# Presentation: 360002

## 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)
- Kellner
- Orthoscopics
- Mysterious French eyepiece - Clave’ Plossl
- Military surplus
- 0.965 or 1.25 eyepiece size

## Design Considerations

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

## 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

## Early EyepiecesHuygens                                               Ramsden

## Upgrade Eyepieces - 1950’s, 1960’sKellner

- 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

- 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

## 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

## Konig - the forgotten wide field

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

## Monocentric

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

## 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

## 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 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

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

## 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

## 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

## Barlow Lens

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

## 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!!!”

## Recommendations

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

## Summary

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