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A Herschel wedge or Herschel prism is an optical prism used in solar observation to refract most of the light out of the optical path, allowing safe visual observation. It was first proposed and used by astronomer
John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wor ...
in the 1830s.


Overview

The prism in a Herschel wedge has a trapezoidal cross section. The surface of the prism facing the light acts as a standard diagonal mirror, reflecting a small portion of the incoming light at 90 degrees into the eyepiece. The trapezoidal prism shape refracts the remainder of the light gathered by the telescope's
objective Objective may refer to: * Objective (optics), an element in a camera or microscope * ''The Objective'', a 2008 science fiction horror film * Objective pronoun, a personal pronoun that is used as a grammatical object * Objective Productions, a Brit ...
away at an angle. The Herschel wedge reflects about 4.6% of the light that passes through one of the prism faces that is flat to 1/10 of the wavelength of the light. The remaining ~95.4% of the light and heat goes into the prism and exits through the other face and out the back door of the housing; thus, the excess light and heat is disposed of and not used for observing. While they decrease the intensity of the light, they do not affect the visible spectra, resulting in a more accurate spectral profile, which can be filtered to bring out certain details. They are an alternative to white light filters, which, despite their name, inherently must block certain visible spectra.


Limitations

While the prism is constructed of special glass which absorbs UV and IR light, it cannot be used with a
reflecting telescope A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...
design. This is because reflecting telescopes usually have a secondary mirror close to focus which may be damaged by the heat caused by IR light. This concentrated IR, in particular, can crack or unglue optical elements in a telescope, especially those located close to the focal plane, resulting in serious injury to the eye or possible blindness. The glass objective lens in a refracting telescope never absorbs these wavelengths (since doing so would make it impossible to image IR/UV wavelengths from dust-clouds and nebula) – indeed, even a normal camera lens will transmit IR, which is why digital camera sensors are fitted with IR blocking filters and why many amateur astronomers go to great lengths to remove these for astro-imaging. It is also important to note that even at 4.5%, (~N.D. 1.35) the light from the sun is still strong enough to burn the retina, and so an appropriate
neutral density filter In photography and optics, a neutral-density filter, or ND filter, is a filter that reduces or modifies the intensity of all wavelengths, or colors, of light equally, giving no changes in hue of color rendition. It can be a colorless (clear) or ...
must still be used.


See also

*
List of telescope parts and construction Hardware Accessories * Finderscope *Iron sight * Reflector (reflex) sight * Cheshire collimator: A simple tool to collimate a telescope Control * Clock drive *GoTo Mechanical construction * Mirror support cell * Serrurier truss *Silvering Mounts ...


External links

{{reflist Astronomical instruments Astronomical imaging Solar telescopes Prisms (optics)