Orion 10-inch f/8 Ritchey-Chrétien astrograph telescope

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Orion 10-inch f/8 Ritchey-Chrétien astrograph telescope

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With similar optics to those found in the leading professional observatories, this telescope is an astro imager’s dream come true

Modern professional observatories use reflector telescopes. However, they don’t use Newtonians, Schmidt-Cassegrains (SCTs) or Maksutov-Cassegrains (MAKs): the instrument of choice is the Ritchey-Chrétien (RC).

Invented early in the last century by the American astronomer George Willis Ritchey and Frenchman Henri Chrétien, this design produces fields of view with the non-elongated, coma-free stars so prized by serious amateur and professional astronomers alike. The pin-sharp views mean it’s the ideal choice of scope if you’re after that perfect astro image, which is why you should be very excited about Orion’s latest: the 10-inch Ritchey-Chrétien astrograph.

Indeed, the telescope wouldn’t look out of place in a professional observatory – the construction and attention to detail are excellent. Finished in a white livery with contrasting black accessories, this substantial scope simply exudes quality. It’s a calibre of craftsmanship that is reinforced when you come to use the dual-speed, 3-inch-diameter Crayford focuser. With our heavy CCD camera fitted in it, we were able to make the very precise tension adjustments needed so we could focus slowly to find the point of maximum sharpness.

A substantial mount is a must with this scope. We had an EQ6 mount with four 5.4kg weights to balance the system. The scope required a small amount of adjustment to the secondary mirror to get it properly collimated – a common thing for a recently delivered telescope. This was a simple task using the supplied collimating eyepiece and the centre-marked primary mirror.

To ensure that the telescope is suitable for use as both a visual and imaging instrument, there are also various adaptors in the form of 4.75-inch-diameter extension tubes in 1- and 2-inch lengths, allowing us to enjoy excellent views of many deep-sky objects. The M13 globular cluster, in particular, was spectacular, with a swarm of stars drawing the eye deep into the cluster. But the optical design of this telescope isn’t really meant for visual use – it’s an astrophotographer’s dream machine. 

Picture perfect

The telescope has been designed to produce good-shaped stars right to the edge of a camera’s sensor. It therefore makes a wonderful deep-sky imaging platform as you can capture celestial objects on large CCD sensors without introducing misshapen stars towards the edge of the field of view.

Telescopes like Newtonians and SCTs produce excellent star images at the centres of their fields of view, but light entering from an angle doesn’t fare so well. The further away from the centre you look, the more star shapes become distorted with the effects of ‘coma’ until, at the field edges, stars take on the comet-like appearance that the name suggests. The Ritchey-Chrétien optics in this scope eliminate this problem.

The long, 2,000mm focal length produces quite a narrow field of view, so this instrument isn’t ideal for imaging larger nebulae, which would benefit from the wider fields of view of shorter focal length tubes. Our test subjects were M13, the Hercules Cluster, chosen so that we could check the centre of the field of view; and M27, the Dumbbell Nebula, which sits in a starry backdrop ideal for testing the sharpness right out to the field edge. We were delighted with the results – both the central and outer regions of the image displayed crisp stars, sitting in a well-lit frame. You can see the images on page 91.

Diffraction spikes aren’t to everyone’s taste, but those produced by the spider vane that holds the secondary mirror in this telescope produced a pleasantly understated effect. The various optical baffles inside the tube minimised stray light and increased contrast very effectively.

Superb though this design of scope is for imaging, all Ritchey-Chrétiens have a weak spot – field curvature. This results in stars being slightly out of focus around the edge of the field of view. However, this is the trade-off against coma, and at this focal length it had no noticeable effect on image quality, while the lack of coma made for very attractive images.

Although a standard finderscope holder is supplied, the finderscope isn’t, but this is a small omission from a telescope that really is a high-quality imaging instrument. As well as being a pleasure to use, it performed very well indeed.

Inside a Ritchey- Chrétien

Images of M13 (left) and M27 (right) taken through the scope

At first glance, a Ritchey-Chrétien (RC) telescope looks like a Schmidt-Cassegrain (SCT), but they are very different from one another optically. An SCT uses a pair of spherical mirrors and a front corrector plate to banish spherical aberration – the inability of a mirror to focus all the light from a star to a single point. An RC, on the other hand, doesn’t have a corrector plate. Instead of simple spherical mirrors, it has hyperbolic primary and secondary mirrors. These produce fields of view that are free from the distortion of coma, which transforms stars from points to elongated smears.

Figuring a hyperbolic mirror is a complex and expensive technique, but thanks to modern, computerised mirror-grinding technology, these telescopes are now no longer the exclusive preserve of professional observatories, and are now within the realms of affordability for advanced enthusiasts. The pin-sharp stars they produce mean that they’re perfect for astro photography.

Cooling fans - There are three cooling fans built into the rear of the telescope tube, all powered by the supplied battery pack. These help to bring the telescope to thermal equilibrium with the surrounding air, which is vital for getting rid of air currents within the tube that could otherwise disturb the light path and result in fuzzy images.

Crayford focuser - The 3-inch diameter Crayford focuser is finely crafted. Its 2-inch adaptor, graduated focus tube and smooth, dual-speed, 1:10 operation make it a joy to use. The large diameter of the focus tube produced a well-illuminated frame, while there was a negligible amount of image shift when focusing and finally locking the focus tube.

Baffles - The relatively large, central obstruction of the secondary mirror could sap some of the contrast from the views through this telescope. However, the optical baffles in the main tube, together with those inside the mirror tube at the centre of the primary mirror, helped to restore brightness and resulted in sharp, high-contrast images.

Losmandy dovetail bar - There are two Losmandy-style dovetail bars installed, one above and one below the tube. This type of bar is the preferred kind for the sturdiest method of mounting telescopes. With the right clamp on a sufficiently substantial mount, it results in a very solid installation.

This review was originally published in the February 2011 issue of BBC Sky at Night Magazine. Prices correct at time of going to press.


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