Celestron C8 SCT XLT CGX Optical Tube Assembly

Further details

There has been much said and written regarding the optics in the SCT. Essentially, the combination of a spherical primary, an aspherized secondary and a Schmidt plate (a thin glass plate, plano (flat) on one side, and with the Schmidt curves on the other), creates a wavefront that is free from Lower Order Spherical aberration (LSA), free from Higher Order Spherical aberration (HSA or zones), a diffraction limited field free of coma and astigmatism, and the field free of Chromatic aberration (CA).

The design has much greater freedom from off-axis coma than the equivalent focal ratio Dall-Kirkham.

Much greater freedom from off-axis coma than the focal ratios usually found in Newtonians.

Does not suffer from the tube currents common in open tube Cassegrain focus designs, such as the Dall-Kirkham, the Klevsov, the sub-aperture meniscus Maksutov and the standard Cassegrain, nor does it require regular cleaning of the primary optics because the closed tube keeps dust and debris out.

It has a smaller secondary than the photographic Ritchey-Chrétien.

It has none of the chromatic aberration associated with achromatic refractors.

There are other ways to design a catadioptric system at f/10 using the full aperture Schmidt type correcting plate, by generating a different aspheric curve on the secondary. However, for production purposes, the difficulty lies in producing a diffraction limited instrument using a different aspheric curve on the secondary, on a consistent basis. Regardless of theoretical advances that other designs may promise, the key lies in the consistent diffraction limited performance that is actually realised when a catadioptric telescope is factory produced. Celestron’s original tried and tested method has worked and worked very well over the decades, one of the reasons the C8 has become a classic instrument, and remains the most recognisable iconic astronomical instrument over the last thirty years. Many competing catadioptric telescopes have found it difficult to match the optical consistency of Celestron.

The Celestron Schmidt Cassegrain design is also beneficial for planetary imagers over other catadioptric designs such as the Maksutov Cassegrain (both the standard Gregory type and the separate secondary configuration), due to the cooling times of the thick meniscus lens on Maksutovs. Maksutovs often show correction errors for long periods of time whilst cooling. On the larger instruments (greater than about 180mm) this can be quite protracted. The problem is often due to the differential cooling periods of both the primary mirror and the meniscus. For planetary imagers, this can sometimes mean an unproductive night waiting for spherical aberration to reach a minimum. The Celestron Schmidt Cassegrains cool at a much faster rate, which means a much longer period of observation with the telescope at optimum correction.

Celestron’s Schmidt Cassegrains offer the amateur astronomer or the optical enthusiast the complete “owning a telescope” experience. The apertures range from five inches to fourteen inches and focal lengths between 1250mm and 4000mm. They are the scope of choice for planetary imagers, and deep-sky imagers because of the range of image scales. They represent the ideal observing instrument with medium to large apertures, medium to longer focal lengths for the comfortable use of higher magnifications, a large diffraction limited field free of aberrations, and rear eyepiece positioning for relaxed long periods of observing. The closed tube design keeps the important optical surfaces clean and prevents image-destroying tube currents seen in some Newtonians. The primary mirror focusing system permits close focusing with minimum spherical aberration, whilst enabling the observer to vary image scale, and all of this potential and performance in a closed compact easy to handle optical tube assembly.

Delivery charge £5.95