Key Takeaways:

  • The RC telescope, part of the Cassegrain system, overcomes coma and spherical aberration with hyperbolic mirrors, improving off-axis clarity in reflecting telescopes.
  • Developed by George Ritchey and Henri Chrétien in 1910, the RC telescope uses hyperbolic mirrors to enhance image quality for astronomical observations.
  • Unlike traditional reflecting telescopes, it avoids spherical aberration, ensuring consistent image quality across a broader field of view.
  • The RC telescope requires precise alignment and maintenance due to its complex mirror shapes, which are more challenging and expensive to produce.

Introduction to RC Telescope

The RC (Ritchey–Chrétien) telescope is a type of reflecting telescope that belogs to the Cassegrain optical system. It was designed by George Ritchey and Henri Chrétien in 1910, and is named after the initials of both men’s names. The RC Telescope is a specialized variant of the Cassegrain telescope that uses a primary hyperbolic mirror and a second hyperbolic mirror designed to eliminate off-axis optical errors (coma). The RC Telescope has a wider field of view and fewer optical errors than traditional reflecting telescopes. This design also elimiates chromatic aberration, which is a natural advantage over refracting telescopes. Additionaly, reflective telescopes are usually metal-coated, allowing for the use of wavelengths across both visible and infrared bands. As a result, the vast majority of astronomical telescopes use a reflective design.

Reflecting Telescope

Newton’s first reflective telescope used a spherical mirror. However, spherical mirrors are well known to have spherical aberration, which is proportional to the third power of the aperture size, limiting both the F-number and image quality are limited. Spherical mirrors offer numerous rotational symmetry axes, resulting in uniform image quality accross the field of view. They are particularly suitable for systems with small relative aperture and low image quality requirements. The spot pattern of the Newton telescope with F=8.3 is presented below. However, due to spherical aberration, the image quality of the entire field of view is not ideal.

coma, spherical aberration, Cassegrain system, RC Telescope
2D Optical Path Diagram
coma, spherical aberration, Cassegrain system
Spot Diagram

F=100mm, F=8.3, spot of Newton telescope 

Cassegrain System & RC Telescope

In the face of  the spherical aberration problem of reflective telescopes, optical designers have proposed different methods to improve it, and various variants have been born. The most classic design is to add a convex mirror, which not only compresses the length of the structure, but also increases the freedom to improve the image quality because of the introduction of the convex mirror. This structure is called the Cassegrain system, as shown below. The Cassegrain system has two mirrors, and generally the concave mirror is called the primary mirror and the convex mirror is called the secondary mirror. Due to the introduction of a secondary mirror, a portion of the light is blocked, and the ratio of the diameter of the light-blocking portion to the diameter of the input beam is called the light-blocking coefficient. The system with a large light block will not only have a large energy loss, but also the image quality will be affected. The design should choose the appropriate blocking ratio.

The classical Cassegrain telescope usually uses paraboloid as the main mirror type, and the paraboloid satisfies the condition of equal optical path, which can solve the spherical aberration problem perfectly. However, the problem of the paraboloid is that there is only one axis of rotational symmetry, if the beam is not vertically incident, it will cause the problem of coma increasing with the increase of the field of view, and there is a certain astigmatism and field curvature, so although the axial image quality of this telescope is perfect, the off-axis image quality is not ideal, and the available field of view is small. In the following figure, F=8.3 Cassegrain telescope, although the axial field of view has reached the diffraction limit, the off-axis field of view has an obvious coma.

coma, spherical aberration, Cassegrain system
coma, spherical aberration, Cassegrain system, RC Telescope
F=100mm, F=8.3, the spot of the Cassegrain telescope

Based on the Cassegrain system, the RC telescope was born by using hyperboloid as the face type of the primary mirror and the secondary mirror. The RC telescope eliminates most of the spherical aberrations and comas, and although the on-axis image mass is slightly reduced compared to parabolic reflection systems, the reduction is not significant. Since the coma is eliminated, the off-axis image quality of the RC telescope is improved and the image quality is uniform. However, the RC telescope has not solved the problem of field curvature and astigmatism, and the light spot generally expands elliptically as the field of view increases. The field of view of the RC telescope is increased relative to the paraboloid-Cassegrain system, but it is also limited, as shown below

What is RC Telescope?
F=100mm, F=8.3, RC telescope spot

For applications requiring a large field of view, a field lens can also be added in front of the image field of the RC telescope to correct aberrations such as field curvature and astigmatism, which can further improve the image quality. The following is the RC telescope with two pieces of flat field lens, and the image quality is relatively good throughout the 1.25° half field of view.

coma, Reflecting telescope , spherical aberration, Cassegrain system, RC Telescope
What is RC Telescope?
F=100mm, F=8.3, spot of RC telescope + correction field lens

RC Telescope Summary

RC telescopes are reflector telescopes with hyperboloid for both primary and secondary mirrors. Compared to other forms of Cassegrain systems, the relative aperture of RC telescopes can be larger, usually between F6-F10. The image quality on the axis can easily reach the diffraction limit, and in a given field of view, the image quality off the axis can also be well maintained, and the image quality of the larger field of view can be further improved with the compensating lens.

The RC telescope also has some drawbacks. Due to the limitation of the structure of the RC telescope, there is a blackout, which will lose some energy, this is a problem that all reflector telescopes will face. At the same time, this structure is easy to introduce large aberrations due to eccentricity, which requires the optical axis of the primary mirror and the secondary mirror to have a good overlap, so the requirements for installation are higher, and even regular correction and maintenance are required. In addition, both the primary and secondary mirrors of RC telescopes are hyperbolic, which is relatively difficult to process and more expensive than spherical or other forms of reflectors.

The RC telescope is a design with both advantages and disadvantages, but it has played an important role in some special fields, especially in astronomical observation.

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