Telescope Eyepieces

Telescope Eyepieces Mike Swanson Eyepiece Basics • The main purpose of the • For example, a scope with eyepiece is to magnify the 1000mm focal length is used image produced by the with a 10mm eyepiece, objective of the telescope. resulting in a magnification • Eyepieces come in various of 100x (1000/10). focal lengths measured in • Prices range from about $20 millimeters (mm). to several hundred dollars. • The magnification provided • Good quality eyepieces are by an eyepiece is determined “multicoated”, better quality by dividing the focal length eyepieces are “fully of the telescope (also multicoated”. measured in mm) by the • Some eyepiece sets are focal length of the eyepiece. parfocal - very little refocusing required when Scope Calculator switching from one eyepiece to another. Eyepiece Basics • Eyepieces come in three • Some eyepieces are heavy sizes: .965”, 1.25” and 2”, and unbalance small scopes. which indicates the size of • The eye relief of an eyepiece the barrel that fits into the indicates the farthest focuser tube (part of the distance your eye can be scope itself). from the first lens and still • The .965 variety is only take in the entire field of available in low quality view (FOV). eyepieces and must be • With most eyepiece designs, avoided. the shorter the focal length, • The majority of eyepieces the shorter the eye relief. are 1.25”. • If you must wear glasses • The 2” models are to allow when using a scope for a wider field of view on (necessary if you have low power eyepieces. astigmatism), a long eye relief will be required. Field of View (FOV) • The amount of sky we can • Eyepieces have a see through a telescope (or characteristic known as binoculars) is measured in Apparent FOV (AFOV). degrees and is called the • The TFOV given by a FOV or more correctly the specific eyepiece and True FOV (TFOV) telescope is calculated as: • The maximum possible AFOV TFOV is determined by the Magnification focal length of the scope and size of the “field stop”. • So if we are using a 1000mm scope and a 10mm eyepiece • Field stop is basically the with a 50 degree AFOV: inside diameter of the 50 eyepiece used, so, 2” = .5 deg eyepieces give a wider 1000/10 possible TFOV than 1.25” True Field of View eyepieces. Scope Calculator Common Eyepiece Types Kellner Usually included with intro scopes. Ok in longer • AFOV: focal lengths. Exhibits chromatic aberration and 45-50 poor performance at the edge of the FOV. • 3 lenses Orthoscopic Good eye relief at short focal lengths. Narrow • AFOV: field of view. Poor performance at the edge of 40-45 the FOV. Excels at planetary views. • 4 lenses Plossl Very good performance to the edge with few • AFOV: 50 optical flaws. Best all-around performer. • 4 lenses Moderate FOV. Poor eye relief at short focal lengths. Wide Angle Various trade names and designs. The most • AFOV: expensive class of eyepiece, prized for wide field 60-100 of view at higher magnifications. Try before you • 5 or more buy. Watch for poor sharpness at the edge, lenses difficulty positioning eye and black-outs when moving head. Some are very heavy. Long Eye Various trade names and designs. Try before you • AFOV Relief designs buy - but very nice for eyeglass wearers. Some varies by users complain of poor optical performance and type difficulty positioning eye. Eye relief typically • 5 or more 20mm. lenses Zoom Eyepieces Pros Cons • Zoom eyepieces allow you • Most have very narrow field to twist the barrel of the of view at lower magnifica- eyepiece to adjust its focal tion – 40 degrees or possibly length, thereby adjusting the less – just when you want a magnification. wider field of view. • Very convenient for tuning • Often, optical quality is in to just the right lower than even a budget magnification for a given Plossl eyepiece. object. • Some are mechanically a • Can save you money – like nightmare - sloppy design, having multiple eyepieces, hard to twist, etc. all in one. Be sure to try before you buy! Warning, Warning!!! There are some designs not previously mentioned, and they are to be avoided. These are some of the earliest eyepiece designs and exhibit many serious optical flaws. They are often shipped with department store scopes: • Huygenian - usually marked as H4 (Huygenian 4mm) • Ramsden - usually marked as R20 (Ramsden 20mm) Barlows and Magnifiers • A Barlow lens effectively • Better Barlow lenses sport stretches the focal length of multicoated optics, precise any scope resulting in higher mechanical tolerances, and magnification from any effective internal blackening. given eyepiece. • Typical Barlows have 2 • The Barlow is inserted into lenses, but better models the eyepiece holder and the include a third lens for edge eyepiece is then inserted into correction and reduced the Barlow. chromatic aberrations. • Barlow lenses are • TeleVue Powermates (4 lens designated by their elements) – available in 2x, magnification factor – 2x, 2.5x, 4x and 5x – are similar 3x, etc. in concept, but provide the best in image fidelity and do not extend the eye relief of the eyepiece. Binoviewers • You have two eyes...use • The greatly added weight them! may cause balance problems • Binoviewers fit into the for some telescopes/mounts. telescope’s eyepiece holder • Problems merging images and has holders for two can be caused by eyepieces. mismatched or poor quality • Prisms inside the binoviewer eyepieces, uncollimated split the light so that half goes to each eyepiece. binoviewer or inability to position the eyepieces close • The added distance between the eyepieces and the focal enough for your plane of the telescope may interpupillary distance. make focusing impossible • Less expensive models may without an adapter. result in vignetting of wide • Pairs of identical eyepieces field images are needed. • Need room for your nose! Clear Skies! Back.

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