Two Mirror Reflector
Introduction
All Cassegrain telescopes consist of a concave primary mirror with a small convex secondary mirror inside the focus of the primary. The secondary redirects light toward the primary. The image, in most cases, lies behind the primary where it is easily accessable for visual observing, photography or CCD imaging.
The convex secondary multiplies the focal length by a factor M. This factor M is termed secondary magnification, and is defined:
M = System Focal Length / Primary Focal Length
The shortest possible configuration for a certain focal length is achived with the Cassegrain-like telescopes. The
disadventages of two-mirror systems are the relatively large obstruction caused by the secondary mirror and the need for
a spider. Because Cassegrain type telescopes are normally not closed, dust, air turbulence and atmpspheric deterioration
of the mirrors can occur. Another factor is the relatively strong field curvature, especially when the diameter of the
secondary must be held small. Field curvature is closely connected with the curvature of the secondary mirror and,
therefore, with the value of the secondary magnification.
The two figures bellow shows the ray tracing and nomenclature in Cassegrain type telescopes.
Of the Cassegrain designs, the Ritchey-Chretien is the most difficult to manifacture because the two hyperbolic mirrors must be exactly matched in shape. It offers the best image quality possible with a two-mirror configuration, and is aplanatic. This type is usually built for photographic applications and is highly regarded ny professional astronomers. Although it has astigmatism and of course, curvature of field, premitting relatively wide field of view.
The Dall-Kirkham is easier to build because the secondary mirror can be kept spherical, but the system has strong coma, so the usable field is small. The elliptical primary of the Dall-Kirkham is also relatively easy to make. The classical Cassegrain lies between the Ritchey-Chretien and the Dall-Kirkham with the respect to both difficulty of fabrication and image quality.
Unknown type of Cassegrain telescope is the Pressmanm-Camichel, with a spherical primary mirror, but the secondary must be strongly deformed to remove spherical aberration. It has coma even stronger than that of the Dall-Kirkham, resulting in a very small usable field.
A significant and interesting variation on the Cassegrain is the Gregorian telescope. Instead of a convex secondary inside the focus of the primary mirror, this system has a concave secondary outside the focus and this leads to a longer tube length The distance between the mirrors is slightly more than the sum of their focal lengths. The image is upright and before the invention of the achromatic objective, this system was often used for terrestial observation.
The classical Gregorian has a parabolic primary and an elliptical secondary. Spherical aberration is eliminated, but the off-axis images suffer from coma, astigmatism, and field curvatire. As is the case with Cassegrain, other mirror shapes are possible. Likewise, although both spherical aberration and coma can be corrected, astigmatim remains present. Unlike the Cassegrain and most other telescopes, the Gregorian has an outward curving focal surface. This suggests the possibility of making an almost aberration/free visual telescope by matching the focal surface to the focal surface of the eyepiece.
The two figures bellow shows the ray tracing and nomenclature in Gregorian type telescopes.
Design and Optimization
Cassegrain type telescopes should be designed in such a way that the resulting system has the following properties:
All Cassegrain telescopes consist of a concave primary mirror with a small convex secondary mirror inside the focus of the primary. The secondary redirects light toward the primary. The image, in most cases, lies behind the primary where it is easily accessable for visual observing, photography or CCD imaging.
The convex secondary multiplies the focal length by a factor M. This factor M is termed secondary magnification, and is defined:
The two figures bellow shows the ray tracing and nomenclature in Cassegrain type telescopes.
Of the Cassegrain designs, the Ritchey-Chretien is the most difficult to manifacture because the two hyperbolic mirrors must be exactly matched in shape. It offers the best image quality possible with a two-mirror configuration, and is aplanatic. This type is usually built for photographic applications and is highly regarded ny professional astronomers. Although it has astigmatism and of course, curvature of field, premitting relatively wide field of view.
The Dall-Kirkham is easier to build because the secondary mirror can be kept spherical, but the system has strong coma, so the usable field is small. The elliptical primary of the Dall-Kirkham is also relatively easy to make. The classical Cassegrain lies between the Ritchey-Chretien and the Dall-Kirkham with the respect to both difficulty of fabrication and image quality.
Unknown type of Cassegrain telescope is the Pressmanm-Camichel, with a spherical primary mirror, but the secondary must be strongly deformed to remove spherical aberration. It has coma even stronger than that of the Dall-Kirkham, resulting in a very small usable field.
A significant and interesting variation on the Cassegrain is the Gregorian telescope. Instead of a convex secondary inside the focus of the primary mirror, this system has a concave secondary outside the focus and this leads to a longer tube length The distance between the mirrors is slightly more than the sum of their focal lengths. The image is upright and before the invention of the achromatic objective, this system was often used for terrestial observation.
The classical Gregorian has a parabolic primary and an elliptical secondary. Spherical aberration is eliminated, but the off-axis images suffer from coma, astigmatism, and field curvatire. As is the case with Cassegrain, other mirror shapes are possible. Likewise, although both spherical aberration and coma can be corrected, astigmatim remains present. Unlike the Cassegrain and most other telescopes, the Gregorian has an outward curving focal surface. This suggests the possibility of making an almost aberration/free visual telescope by matching the focal surface to the focal surface of the eyepiece.
The two figures bellow shows the ray tracing and nomenclature in Gregorian type telescopes.
Design and Optimization
Cassegrain type telescopes should be designed in such a way that the resulting system has the following properties:
- Short tube length
- Small secondary mirror
- Flat focal surface
- Accessible focal surface
- For given focal length, field of curvature increases as the diameter of the secondary mirror decreases and the separazion of the mirrors decreases.
Applet Tag:
- param name = "Title" value = "Two Mirror: Cassegrain Telescope"
- param name = "Telescope Type Index" value = "0"
- param name = "Primary Diameter" value = "150.0"
- param name = "Primary Focal Length" value = "500.0"
- param name = "System Focal Length" value = "2000.0"
- param name = "Primary Eccentricity" value = "1.000"
- param name = "Secondary Eccentricity" value = "2.778"
- param name = "Back Working Distance" value = "200.00"
- param name = "Half Field Angle" value = "0.45"
- param name = "Central Obstruction" value = "0.00"
- param name = "Plot Scale" value = "0.10"
- param name = "TF Start Angle" value = "0.00"
- param name = "TF End Angle" value = "0.45"
- param name = "TF Defocus" value = "0.05"
- param name = "Number of Arms" value = "36"
- param name = "Number of Rings" value = "10"
- param name = "Ray Density" value = "10"
- param name = "Scale Type Index" value = "3"
- param name = "Ray Pattern Index" value = "0"
- param name = "Merrit Function Index" value = "2"
- param name = "Glass Catalog Index" value = "2"
- param name = "Correction Index" value = "0"
- param name = "Monochr. Color Index" value = "10"
- param name = "WL1 Color Index" value = "1"
- param name = "WL2 Color Index" value = "4"
- param name = "WL3 Color Index" value = "9"
- param name = "Text Color Index" value = "10"