Herschel Wedge
Encyclopedia
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.
reflecting a small portion of the incoming light at 90%. The the trapezoidal prism shape refracts the remainder light gathered by the telescope's the objective
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 a wave. 95.4% of light and heat goes into the prism and exits through the other face and out the backdoor of the housing, thus the excess light and heat is dispensed 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.
designs. This is because a reflecting telescope will focus all wavelengths equally, including UV and IR, and the concentrated IR, in particular, can crack or un-glue optical elements in a telescope, which can result in injury/blindness. The glass objective lens in a refracting telescope absorbs these wavelengths (as does a Maksutov reflector
, with it's a glass corrector plate, though it is not recommended for use with a Herschel wedge).
It is also important to note that even at 4.5%, the light from the sun is still strong enough to burn the retina, and so an appropriate neutral density filter must still be used.
Solar telescope
A solar telescope is a special purpose telescope used to observe the Sun. Solar telescopes usually detect light with wavelengths in, or not far outside, the visible spectrum.-Professional solar telescopes:...
to refract
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
most of the light out of the optical path, allowing safe visual observation.
Overview
The prism in a Herschel wedge is a trapezoidal cross section. The surface of the prism facing the light acts the same as a standard diagonal mirrorStar diagonal
A star diagonal is an angled mirror or prism used in telescopes that allows viewing from a direction that is perpendicular to the usual eyepiece axis. It allows more convenient and comfortable viewing when the telescope is pointed at, or near the zenith . Also, the resulting image is right side...
reflecting a small portion of the incoming light at 90%. The the trapezoidal prism shape refracts the remainder light gathered by the telescope's the objective
Objective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...
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 a wave. 95.4% of light and heat goes into the prism and exits through the other face and out the backdoor of the housing, thus the excess light and heat is dispensed 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 that absorbed UV and IR light, it cannot be used with reflecting telescopeReflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
designs. This is because a reflecting telescope will focus all wavelengths equally, including UV and IR, and the concentrated IR, in particular, can crack or un-glue optical elements in a telescope, which can result in injury/blindness. The glass objective lens in a refracting telescope absorbs these wavelengths (as does a Maksutov reflector
Maksutov telescope
The Maksutov is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus lens in a design that takes advantage of all the surfaces being nearly "spherically symmetrical". The negative lens is usually full diameter and placed at the entrance pupil of the...
, with it's a glass corrector plate, though it is not recommended for use with a Herschel wedge).
It is also important to note that even at 4.5%, the light from the sun is still strong enough to burn the retina, and so an appropriate neutral density filter must still be used.