RC Optical Systems - Ritchey-Chrétien Overview

Why a Ritchey-Chrétien...

An example of off-axis coma typical in reflective optical systems. A Ritchey-Chrétien is a specialized Cassegrain reflector utilizing true hyperbolic surfaces. All reflectors typically have an aberration called coma - all except for the Ritchey-Chrétien. The hyperbolic mirrors of the Ritchey-Chrétien make this design coma free, which results in a much smaller spot size. This is the #1 reason why nearly all professional observatories have chosen the Ritchey-Chrétien design.

Since Ritchey-Chrétien optics are two mirror optical systems, they have no Spectral Dispersion or Chromatic Aberration. Chromatic Aberrations are common in Catadioptric Systems and caused by their refractive element known as a "corrector plate". See diagrams of Chromatic Aberration on and off axis in a Catadioptric system.

In addition to being coma free and chromatic free, a true Ritchey-Chrétien offers:

Only two surfaces for less light loss - Catadioptric type optics have 4 surfaces (two are glass!). Some APOs have 6 surfaces. Each surface degrades the amount of light getting to the focal point.
No refractive elements - Glass scatters light, especially in IR where CCD cameras are most sensitive. A Ritchey-Chrétien has no refractive elements, such as a corrector plate.
Flatter Field - The lower the amplification factor of the secondary mirror, the flatter the field. For example, the RC has a 2.7x secondary, whereas, a Schmidt-Cassegrain has a 5x secondary. The RC has a much flatter field!
Smaller Spot Size on and off axis. Examine this spot diagram to see the difference!

For these reasons, the Ritchey-Chrétien is ideal for medium to large format CCD or film photography, where a large aberration-free field of view is required. It is also a superb visual instrument.

A Historical Perspective of the Ritchey-Chrétien Optics...

Read more about George Willis Ritchey.

Why an RCOS Ritchey-Chrétien...

Stress test results on the RCOS Fork Mount. Engineering, CAD, and Solid Modeling:

EVERYTHING we manufacture is designed in CAD (Computer Aided Design) and then modeled using SolidWorks 2008. Rather than a "trial and error"approach, we design, engineer, and model all of our critical components. This ensures that you get the best product or part for its intended use. For example, we needed a low frequency, high strength fork mount for out latest .6m class telescope. By engineering and modeling, we were able to get the frequency and flexure to remarkable limits!

Once the component is designed and modeled, its machined and or fabricated using 6061 or 7075 aluminum, or steel alloy. All components are CNC Machined to ensure precision and repeatability.

Customer Support:

RCOS products are backed with unparalleled customer support. From personal one-on-one service to full product documentation, we strive to make certain you get the most out of your investment. In addition, RCOS owners enjoy the support of hundreds of other RCOS customers, a resource that is second to none.

RCOS Optical Tube Options...

Astronomy Use Truss verses Tube Design:

Carbon Truss Telescopes: We have been able to quantify a truss telescope does have better intrinsic seeing than a tube telescope. There is little chance of trapping a micro climate (heat causing tube currents) inside of a truss. This is the foremost reason why most all large professional telescopes are trusses. Truss telescopes also offer a lower cross section or "sail" area than a tube telescope, so they are less vulnerable to wind. And a truss is easily maintained as all components are readily accessible.

Carbon Tube Telescopes: The tube telescope is slightly better athermalized than a truss as carbon tubes offer the highest stability since there is less aluminum. They also offer better protection from stray light and are especially useful for daytime imaging.

Military (Ruggedized) Carbon Nomex Tube Telescopes:

Reinforced with Nomex honeycomb, these are the most durable carbon scopes made in their class. Designed for Mobile Missile Tracking Mounts, they can withstand the most punishing conditions. These instruments are designed for day or night use, visible to long wave infrared (demonstrated out to 12 microns), and endure high speed slewing up to 90 degrees per second.

Optics...

Ion milling process. Powered by True Ritchey-Chrétien Optics:

We use only the finest optics available in the Ritchey-Chrétien design featuring:

Expert Manufacturing- Our optics are manufactured by some of the most experienced opticians in the world, and guaranteed to be at least 1/25th wave RMS. Final figuring is done by either computerized polishing, or Ion Milling.
Zero Expansion Materials - All mirrors utilize zero expansion materials such as Astro-Sitall, Zerodur, or ULE. These materials ensure the mirror will maintain shape during changing temperatures.
Certified Optics - Interferometric testing and fringe analysis data is provided with every set of optics.
Flexibility - Available in any configuration or f/ratio, these true Ritchey-Chrétien optics are the finest in the world!

Collimation of Ritchey-Chrétien Optics:

"Is it difficult to collimate a Ritchey-Chrétien?" Our competitors would suggest that it is. Collimation can be challenging due to the fact that there are two mirrors to adjust - the primary and secondary. So we developed a procedure that allows you to collimate your Ritchey-Chrétien telescope quickly and easily. We also make this procedure available to owners of other brands of Ritchey-Chrétien and Cassegrains telescope (see: Collimation using the Takahashi Collimation Scope on the RCOS web site). This is an easy 10 minute procedure using the Takahashi Collimating Scope. How easy? Play the "Collimation Video" by Ken Crawford. Also see John Smith's 2003 Expanded Collimation procedure.

Optical Tube...

Focus Stability and low thermal mass:

The telescope and telescope tube also play a vital role. Everything matters. So we carefully considered all aspects of the design to develop a system that functions to it's fullest potential.

Carbon Fiber = Low thermal mass, low expansion, and stable focus

Coefficient of Expansion... (x / unit length / Deg. C x 10 -6 power)

Aluminum = 18.35
Steel = 11.0
Titanium = 8.5
Invar 1.1

Carbon Fiber (High Modulus with Epoxy) = 0.9

Typical aluminum and fiberglass telescope tubes both have a high coefficient of expansion and unstable mirror spacing. Telescopes made of such products require frequent refocusing during the night as the temperature changes. As an example, an F/9 Cassegrain has a 9x affect in back focus relative to mirror spacing. That is, a 1x change in mirror spacing changes the back focus 9x. So when the temperature changes, so does the telescope's focus.

Carbon does not suffer from these instability problems. That's why we choose to use an optical tube made of carbon graphite fiber. Carbon has a very low coefficient of expansion and also has excellent thermal properties. Focus and forget. Night after night, month after month!

RC Optical Systems was the first manufacturer to introduce commercially available Carbon Tubes / Carbon Trusses for telescopes. And we continue to be the worlds largest manufacturer of Carbon Tube / Carbon Truss Ritchey-Chrétien Telescopes!

Ask for Carbon only:

Other manufacturers have since introduced carbon fiber tubes. Some of these other carbon tubes are made with a high epoxy content, fiberglass, layers of Styrofoam, or even metallic components. These additional materials are not consistent with the low expansion and thermal properties of carbon fiber and may even negate the reason for using carbon.

Secondary Focuser...

Cassegrains have a two mirror, amplified optical system. Mirror spacing and back focus are directly related to each other. If you change one, it affects the other. Different accessories or cameras will change the back focus, which will change the required mirror spacing.

Since a Ritchey-Chrétien is part of the Cassegrain family, a secondary mirror focuser is required for optimum optical performance. All of our telescopes are shipped with a secondary focuser as standard equipment.

Our secondary focuser is an original RCOS design that allows the secondary mirror to move in precise increments without backlash, wobble, or play. This is a true "zero image shift" focuser. The absolute precision is due to twin stainless steel linear races riding on 4 linear ball bearings. Solid and very, very precise.

Laser aligning the secondary of an RCOS Ritchey-Chretien. In 1999, we offered a motorized version of the Secondary Focuser to control the secondary's position. This controller was developed by RCOS to precisely move the mirror in increments of 1/10,000 of an inch!

In 2001, we have gone a step further with the introduction of the "Telescope Command Center" (TCC). The TCC can now control the Servo DC Secondary Focuser by use of your PC. It can be operated remotely. The TCC powered linear actuator now has a resolution of 1/40,000 of an inch!

Using the "Telescope Command Center" on your PC, there are unlimited programmable positions which can be stored into memory and a "Home" position which is set at a mechanical limit. Other functions of the TCC are field rotation (of the image plane), temperature and fan speed control.

As with most of our components, the secondary support is machined on CNC mills to ensure precision. Each secondary is then laser aligned when installed.

Infrared Thermography...

RCOS Robotic Observatory showing our 24RC with STL-6303. Note the "hot" camera power supply in the lower left.

Where are the heat sources in your observatory? RCOS uses an 8-12mu 60Hz Long Wave Infrared (LWIR) camera to find out...

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RC Optical Systems, Inc. An Arizona Corporation
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