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Perspectives on Telescope Eyepieces

Perspectives on Telescope Eyepieces. November 13, 2007 James Chen. Objectives of this Presentation. Historical Perspective Design Considerations Eyepiece Designs Selection Criteria Value versus Cost. Forty Years Ago. Limited Choices Huygens or Ramsden (standard eps w/telescope)

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Perspectives on Telescope Eyepieces

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  1. Perspectives on Telescope Eyepieces November 13, 2007 James Chen

  2. Objectives of this Presentation • Historical Perspective • Design Considerations • Eyepiece Designs • Selection Criteria • Value versus Cost

  3. Forty Years Ago • Limited Choices • Huygens or Ramsden (standard eps w/telescope) • Kellner • Orthoscopics • Mysterious French eyepiece - Clave’ Plossl • Military surplus • 0.965 or 1.25 eyepiece size

  4. Design Considerations • Resolution • Apparent Field of View (AFOV) • Focal Length • Eye Relief • Contrast • Characteristics of telescopes (i.e. f/ratio)

  5. Chromatic Aberration Field curvature Angular magnification distortion Rectilinear distortion Astigmatism Spherical aberration Spherical aberration of the exit pupil Transmission anomalies by wavelength Vignetting Coma Light loss Wavefront aberrations Loss of contrast due to light scatter Thermal issues Aberrations

  6. Early EyepiecesHuygens Ramsden

  7. 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’sKellner

  8. Classic high contrast planetary eyepiece AFOV 45 to 50 degrees poor eye relief at high powers. 1mm less f.l. high cost in the past, now affordable Slight loss of edge sharpness for low f/ ratios Upgrade Eyepieces - 1950’s, 1960’s Abbe Orthoscopic

  9. Our Friend - Plossl • 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 add addition elements - not truly Plossls • 1.25” 40mm AFOV limited

  10. Konig - the forgotten wide field • Limited availability • 60 to 70 degrees AFOV • Eye relief like Kellner • Images tend to soften towards the edges

  11. Monocentric • High contrast • No ghosting • Modern versions use improved glass • narrow AFOV 25 degrees • Planetary and double stars uses

  12. Early Wide-Field - Erfle • 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

  13. Brandon Eyepiece

  14. 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 28JAN1938 & US Patent 2,217,281 filed 18JAN1939. 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.

  15. Ultra-Wide Fields • Naglers • Pentax XW • Panoptics • Meade Series 5000 UWA, SWA • Antares W70, SW • Proxima • 1.25” and 2”

  16. Ultra-Wide Fields continued . • 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

  17. Zoom Eyepiece • General Purpose Zooms - • 8mm to 24mm • 7mm to 21mm • 6.5mm to 19mm • slightly narrow AFOV at low power typically 35 to 40 degrees • good AFOV at higher powers 50 to 60 degrees • High power Zooms - consistent AFOV approx. 50 degrees • 2mm to 4mm • 3mm to 6mm • 5mm to 8mm • Great for star parties, travel scopes, little kids

  18. a concave achromatic lens with negative focal length 2X, 3X enables high magnification with a longer eye relief eyepiece Barlow Lens

  19. Selection Criteria • What type of Viewing? Planets or DS • What type of Telescope is used? • Do you wear glasses? Astigmatism? • What aberrations are most noticeable to you? Curvature of field, pin-cushion, etc. • Cost and Budget. Cheap, reasonable, moderate, and “Oh My Lord!!!”

  20. Recommendations • Depends on your requirements • Planetary • Deep Sky • Budget • Value versus snob appeal

  21. Summary • Today is the Golden Age for Eyepieces • Buy for your requirements • Plossls and orthoscopics are still fine eyepieces • Barlows add versatility

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