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.

Afocal Systems: Incident light emerges collimated, 
Focal Systems: Incident light is brought directly to a real image.

Within each family, various methods for producing desired results are possible, providing the potential user guidance in selecting an approach.

Afocal Systems

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. 


Focal Systems

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. 

1. Resolution: As with any optical system, resolution is perhaps the single most important design consideration. For a telescope, on-axis resolution will depend on the figuring and alignment accuracy of the optical components, atmospheric turbulence, and diffraction. Diffraction effects arise from both the limiting primary mirror aperture and the central obscuration . Off-axis resolution over a given field-of-view will further depend on the design adopted. 

2. Mirror Apertures: The relative size of the primary and secondary mirror apertures are main considerations in the light collecting power and diffraction characteristics of the system. Light collection varies directly with the unobscured primary aperture area. Diffraction depends both on primary mirror aperture and size of secondary obscuration. 

3. Effective Focal Length: Effective focal length of the telescope system will determine its geometrical imaging properties. Effective f-number will affect image quality. Since effective focal length is determined by the degree to which the primary mirror focal length is magnified by the secondary, flexibility between these focal lengths and the overall required package size is possible. Back focal length should be chosen convenient to the application. 

4. Materials: As discussed earlier, a wide range of choices are possible, including glass, ceramics, and metals. Environment, weight, and accuracy requirements must be considered. 

5. Optical Coatings: May be selected from a variety of evaporated metals and dielectric materials for optimal performance in the wavelength region, optical power level, and environment of intended operation. 

6. Housing Design and Mounting Provisions: Careful consideration must be given to thermal and mechanical stresses, which may affect the optical components directly or their relative alignment. 

Manufacturing Difficulty: A number of design/cost trade-offs are available to the designer of a telescope system: 

1. Within the practical limits of structural stability, the cost of a typical system will vary directly with number, accuracy, type and speed of the aspheric components required to meet design specifications. This indicates the amount of material to be removed from "best-fit-sphere" when generating that surface. As the asphericity increases, so does the cost in manufacturing. When testing requirements are also considered, it is found that spherical components are least expensive, followed by paraboloids, ellipsoids, hyperboloids, and generalized aspherics, in that order. 

2. Alignment tolerances become very critical and difficult to maintain in extremely fast systems, and in systems involving two or more aspherics. Thus, it is recommended that reasonable space be allowed for length of system; that apertures of the components be kept at a minimum to meet light collection and resolution requirements; and that operational wavelength region be considered when specifying the figuring accuracy of components and field-of-view performance of the system. In the Infrared, it is often found that the least expensive available Newtonian or Dall-Kirkham design is adequate for performance requirements.

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.