Herschel wedge

The Herschel wedge Herschel prism is an optical device for astronomical observation of the sun. The operating principle is derived from the German - English astronomer William Herschel.

The design uses the law of reflection and the refraction of light. Due to the 45 ° position of the first glass / air surface about 4 percent of the sunlight is reflected to the eyepiece. The exact value for the reflected portion is described by the Fresnel formulas and depends on the refractive index.

The entering into the glass remaining light, or about 96 percent, is broken down to the normal line. The second glass / air surface, so the exit surface must be in a minimum angle to the entrance surface. This minimum angle is determined by that of the exiting light beam is incident perpendicularly, so that the can at the exit surface reflected light component not in the area of the reflected beam to reach the entrance surface. The minimum of the wedge angle is, therefore, the refractive index of the glass being used. When using BK7, having a refractive index of 1.52 results in a minimum angle of 27.7 °.

For the visual observation of the incoming light in the eyepiece is too strong, so that the remaining portion must be reduced by a neutral density filter with a neutral density of ND 3.0. This filter is absolutely necessary for the visual observation. In combination with a polarizing filter can be the amount of light adjusted continuously.

For photography gray lower density filter can be used to keep the exposure time low.

In addition to the pentaprism, the lens solar filters and solar projection method of Herschel wedge is a way to observe the Sun without risk of sight. A Herschel wedge, in conjunction with a telescope detailed images of the photosphere of the sun. It may be sunspots, flares, and the granulation observed or photographed. With reflecting telescopes of Herschel wedge should not be used because the secondary mirror could heat up too much and thereby forfeits.

  • Astronomical instrument
  • Prism
  • William Herschel
Astronomy Borosilicate glass Granule (solar physics)
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