A Prismatic CircleTh. Wegener, Berlin No. 1948

Brad Morris5 June 2010

Provenance of this Circle

A Guided Tour

• Telescopes (3)• Two Verniers on one

Index Arm• Index Mirror• Prism• Shades• Arc Inspection

Microscope• Telescope Shade• Telescope Prism• Adjusting Tools and Brush

•Matches Chauvenet’s description• “A Manual of Spherical and Practical Astronomy” Volume II, Article 116

•All components mentioned are present

Telescope Details

Prism Attachment

Shade Attachment

Cross Hairs

• Rotating disk• 4 Positions• Clear to Dark

Allows head to be out of the way of measurements close to 180⁰

How do the Shades Work?

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!0

0⁰

-20

-15

-10

-5

0

5

10

15

20

-20 -15 -10 -5 0 5 10 15 20

Critical Angle 45.60224AngleA 9.00B 51.32C 8.87

AngleX AxisY Axis

Prism Planar Mirror

Index Adjustment with a Prism

Horizon to Horizon Measurement•First Section: -5⁰ to +114⁰•Second Section: +175⁰ to +276⁰

•Rotate Circle About Telescope for second section•Subtract from 360⁰ •Yields +84⁰ to +185⁰ for second section

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”

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.

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

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