Telescope Systems
Numerous telescope designs exist for a wide variety of applications. Two basic families of designs emerge when described by the characteristics of their operation.
Within each family, various methods for producing desired results are possible, providing the potential user guidance in selecting an approach.
Many applications involving lasers require the use of optical systems which operate between infinite conjugates. Such systems, commonly referred to as Beam Expanders, are, in fact, telescopes. They are used in controlling the energy of laser beams, correcting beam divergence, LIDAR systems, beam propagation studies, and in reducing the field of view and increasing the magnification of FLIR systems.
The most common, the classical Cassegrain, (sometimes known as Merseinne Telescope) employs two confocal paraboloids. The expansion ratio is given simply by the ratio of the focal lengths of the mirrors. The energy density varies with the square of this ratio. A less expensive version is obtained using a Dall Kirkham design comprised of elliptical primary and convex spherical secondary giving good on-axis performance.
A Gregorian telescope, employing a concave parabolic secondary mirror is sometimes used in combination with a pinhole placed at the common focus to "spatially filter" the laser beam.
In most telescope applications it is desired that light from a distant object or source be brought to focus where it may be detected, photographed, or measured. Conversely, these systems are often used as collimators or target projectors, where a target or source placed in the focal plane is imaged to some distant point. Designs commonly used for this purpose are described below:
Single Mirror Designs
In many cases, one-mirror optical designs provide an ideal solution. Conic surfaces of revolution possess two focal points between which perfect on-axis imaging can be obtained. In the case of the paraboloid, light from an infinitely distant point source is brought to a perfect, on-axis point image. Ellipsoids focus light similarly between two finite conjugate positions.
In the Newtonian Telescope, a small diagonal mirror is inserted in the focusing beam to bring it out at a right angle to the incoming beam. This yields a more accessible focused spot, but produces a central obscuration in the aperture which increases the system diffraction spot size.
To eliminate obscuration effects, an off-axis section of the primary mirror can be used. Known as a Herschelian Telescope, this design is common in collimation and target projection systems.
SORL offers three On-Axis Newtonian designs and a complete line of 19 Off-Axis TOAN Series Collimators, as well as full custom capabilities to meet your requirements.
Two Mirror Focusing Designs
The addition of a second mirror to the optical design allows the designer to improve the system field-of-view, increase the system focal length within a given package size, or reduce the package size while maintaining a given focal length and performance characteristics.
The Classical Cassegrainian Telescope employs a parabolic primary mirror, and a hyperbolic secondary positioned such that the parabolic and virtual hyperbolic focii coincide. In this configuration the on-axis image produced at the real hyperbolic focus is perfect, but off-axis performance suffers.
An increased field-of-view can be obtained by using two hyperboloids in a similar configuration. This is known as a Ritchey-Chretien design, which is completely corrected for spherical aberration and coma.
A less expensive design than either the Cassegrain or Ritchey-Chretien, the Dall Kirkham, uses an ellipsoid primary mirror and a spherical secondary mirror. Here the paraxial focii of the two mirrors are slightly separated, and spherical aberration is corrected by the ellipse. On-axis performance of this system is quite good, but degrades rapidly off-axis. For infrared applications, however, off-axis performance is often adequate.
Using a concave elliptical secondary mirror and parabolic primary results in a Gregorian Telescope. Designs of this nature, however, are not frequently encountered.
Off-axis versions of each of the above are possible, but only the Off-Axis Dall-Kirkham is common.
SORL offers a variety of standard telescope models employing these designs, or will manufacture a system to your specifications. Every model in the SORUCOAR Series offers focusing control to cover application requirements fulfilled by an Off-Axis Dail-Kirkham design.
Catadioptric Systems
The introduction of refractive elements to the optical design can, in large part, eliminate the off-axis aberration and field curvature characteristics of reflective telescopes. This is achieved at the expense of the all-reflective broad band spectral and achromatic performance advantages, but is often justified in systems where good imaging over a wide field and discrete wavelength band is desired. The combinations here are limitless, but include such familiar designs as the Schmidt, Schmidt-Cassegrain, and Bouwers-Maksutov. Such designs are often employed in astronomical and reconnaissance applications.
Designing a Telescope
Several parameters must be defined when designing a telescope. Some of these will be determined by the system performance requirements; others must be defined within practical limits such as the available space and operating environment. Here we will discuss some of these factors.
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