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Perspectives on Telescope Eyepieces November 13, 2007 James Chen.

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Presentation on theme: "Perspectives on Telescope Eyepieces November 13, 2007 James Chen."— Presentation transcript:

1 Perspectives on Telescope Eyepieces November 13, 2007 James Chen

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

3 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

4 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)

5 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

6 Early Eyepieces Huygens Ramsden

7 Upgrade Eyepieces - 1950’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

8 Upgrade Eyepieces - 1950’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

9 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

10 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

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

12 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

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

16 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

17 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

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

19 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!!!”

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

21 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

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