The f/10 and f/11 Schmidt Cassegrain

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Perhaps the most notable success story in the history of modern astronomical telescopes for amateurs has been the Schmidt Cassegrain or SCT as it has become known. For several decades now, the two US manufacturers Celestron and Meade have led the world in consistent marketing, producing and re-inventing the design of a single telescope type. The SCT, as popularised by these two companies, has become the telescope that defines the hobby of modern amateur astronomy. The two companies settled on a common focal ratio of f/10 with their range of SCTs (apart from Celestron's C14 which is f/11). This meant they could keep the telescopes highly compact, light and portable, a desirable feature that increased sales potential. Other design work by one or two different companies using the original f/18 or f/20 SCT design had led to optically superior instruments, but with less appeal as commercial products because of their size. Although an f/10 SCT is then a compromise, it is a compromise that has proved extremely popular.

Before we look at the advantages and any disadvantages of the optical design, lets familiarise ourselves with what an SCT is and how it works.

The Schmidt Cassegrain telescope is an example of a catadioptric. That is to say, a telescope that uses refraction and reflection to bring light to a focus. It is a variation of the Cassegrain telescope (named after its French inventor), in that it has a "Cassegrain focus." The family of Cassegrain telescopes (some argue there is only one true Cassegrain telescope) share little in common with each other apart from a common focus position at the rear of the telescope. Another example is the Maksutov Cassegrain. 
The main light-collecting optic of the SCT is the primary mirror. This mirror sits on a movable cell toward the rear of the instrument, and its optical surface is concave and  spherical in curvature. Spherical optics can suffer from an image-degrading aberration known as spherical aberration, and the severity increases with decreasing focal ratio. With f/10 and f/11 SCTs the primaries have focal ratios of f/2 to f/2.25, so spherical aberration is severe.

 

To correct for this, a full aperture refracting glass plate known as a Schmidt Plate (after Bernhard Schmidt), intercepts the incoming light at the front of the telescope, and makes the appropriate correction prior to the light reaching the primary. A Schmidt Plate is a rather neat idea for controlling spherical aberration, in that it only needs to be thin relative to its diameter. This benefits astronomers because it cools much faster than a lens of the same diameter. The outside of the Schmidt plate is optically flat (a plano surface). The inside surface has a very subtle but complicated curvature variation known as the Schmidt Curve. To the naked eye, both sides appear flat, but the subtle Schmidt curves on the inside surface can be described as working a little like a weak diverging lens toward the periphery of the plate, and a weak converging lens toward the centre.  These curves then, are there purely as an aberration control. At the centre of the Schmidt Plate is a small cell that holds another much smaller mirror that has a convex surface. This mirror is known as the secondary, and its surface curvature is usually aspherized (figured during the polishing stage to a surface that is no longer a pure spherical surface). The secondary mirror and its cell in an f/10 SCT produces quite a sizable obstruction in the light path. Of all the SCTs the smallest secondary diameter is about 33% that of the primary. Although this may mean the SCT image loses a small amount of subtle lunar and planetary detail compared to a refractor of the same aperture, the larger secondary is necessary for providing a usable photographic field of view with minimal vignetting.

Light then travels through the Schmidt Plate, and is reflected off the primary. The converging cone from the primary is reflected again off the secondary, and converging onwards through a hole at the centre of the primary. The image comes to focus outside the rear of the telescope where the eyepiece sits. Unlike the Newtonian or refractor, the light path in the SCT is folded, reducing the physical length of the telescope. The layout of the optical components in this manner permits a useful and convenient way to focus. Tight within the hole at the centre of the primary is a short metal tube. This tube fits around another longer tube known as the primary baffle. One tube fits very tightly over the other tube, and can only move against the other via a small amount of lubrication between the two. This controlled lubricated sliding motion of one tube over another is the focus mechanism of the telescope. At one end of the tube travel, objects at infinity are brought to focus at the eyepiece, and at the other, objects at close focus are brought to focus. The eyepiece does not move its position, just the primary. The mechanism moving the primary at a controlled rate varies between manufacturers, but is essentially a mechanical slow movement of the primary cell being pushed up or pulled back down by the manual turning of a threaded rod or studding. The focus knob at the rear of the telescope is simply turned one way or the other to move the primary to a position where the image at focus at the eyepiece is either stars, a bird in the garden, or somewhere in between.

The mirrors in an SCT are glass or Pyrex, and aluminised (coated in vaporized aluminium) to make the reflecting surface, and over-coated with either Silicon di-oxide or other metal combinations to increase reflectivity and protect the aluminium coating. The Schmidt Plate usually has multi-layer transmission coatings applied to both sides to maximise transmission and minimise reflection.

This design has obvious advantages for practical use. The telescope is a closed tube design, meaning that tube currents are minimal. The convex secondary slows the convergence of the light cone from the primary from an f/2 to f/10 for the whole telescope. The secondary then increases the focal ratio 5X to f/10.  A 200mm aperture Newtonian with an f/10 focal ratio would have a tube two metres long and would be rather unstable on a small equatorial mount. A 200mm f/10 SCT is less than 2ft long, and is lightweight and compact. It requires nothing more than a small equatorial or alt-azimuth mount for successful use. Observing with an SCT is simple and convenient. The eyepiece position is always the same regardless of the region of the sky the telescope is pointed at. A lot of observing can be carried out whilst seated, which is useful for astronomers as more productive observing comes from a more relaxed viewing position. There is no rotating of the main tube required nor racking out of the eyepiece for terrestrial viewing. Terrestrial photography and astrophotography are convenient in that the camera remains in one position and is therefore easily accessible in the dark.  The baffling of stray light (prevention of internal reflections and off-axis light producing veiling glare) is taken care of by two baffles. One is the main baffle tube at the centre of the primary, the other is a wider cup baffle around the secondary. This is positioned so that when the eye is placed on-axis at the rear of the telescope, all it can see is the surface of the secondary. The secondary in turn receives light from the primary only, the secondary baffle prevents off-axis reflections from reaching it. The advances in modern optical transmission and reflection coatings, plus effective baffling, means that the modern SCT is an excellent deep-sky telescope, the images providing a darker background sky than many Newtonians of the same aperture. Even 10 years ago this was not the case.

The SCT is remarkably adaptable as a photographic tool. A 200mm aperture f/10 SCT is also a 2000mm f/10 lens for a camera, and as these instruments are designed for astronomical use, they are considered to be of diffraction limited optical quality, something that normal telephoto lenses don't need to be, and usually are not. The focal length of the SCT can be altered to a specific visual or photographic application, by the addition of a focal reducer or focal extender. An f/6.3 reducer (a positive lens group added at the rear of the telescope), turns the 2000mm f/10 into a 1260mm f/6.3 lens, with obvious benefits for wider field deep-sky imaging. Alternatively, a focal extender known as  Barlow lens (a negative lens group) can be added in place of the focal reducer. A 2X Barlow converts the 2000mm f/10 into a 4000mm f/20 lens, with an increased image scale that benefits lunar and planetary imaging. One other adaptation that can be made for astrophotography is to remove the secondary assembly completely from the telescope and replace it with a lens group that makes corrections which allows a camera to be placed at the front of the telescope, suspended in the centre of the Schmidt plate. Light passes through the Schmidt plate, reflects off the primary, passes through the correcting lens group where the secondary mirror was, and then focuses onto the chip of a camera. This is known as aHyperstar corrector. The huge benefit of this is that the telescope becomes a lens whose focal length is the focal length of the primary mirror alone. Our 2000mm f/10 is then converted to a 400mm f/2. This has enormous implications for the type and richness of the images the camera can record. An SCT fitted with a Hyperstar can record bright detailed deep-sky wide field images in a tiny fraction of the exposure time required for an f/10 or even f/6.3 lens.

Currently, SCTs are manufactured in the following apertures as optical tube assemblies. 5" f/10, 6" f/10, 8" f/10, 9.25" f/10, 11" f/10, and 14" f/11.

As complete telescopes, with mounts, tripods and accessories, the above aperture SCTs are available on alt-azimuth single arm and dual arm fork mounts as computerised GOTO telescopes, German equatorial GOTO mounts, and the 5" SCT is also available on a non-motorised German equatorial mount.

A recent development in catadioptric design by Meade is the Aplanatic SCT. An aplanat is an optical system that is free from spherical aberration and off-axis coma. The standard SCT has a little off-axis coma that becomes noticeable on wide field photographic images of stars. By changing the geometrical figure of one or more optical surface, the image can be made to be coma free up to a certain field size, that corresponds to the chip size in popular SLR cameras.

Currently, Meade's aplanatic SCTs (named ACF - Advanced Coma Free), are manufactured as optical tube assemblies and as complete telescopes on mounts. ACFs are available in 6" f/10, 8" f/10, 10" f/10, 12" f/10, 14" f/10 and 16" f/10. 
An even more recent development by Celestron is also an aplanatic SCT but with the addition of an optical correction that provides a flat focal plane for astrophotography. Telescope focal planes are generally curved, so a flat focal plane is beneficial for astrophotography because the receptor (the camera CCD or CMOS chip) is flat. Celestron have adapted their current SCT design into a flat focal plane aplanat by the addition of correcting lenses inside the rear portion of the primary baffle tube. These telescopes are named the Edge HD SCTs, and are available as complete telescopes or just optical tube assemblies in the following apertures - 8" f/10, 9.25 f/10, 11" f/10 and 14" f/11.

With all these SCTs collimation is achieved by tilting the secondary mirror only.