Perspectives on Telescope Eyepieces November 13, 2007 James Chen
Objectives of this Presentation Historical Perspective n Design Considerations n Eyepiece Designs n Selection Criteria n Value versus Cost n
Forty Years Ago n Limited Choices – Huygens or Ramsden (standard eps w/telescope) – Kellner – Orthoscopics – Mysterious French eyepiece - Clave’ Plossl – Military surplus n 0. 965 or 1. 25 eyepiece size
Design Considerations Resolution n Apparent Field of View (AFOV) n Focal Length n Eye Relief n Contrast n Characteristics of telescopes (i. e. f/ratio) n
Aberrations n n n n Chromatic Aberration Field curvature Angular magnification distortion Rectilinear distortion Astigmatism Spherical aberration of the exit pupil n n n n Transmission anomalies by wavelength Vignetting Coma Light loss Wavefront aberrations Loss of contrast due to light scatter Thermal issues
Huygens Early Eyepieces Ramsden
Upgrade Eyepieces - 1950’s, 1960’s Kellner n n n AFOV 40 to 50 degrees Some ghosting Poor eye relief at high powers Modern versions (RKE, MA, etc. ) relatively low cost Not suitable for low f/ ratios
Upgrade Eyepieces - 1950’s, 1960’s Abbe Orthoscopic n n n Classic high contrast planetary eyepiece AFOV 45 to 50 degrees poor eye relief at high powers. 1 mm less f. l. high cost in the past, now affordable Slight loss of edge sharpness for low f/ ratios
Our Friend - Plossl n n n Approx. 50 degree AOV Available wide range of focal lengths 1. 25” or 2” barrel size Poor eye relief at high powers Some modern variants addition elements not truly Plossls 1. 25” 40 mm AFOV limited
Konig - the forgotten wide field n n Limited availability 60 to 70 degrees AFOV Eye relief like Kellner Images tend to soften towards the edges
Monocentric n n n High contrast No ghosting Modern versions use improved glass narrow AFOV 25 degrees Planetary and double stars uses
Early Wide-Field - Erfle n n Typically 60 to 65 degrees WW II surplus some astigmatism and edge distortion, worse with low f-ratios modern versions use new glass and different curves and spacing for better performance
Brandon Eyepiece
Brandon Continued The Brandon Orthoscopic is a reversed asymmetric Abbe doublet, designed by Chester Brandon, an American optical and instrument designer, in 1942. Brandon's design is the reverse of that of a doublet Orthoscopic eyepiece designed by Albert Konig, on behalf of Carl Zeiss, Jena, in late 1937, filed in Germany 28 JAN 1938 & US Patent 2, 217, 281 filed 18 JAN 1939. Brandon's design comprises a doublet field lens with an almost flat first surface negative meniscus flint in contact with a bi-convex crown and an eye lens with an almost equi-convex crown, nearly touching the second surface of the field lens, and a negative meniscus flint in contact , shallower convex side facing the eye.
Ultra-Wide Fields n n n n Naglers Pentax XW Panoptics Meade Series 5000 UWA, SWA Antares W 70, SW Proxima 1. 25” and 2”
Ultra-Wide Fields continued. n n n n Computer-aided designs 5, 6, 7, or even 8 element optical systems Individual elements not restricted to simple plano convex and plano concave units Exotic glass and coatings used Some equipped with integral Barlow lenses Designed to be used with short f/ratio Due to complexity and multiple optical surfaces and elements, some light loss and possible contrast loss HIGH COST
Zoom Eyepiece n General Purpose Zooms – – – 8 mm to 24 mm 7 mm to 21 mm 6. 5 mm to 19 mm slightly narrow AFOV at low power typically 35 to 40 degrees good AFOV at higher powers 50 to 60 degrees n High power Zooms - consistent AFOV approx. 50 degrees – 2 mm to 4 mm – 3 mm to 6 mm – 5 mm to 8 mm n Great for star parties, travel scopes, little kids
Barlow Lens n n n a concave achromatic lens with negative focal length 2 X, 3 X enables high magnification with a longer eye relief eyepiece
Selection Criteria What type of Viewing? Planets or DS n What type of Telescope is used? n Do you wear glasses? Astigmatism? n What aberrations are most noticeable to you? Curvature of field, pin-cushion, etc. n Cost and Budget. Cheap, reasonable, moderate, and “Oh My Lord!!!” n
Recommendations n Depends on your requirements – Planetary – Deep Sky Budget n Value versus snob appeal n
Summary Today is the Golden Age for Eyepieces n Buy for your requirements n Plossls and orthoscopics are still fine eyepieces n Barlows add versatility n
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