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Finderscopes Part I Optical Systems: Refractors and Eyepieces

Finderscopes Part I Optical Systems: Refractors and Eyepieces. Refractors. Objective lens; certain aperture and focal length Focal length = distance from lens to focal plane Focal ratio = length / aperture F8 – F10 considered “long” or “slow” Under F6 considered “short” or “fast”.

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Finderscopes Part I Optical Systems: Refractors and Eyepieces

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  1. Finderscopes Part IOptical Systems: Refractors and Eyepieces

  2. Refractors • Objective lens; certain aperture and focal length • Focal length = distance from lens to focal plane • Focal ratio = length / aperture • F8 – F10 considered “long” or “slow” • Under F6 considered “short” or “fast”

  3. Refractors: Take-Aways • F-Ratio expresses how steep the light cone is • Aperture determines how much light is gathered • Aperture and F-Ratio determine focal length (any 2 determines the 3rd) • Focal length (with ep focal length) determine magnification

  4. Refractors: Pop Quiz • Q: How many variables discussed so far?

  5. Refractors: Pop Quiz • Q: How many variables discussed so far? • A: 3: aperture, ob.lens fl, ep fl

  6. Refractors: Pop Quiz • Q: How many variables discussed so far? • A: 3: aperture, ob.lens fl, ep fl • Is F-ratio a separate variable? (or just an expression of first 2)

  7. Refractors: Pop Quiz • Does F-ratio affect anything other than aperture or lens fl?

  8. Eyepieces - 3 things you care most about • 1) ep focal length – determines magnification (with length of ob.lens) • 2) field of view (fov) determines viewable area • 3) how “well-corrected” • determines the ep’s ability to bring entire field of view into focus at the same point • If not, at the faster focal ratios, coma and pincusioning at edges (okay F8+)

  9. Eyepieces – well-corrected • Where rubber+road ($$) • 40o fov: correction not a problem, shows only middle of field • 60o fov: usually good balance point • Fast F(F5-F6) doesn’t have to be well corrected to have tight edges • 70-80o fov: mess at F6 and faster unless well-corrected • Example: 1RPD vs Terminagler

  10. Eyepieces – field stop (and vignetting) • Field Stop: max diameter of light cone allowed to hit the bottom lens of the ep • Cone light outside the field stop gets blocked – wasted aperture • Rule of thumb: 1 to 1 match between ep’s focal length and field stop • (max focal length for 1.25” ep is about is about 25mm) • If you have a 30mm or 40mm 1.25 ep… • Fast F-ratio scopes – optical path vignetting possible “stopped down”

  11. Definitions with Examples • Focal Length: Aperture x F-ratio • 100mm scope: • F7 = 700mm F10 = 1,000mm • Magnification: (obj.lens FL) / (ep’s FL) • 700mm fl / 20mm ep = 35x mag • 1,000mm fl / 20mm ep = 50x mag • FOV: ep fov / magnification • (assume 20mm 60o fov ep) • 60o / 35x = 1.7o fov • 60o / 50x = 1.2o fov

  12. Want bigger fov? All other things being equal…

  13. Reduce magnification • Ep with same ep fov but longer ep fl • downside: brightness, resolvability • Need the mag? • Ep with same ep fl but wider ep fov • downside: ep’s for fast scopes require correction, $ • Use scope with shorter ob.lens fl • Get same aperture, use same ep • downside: magnification, faster scope constraints • Use scope with smaller objective lens • Keep f-ratio, keep same ep • downside: less light, less magnification

  14. Definitions with Examples • Exit Pupil: Ep fl / F-ratio • 100mm scope: • F7 = 700mm 20mm ep / F7 = 2.8 pupil • F10 = 1,000mm 20mm ep / F10 = 2.0 pupil • 127mm scope: • F7 = 890mm 20mm ep / F7 = 2.8 pupil • F10 = 1,270mm 20mm ep / F10 = 2.0 pupil

  15. Exit Pupil • If scope and ep combo > your pupil • Light loss • Reflector – darkening of center of fov from secondary mirror • Upside: can move your head around and see different parts of the fov • Generally, don’t want exit pupil over 5mm (quick calc: F-ratio * 5 is ep fl cross-over) • Becomes another constraint

  16. Constraints and Trade-Offs • Variables you Control • Aperture of scope • Magnification • F-Ratio of scope • EP focal length • EP field of view • How well-corrected ep is

  17. Constraints and Trade-Offs • Performance Attributes • Image brightness • Aperture • Magnification • Wide field of view • Flat field, edges sharp • Exit pupil not beyond 5-7mm • What enhances your observing? • What observing do you do (or use that scope for)?

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