The Houghton telescope or Lurie–Houghton telescope is a
catadioptric
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses ( dioptrics) and curved mirrors ( catoptrics). Catadioptric combinations are used in focusing systems such as searchlights ...
telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...
. Houghton's original design was patented in 1944. Instead of the fairly hard to make
Schmidt and heavy
meniscus (Maksutov) corrector
lenses
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...
, the corrector for the Houghton is relatively easy to make. It consists of two lenses: a positive and a negative, set at the front of the telescope which fixes the telescope's aperture. All lens and mirror surfaces are
spheroidal, which eases construction. These lenses are relatively thin, though not as thin as the
Schmidt corrector. With a good
anti-reflective coating
An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the eff ...
, light loss and "ghost" reflections are minimal.
Lurie slightly modified Houghton's original design by adding a diagonal mirror to direct the focused light outside the telescope tube in the same way as a
Newtonian telescope
The Newtonian telescope, also called the Newtonian reflector or just a Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror. Newton' ...
.
The corrector
Each surface of the lenses in the corrector
[The total power of a "corrector lens" is 1, by definition.] creates a degree of freedom to correct optical aberrations. There are enough degrees of freedom to correct for
spherical aberration
In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strike a ...
,
coma
A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
, and field curvature. It is placed in the path of the incoming light rays, which are parallel, so the residual
chromatic aberration
In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the ...
is very nearly zero. The Houghton corrector can be made of the same type of glass (usually BK7) which reduces cost. The design tolerances are very relaxed, compared to the similar
Maksutov telescope
The Maksutov (also called a "Mak") 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 ...
s.
There are two types of correctors: symmetric and asymmetric. In the symmetric version ''R''
1 = −''R''
3 and ''R''
2 = −''R''
4 which allows for
interferometry
Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...
testing of the surfaces against each other. However, the correction is slightly worse than for the asymmetric corrector, which has four different radii.
The following figure shows the equations necessary to design the symmetric-form of the corrector.
[Note that in these equations ''D'' is not the diameter of the primary mirror; rather, it is the distance from the mirror to the corrector as a fraction of the focal length of the mirror.]
The mirrors
Unlike the paraboloidal mirror used in the Newtonian telescope, the Houghton uses a spheroidal primary mirror. A spheroidal mirror is much easier to make because the entire surface appears to uniformly "black out" when checked with a
Foucault test
The Foucault knife-edge test is an optical test to accurately measure the shape of concave curved mirrors. It is commonly used by amateur telescope makers for figuring primary mirrors in reflecting telescopes. It uses a relatively simple, inexp ...
. In the Houghton and the Lurie–Houghton, the radius of curvature of the primary mirror is slightly less than that of the total system. The diameter of the primary mirror should be larger than the aperture set by the corrector, to reduce
vignetting
In photography and optics, vignetting is a reduction of an image's brightness or saturation toward the periphery compared to the image center. The word '' vignette'', from the same root as '' vine'', originally referred to a decorative bor ...
.
The secondary mirror in the Lurie–Houghton is identical to the secondary mirror in a Newtonian telescope. An advantage of the Lurie–Houghton over the Newtonian is that the secondary mirror can be mounted to one of the corrector lenses, thus eliminating the spider mount. This eliminates star image
diffraction spike
Diffraction spikes are lines radiating from bright light sources, causing what is known as the starburst effect or sunstars in photographs and in vision. They are artifacts caused by light diffracting around the support vanes of the secondary m ...
s, caused by the
vanes of the spider mount.
Notes
References
See also
*
List of telescope types
The following are lists of devices categorized as types of telescopes or devices associated with telescopes. They are broken into major classifications with many variations due to professional, amateur, and commercial sub-types. Telescopes can be ...
*
Newtonian telescope
The Newtonian telescope, also called the Newtonian reflector or just a Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror. Newton' ...
External links
Example of an amateur built Lurie–Houghton telescope
{{DEFAULTSORT:Lurie-Houghton telescope
Telescope types