Herschel wedge
The Herschel wedge, also known as the Herschel prism, is an optical device for astronomical observation of the sun . The operating principle comes from the German-English astronomer Wilhelm Herschel .
The construction uses the law of reflection and the refraction of light . Due to the 45 ° position of the first glass / air surface, approx. 4 percent of the sunlight is reflected to the eyepiece. The exact value for the reflected portion is described by Fresnel's formulas and depends on the refractive index .
The remaining light entering the glass , i.e. approx. 96 percent, is refracted towards the angle of incidence. The second glass / air surface, i.e. the exit surface, must be at a minimum angle to the entry surface. This minimum angle is determined in that the exiting light beam strikes perpendicularly, so that the light component reflected on the exit surface cannot reach the area of the beam reflected on the entry surface. The minimum angle of the wedge is therefore dependent on the refractive index of the glass used. Using BK7 with a refractive index of 1.52 results in a minimum angle of 27.7 °.
The light arriving in the eyepiece is still too strong for visual observation, so the remaining portion has to be reduced using a gray filter with a neutral density of ND 3.0. This filter is essential for visual observation. In combination with a polarization filter, the amount of light can be continuously adjusted.
Gray filters of lower density can be used for photography in order to keep the exposure time low.
In addition to the pentaprism , the lens solar filter and the sun projection method , the Herschel wedge is a way of observing the sun without endangering the eyesight. A Herschel wedge, in conjunction with a telescope , provides detailed images of the sun's photosphere . It can sunspots , flares and the granulation can be observed and photographed. The Herschel wedge should not be used with reflector telescopes, as the secondary mirror could heat up too much and be damaged as a result.