Telescope types · Reflector · Cassegrain

Ritchey-Chrétien telescopes

Two hyperbolic mirrors and no lenses. The design is free of coma, which is why the Hubble Space Telescope, Keck and most large research telescopes use it, and why amateurs use it to image small galaxies.

Hyperbolic primary Hyperbolic secondary Focus Field flattener

Swipe sideways to see the full diagram.

Light reflects off the concave hyperbolic primary to a convex hyperbolic secondary, then back through a hole in the primary to focus behind it. A field flattener in front of the camera removes the remaining field curvature.
Typical focal ratio
f/8slow; reducers to f/6
Central obstruction
About 47%iOptron Photron RC8, by diameter
Coma
Noneboth mirrors hyperbolic
Visual use
Fairlow planetary contrast
8-inch price
About $1,180tube only

Two hyperbolic mirrors cancel coma without any lens in the light path

George Ritchey and Henri Chrétien designed the system around 1910. In a classic Cassegrain the primary mirror is a parabola and leaves coma off-axis; making both mirrors hyperbolic cancels coma and spherical aberration together. With no glass for the light to pass through, an RC shows no color error at any wavelength, from ultraviolet to infrared.

What it leaves is astigmatism and a curved field, so amateur RCs are used with a field flattener or a flattener-reducer for cameras larger than about APS-C. The large convex secondary needs careful collimation: a slight tilt shows up as stars that are elongated in one corner of the frame.

Taiwan's GSO makes most amateur RCs under several brand names

ModelMakerApertureFocal lengthWeightPrice
Photron RC8iOptron (optics from GSO, Taiwan)203 mm1,624 mm (f/8)18.7 lb$1,178
AT8RCAstro-Tech (GSO, Taiwan)203 mm1,625 mm (f/8)18.9 lb$1,199, sold out
10-inch to 16-inch truss RCsGSO, sold as iOptron and Astro-Tech254 mm to 406 mmf/8heavydealer quote
RiDK and RC seriesOfficina Stellare (Italy)300 mm and upvariesobservatoryspecial order

Prices are US list prices checked in November 2026. Special-order instruments are quoted by the dealer.

Large professional RCs come from PlaneWave (USA), ASA (Austria) and China's Nanjing Institute of Astronomical Optics and Technology, which builds research telescopes including the LAMOST survey telescope. PlaneWave's popular CDK line is a different design, the corrected Dall-Kirkham.

An 8-inch RC frames more galaxies than any shorter focal length

On an APS-C camera with the Sony IMX571 sensor (23.5 x 15.7 mm, 3.76-micron pixels), an 8-inch f/8 RC at 1,624 mm gives a field of 0.83 x 0.55 degrees at 0.48 arcseconds per pixel. 92 of the 110 Messier objects fit in the frame, and 61 of them fill at least a quarter of its short side, among them M101 (Pinwheel Galaxy), M20 (Trifid Nebula), M81 (Bode's Galaxy) and M6 (Butterfly Cluster).

Most backyard skies blur stars to 2 to 3 arcseconds. Image scales below about 0.7 arcseconds per pixel record no extra detail, so cameras on long focal lengths are usually binned 2x2.

Do not buy an RC for the eyepiece

An RC shows sharp stars visually, but its secondary blocks nearly half the aperture's diameter, which softens planetary contrast, and at f/8 with a large secondary it shows narrow fields. An 8-inch Schmidt-Cassegrain costs about the same, shows more on the planets and finds objects for you. Buy an RC for imaging small galaxies and planetary nebulae on a solid equatorial mount with autoguiding.