Perspectives on Telescope Eyepieces

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Perspectives on Telescope Eyepieces

• November 13, 2007
• James Chen

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Objectives of this Presentation

• Historical Perspective
• Design Considerations
• Eyepiece Designs
• Selection Criteria
• Value versus Cost

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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

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Design Considerations

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

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Aberrations

• 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

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Early EyepiecesHuygens
Ramsden
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Upgrade Eyepieces - 1950s, 1960sKellner

• 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

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Upgrade Eyepieces - 1950s, 1960s Abbe
Orthoscopic

• 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

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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

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Konig - the forgotten wide field

• Limited availability
• 60 to 70 degrees AFOV
• Eye relief like Kellner
• Images tend to soften towards the edges

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Monocentric

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

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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

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Brandon Eyepiece
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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.
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Ultra-Wide Fields

• Naglers
• Pentax XW
• Panoptics
• Meade Series 5000 UWA, SWA
• Antares W70, SW
• Proxima
• 1.25 and 2

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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

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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

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Barlow Lens

• a concave achromatic lens with negative focal
  length
• 2X, 3X
• enables high magnification with a longer eye
  relief eyepiece

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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!!!

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Recommendations

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

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Summary

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