The Ritchey-Chrétien Telescope for amateurs (1): Introduction

The Ritchey-Chrétien optical design is a form of Cassegrain telescope designed to eliminate coma (which is suffered by standard Schmidt-Cassegrains (SC)), and provide a relatively large field of view.  They do, however, still suffer from astigmatism.  A RC telescope employs primary and secondary hyperbolic shaped mirrors.  Since there are only two mirrors, they have no spectral dispersion or chromatic aberration and, as there are only two optical surfaces (SC’s have 4), there should be less degradation of the wavefront passing through the telescope. Using no lenses (which also scatter some light), they can cover a wider part of the visible and Infrared spectrum.  RC telescopes also give a flatter field than a SC as the secondary mirror has less curvature and thus a lower amplification factor – typically 2.7x rather than 5x.  Their focal ratio is typically f/8 rather than f/10 for a SC. One good feature is that there is no corrector plate to dew up.

It is thus not surprising that many professional telescopes (including the Hubble Space Telescope) use the RC design.  The first RC telescope, built in 1927, had a diameter of 24 inches and this was followed by a 40 inch instrument for the United States Naval Observatory.  For many years smaller RC telescopes have been made for University Observatories and affluent amateur astronomers by RC Optical systems but these are beyond the reach of most amateurs.

However,  Ritchey-Chrétien  telescopes can now be bought for reasonable sums  made by a Taiwanese Company, Guan Sheng Optical (GSO).  These are sold under a wide variety of brand names.  The 6 and 8 inch telescopes are mounted in solid tubes which can be either made of steel or carbon fibre.  Some have 96% and some 99% reflectivity mirror coatings so variants can be bought at a range of prices starting at around £450 for the 6 inch model and £950 for the 8 inch model. 

8 -inch Stellar Lyra Richey-Chrétien Telescope.
8 -inch Stellar Lyra Richey-Chrétien Telescope.

The 10 inch models can be obtained with either a solid steel or open truss tube construction at around £2,500 or £2,800 respectively.  A 12 inch steel tube model is available for around £3,000 with an open truss model for £3,800.

10 inch Truss Tube Stellar Lyra Richey- Chrétien Telescope

In September 2021 I was able to acquire a Stellar Lyra 8 inch RC (sold by First Light Optics) having a steel tube and 96% reflectivity mirrors.  Pleasingly, it came with both a x0.7 focal reducer and also a tilt adjuster to ensure that the camera sensor can be accurately aligned on the image plane.  The focal plane is well away (around 10 inches) from the rear of the telescope tube and so a set of three extenders are supplied to be placed in front of the focuser and so enable it  to be mounted as far to the rear as possible to help remove any droop from the extenders placed beyond the focuser.  This will also allow the use of filter wheels if a monochrome camera was used.

The first task having acquired a Ritchey-Chrétien is to check and perhaps adjust its collimation.  By all accounts, this is a rather tricky process.  One reason why it is more difficult than for a Schmidt-Cassegarain is that a SC uses a spherical main mirror so it does not need to be precisely parallel to the secondary whereas using an RC with two hyperbolic mirrors this is really important.  Looking on the WEB, there are many ways to carry this out using a variety of collimation tools.  I have outlined my experiences with this scope in an article in the digest ‘Experiences with collimating a Ritchey-Chrétien  Telescope.

The next task, which can be largely carried out in daylight is to find the optimum focus position for the camera using the provided extenders.  I had to use all three of the extension tubes before the focuser and a short barrel extender following it.  The balance point of the telescope and camera is then right at the rear of the tube assembly which means that rear of the camera is some 12 inches below the attachment point.   This could possibly be a problem when slewing the telescope;  I found that the camera came very close to the detachable telescope control unit of my mount but I suspect will not be a problem with most mounts where the control electronics are integral with the mount.

Part 2 will describe the performance of the telescope.