance is called the eye relief. A larger eye relief means that the optimum position is farther from the eyepiece, making it easier to view an image. However, if the eye relief is too large it can be uncomfortable to hold the eye in the correct position for an extended period of time, for which reason some eyepieces with long eye relief have cups behind the eye lens to aid the observer in maintaining the correct observing position. The eye pupil should coincide with the exit pupil, the image of the entrance pupil, which in the case of an astronomical telescope corresponds to the object glass. Eye relief typically ranges from about 2 mm to 20 mm, depending on the construction of the eyepiece. Long focal-length eyepieces usually have ample eye relief, but short focal-length eyepieces are more problematic. Until recently, and still quite commonly, eyepieces of a short-focal length have had a short eye relief. Good design guidelines suggest a minimum of 5–6 mm to accommodate the eyelashes of the observer to avoid discomfort. Modern designs with many lens elements, however, can correct for this, and viewing at high power becomes more comfortable. This is especially the case for spectacle wearers, who may need up to 20 mm of eye relief to accommodate their glasses. Eyepiece designs Technology has developed over time and there are a variety of eyepiece designs for use with telescopes, microscopes, gun-sights, and other devices. Some of these designs are described in more detail below. Negative lens or "Galilean" The simple negative lens placed before the focus of