Perspectives on Telescope Eyepieces November 13, 2007 James Chen

Objectives of this Presentation n Historical Perspective n Design Considerations n Eyepiece Designs n Selection Criteria n Value versus Cost

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

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

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

Early Eyepieces Huygens Ramsden

Upgrade Eyepieces ’s, 1960’s Kellner n AFOV 40 to 50 degrees n Some ghosting n Poor eye relief at high powers n Modern versions (RKE, MA, etc.) n relatively low cost n Not suitable for low f/ ratios

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

Our Friend - Plossl n Approx. 50 degree AOV n Available wide range of focal lengths n 1.25” or 2” barrel size n Poor eye relief at high powers n Some modern variants add addition elements - not truly Plossls n 1.25” 40mm AFOV limited

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

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

Early Wide-Field - Erfle n Typically 60 to 65 degrees n WW II surplus n some astigmatism and edge distortion, worse with low f-ratios n 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 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.

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

Ultra-Wide Fields continued. n Computer-aided designs n, 6, 7,or even 8 element optical systems n 5, 6, 7,or even 8 element optical systems n Individual elements not restricted to simple plano convex and plano concave units n Exotic glass and coatings used n Some equipped with integral Barlow lenses n Designed to be used with short f/ratio n Due to complexity and multiple optical surfaces and elements, some light loss and possible contrast loss n HIGH COST

Zoom Eyepiece n 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 n High power Zooms - consistent AFOV approx. 50 degrees –2mm to 4mm –3mm to 6mm –5mm to 8mm n Great for star parties, travel scopes, little kids

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

Selection Criteria n 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!!!”

Recommendations n Depends on your requirements –Planetary –Deep Sky n Budget n Value versus snob appeal

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