| This 20mm Astro-Tech Series 6 is a good medium power/wide field (65°) eyepiece at a very little price. The calculator below will let you see just how wide the field will be with your particular scope. For example, it will give you about a wide 1.17° field and a bright 4.4mm exit pupil at 57x with a 10” f/4.5 Dobsonian. This will give you almost four and a half minutes with such a scope to observe an object as it drifts from one side of the eyepiece’s flat field to the other. That’s plenty of time for multiple observers to look at the object or for you to absorb as much detail as possible before having to reposition the scope. With short focal length refractors, it’s a low power, very wide field eyepiece. With the 400mm focal length Astro-Tech AT-66, for example, it provides 20x, a 3.3mm exit pupil, and a huge 3.25° field of view that’s six and a half times as wide as the Moon. While it’s particularly well-suited for use with Dobsonian scopes and short focal length refractors, it can provide “picture window onto space” images with any telescope type. The low price of this eyepiece also makes it a very affordable choice to buy in pairs for binoviewer use. Its 13mm eye relief is quite reasonable for this focal length, although it can somewhat limit the field for eyeglass wearers. Since the Astro-Tech field is substantially wider to begin with than most other eyepieces in this price range, most eyeglass wearers won’t find the minor vignetting to be a drawback. Those who don’t wear glasses will see a huge field at only a fraction the cost of a more exotic wide field eyepiece.
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