Lurie–Houghton telescope
Catadioptric telescope with spherical surfaces and Newtonian-style diagonal.
The Lurie–Houghton telescope is a modification of the Houghton telescope, a catadioptric design that uses a two-lens corrector at the front and a spherical mirror at the back. It is notable for using only spherical surfaces, which eases construction, and for incorporating a diagonal mirror on the corrector to direct light outside the tube, similar to a Newtonian telescope, allowing a shorter focal length and wider field of view.
- Patent year
- 1944
- Corrector lenses
- 2
- Mirror type
- spheroidal primary
- Corrector types
- symmetric and asymmetric
- Glass type
- usually BK7
Lore & Background
The Lurie–Houghton telescope is a modification of the Houghton telescope, which was patented in 1944. Houghton's original design uses a two-lens corrector at the front and a spherical mirror at the back. The corrector consists of a positive and a negative lens, all surfaces spherical, which eases construction compared to the Schmidt corrector plate or Maksutov meniscus lens. The corrector is placed in the path of parallel incoming light rays, so residual chromatic aberration is very nearly zero. The design tolerances are very relaxed compared to similar Maksutov telescopes.
Lurie's modification places a diagonal mirror on the corrector to direct the focused light outside the telescope tube, like a Newtonian telescope. This allows a shorter focal length and wider field of view. The secondary mirror in the Lurie–Houghton is identical to that in a Newtonian telescope, but can be mounted to one of the corrector lenses, eliminating the spider mount and thus star image diffraction spikes caused by spider vanes.
The corrector has enough degrees of freedom to correct for spherical aberration, coma, and field curvature. There are two types of correctors: symmetric and asymmetric. The symmetric version has r1 = −r3 and r2 = −r4, allowing interferometry testing of surfaces against each other, though correction is not quite as good as with the asymmetric corrector, which has four different radii.
Reader's Guide
The Lurie–Houghton telescope represents a practical alternative to other catadioptric designs, as it uses only spherical surfaces that are relatively easy to make. The two-lens corrector, while not as thin as the Schmidt corrector, avoids the heavy meniscus lens of the Maksutov and the intricately shaped compound curve of the Schmidt corrector plate. Light loss and ghost reflections, troublesome in the past, are minimal with modern anti-reflective coatings. The design's relaxed tolerances and use of the same type of glass (usually BK7) reduce cost. The Lurie–Houghton's elimination of the spider mount for the secondary mirror removes diffraction spikes, a notable advantage over the Newtonian telescope. The primary mirror's radius of curvature is slightly less than that of the total system, and its diameter should be larger than the aperture set by the corrector to reduce vignetting.
Did You Know?
- The Lurie–Houghton telescope uses a two-lens corrector with all spherical surfaces, easing construction.
- The corrector can be made of the same type of glass, usually BK7, which reduces cost.
Frequently Asked Questions
What is the Lurie–Houghton telescope?
It is a catadioptric telescope design that builds on the Houghton layout by adding a Newtonian-style diagonal mirror to the front corrector assembly. This modification lets the light exit the tube at a right angle rather than straight through the back.
How does the Lurie–Houghton telescope differ from a standard Houghton telescope?
While a Houghton telescope sends light straight back through the corrector to the eyepiece, the Lurie–Houghton variant places a diagonal mirror on the corrector to redirect the beam sideways. This allows a shorter tube length and a wider usable field of view.
Why does the Lurie–Houghton telescope use only spherical surfaces?
Every optical surface in the design—both corrector lenses and the primary mirror—is spherical, which makes grinding and testing far simpler than figuring aspheric elements. This keeps manufacturing costs and tolerances manageable.
What materials and corrector configurations are used in the Lurie–Houghton telescope?
The two-lens corrector is typically made from BK7 glass and can be built in either a symmetric or an asymmetric arrangement. The primary mirror at the rear is a spheroidal reflector.
When was the Lurie–Houghton telescope patented?
The design received its patent in 1944. It has since been recognized as a practical catadioptric option for hobbyists who want a wide field without aspheric figuring.
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