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A Prismatic Circle Th. Wegener, Berlin No. 1948

A Prismatic Circle Th. Wegener, Berlin No. 1948. Brad Morris 5 June 2010. Provenance of this Circle. A Guided Tour. Matches Chauvenet’s description “A Manual of Spherical and Practical Astronomy” Volume II, Article 116 All components mentioned are present. Telescopes (3)

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A Prismatic Circle Th. Wegener, Berlin No. 1948

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  1. A Prismatic CircleTh. Wegener, Berlin No. 1948 Brad Morris 5 June 2010

  2. Provenance of this Circle

  3. A Guided Tour • Matches Chauvenet’s description • “A Manual of Spherical and Practical Astronomy” Volume II, Article 116 • All components mentioned are present • Telescopes (3) • Two Verniers on one Index Arm • Index Mirror • Prism • Shades • Arc Inspection Microscope • Telescope Shade • Telescope Prism • Adjusting Tools and Brush

  4. Telescope Details Prism Attachment Allows head to be out of the way of measurements close to 180⁰ Shade Attachment • Rotating disk • 4 Positions • Clear to Dark Cross Hairs

  5. How do the Shades Work?

  6. Optical Path • Path 1 • Green Dashed Line • Over top of Prism • Path 2 • Solid red line • Thru Prism • Reflected off of Index Mirror • Arc Graduations • -5 degrees • +280 degrees • Actual Travel of Index Mirror • -5 degrees • +277 degrees • Optical Path Occlusion • +114 degrees • +175 degrees • Live Demonstration using Slider!

  7. Index Adjustment with a Prism

  8. Horizon to Horizon Measurement • Rotate Circle About Telescope for second section • Subtract from 360⁰ • Yields +84⁰ to +185⁰ for second section • First Section: -5⁰ to +114⁰ • Second Section: +175⁰ to +276⁰

  9. Vernier to Vernier Mapping • Two Verniers, A & B • Null Index Error on Vernier A • Set Vernier A to zero • Defines Index Error on Vernier B • Set Vernier A to discrete values throughout travel • Read Vernier B values • Subtract Index Error B from all of Vernier B values • Chauvenet indicates (and chart shows) the average of vernier A and B as “true value”

  10. Star – Star SightsInformation on this page removed • Effort to Confirm Arc • Referenced article claims 5” accuracy • Star – Star Observations using • USNO Bright Stars List • Refraction and Aberration Correction • Results show objects pass thru each other • No correction required to date. • Set using only primary vernier The numbers refer to star to star distances on the arc of the circle, using a now discredited method. Current discussions now on NAVLIST to determine appropriate methodology Sept 18 2012.

  11. Chauvenet:“A Manual of Spherical and Practical Astronomy” Volume II, Article 116 Chauvenet’s Opinion Prismatic Circle versus a Sextant (It is unknown if this is just repeated promotional hype or if Chauvenet actually determined this) • Magnitude of Angular Measurement • Prismatic Circle: All • Sextant: Limited • Errors due to Eccentricity • Prismatic Circle: Errors eliminated by the use of mean of two verniers • Sextant: Eccentricity errors not eliminated • Brightness of Images • Prismatic Circle: • Brighter because of angle of incidence on central mirror • Brighter because Prism reflects better than a silvered mirror • Sextant: not as bright • Prismatic Error (parallelism of optics in path) • Prismatic Circle: • prism axes of rotation are independent and are thus more easily adjusted • Shades can be inverted • Sextant: • Coupled in horizon mirror Figure 34, showing the prismatic circle set to 180⁰ and the telescope prism

  12. Wanted: The Stand • Smithsonian • National Museum of American History • Artifact 1983.0245.02 • Cost in 1877 was $198.50 • Article was complete • Prismatic Sextant • Counterpoise Stand • About $4000 in 2010 dollars • Clearly shows that this device was used on land • A stand would never work at sea

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