The Cassegrain design is very popular amongst amateur astronomers, and almost every amateur astronomer will own, at some point in life, the Cassegrain design telescope. We have to types- Schmidt(SCT) and Maksutov(MAK).
The main difference between the Schmidt Cassegrain and the Maksutov Cassegrain is the corrector lens at the front of the tube and secondary mirror. Schmidt has thin complex shaped corrector lens with a secondary mirror, and Maksutov has thick spherical corrector lens and the secondary mirror that is not a mirror but an aluminized small spot on the inside of the Maksutov corrector lens. Another difference is in the size of the aperture. Schmidt Cassegrains are made to have big apertures, and Maksutov Cassegrains usually have small apertures.
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First, let me explain what these two have in common. They share the same Cassegrain design. This design is catadioptric using mirrors to produce an image like the Newtonian reflector, but it is an entirely different type of design than the Newtonian.
The Newtonian reflector has a primary mirror at the end of the optical tube, and 45degree tilted secondary mirror at the front sending light to the eyepiece mounted right above the secondary mirror at the front of the optical tube.
The catadioptric design also has a primary mirror at the end of the optical tube and convex secondary mirror at the front. However, the primary mirror has a hole in it, and convex secondary mirror has a flat position to the primary. The light is traveling from the primary mirror to the secondary mirror, and then it is reflected into the hole of the primary mirror where is the eyepiece. The focuser with eyepiece holder is mounted at the back of the optical tube like on refracting telescope.
Almost all Cassegrain telescopes are using spherical primary mirrors. The spherical mirrors are cheaper to make compared to parabolic mirrors, but they suffer from spherical aberration, and that’s why most Newtonian reflectors are using more expensive parabolic mirrors. On the other hand, the Cassegrain telescopes can use spherical mirrors because they have a correcting lens at the front of the optical tube which is correcting the spherical aberration for them. And the secondary mirror is mounted on the correcting lens which results in less light obstruction and no diffraction spikes because of no need for secondary mirror spider mount.
Cassegrain Focusing: How It Works
Another huge difference compared to standard reflector or refractor is the focusing. The focuser on the refractor or reflector is build so that you are moving the draw tube with the eyepiece in and out. This is how you achieve the focus. But in the Cassegrain design, the eyepiece holder is stationary. When you are rotating the focus knob, you are moving the whole primary mirror inside the tube up and down. It used to be a big issue for early Cassegrains because the mirror was going out of collimation while focusing- the movement was not equal on the sides. But now we have mirror lock that prevents that, and the collimation is not an issue anymore.
Portability And Focal Length
All Cassegrains are known for their long focal length. The design allows having a pretty short tube with a very long focal length. The effective focal length is much higher than the primary focal length. For example, this Celestron NexStar 6 SE with a short tube and 6″ aperture has an effective focal length of 1500mm. The reason behind it is that the light is traveling twice the distance and convex curvature of the secondary mirror, and the mirror is magnifying the focal length making it much longer than it is. This is what makes the catadioptric design perfect for observing planets and the Moon because you can achieve high magnification.
However, they are not as good for deep sky objects because of the high focal ratio and the narrow field of view. As we know, the high focal ratio is not good for faint objects. There are some deep sky objects like galaxies or planetary nebulas that are small and bright so they can be seen with this type of telescope, but typically, the Cassegrain is mainly for viewing planets and the Moon in the high detail.
Because of the small tube, even big aperture weights much less than the equivalent of the same aperture of reflector or refractor. Thank’s to that they are very portable and very easy to transport. You also don’t need a heavy duty mount for big aperture like with a standard telescope. You can mostly see them used on the altazimuth fork type mounts.
The corrector plate of the Schmidt Cassegrain is an aspheric lens. This shape is correcting the spherical aberration of the primary mirror because it has equal but opposite spherical aberration to the primary mirror. This type of correcting plate allows having big apertures because the plate is thin and simple to manufacture. Another advantage is that it leads to a shorter time required for a Schmidt Cassegrain to thermally equalize to outside temperature. The MAKs correcting plate is much thicker, so they have mostly small apertures. It is not efficient and not at all economical to make these thick plates for the big aperture.
Schmidt Cassegrain has a convex secondary mirror mounted on the inside of the correcting lens. That’s why it’s free of diffraction spikes. The secondary mirror has three collimation screws so it can be collimated same like on the reflector telescope. There are also collimation screws for the primary mirror at the back but the collimation is usually solid, and there is no need for doing the collimation very often because it is a closed system.
The example of the Schmidt Cassegrain telescope is Celestron NexStar 8 SE.
The Maksutov corrector plate is a meniscus corrector which is a highly curved spherical lens. This corrector lens is correcting a coma found in all reflecting telescopes while also correcting spherical aberration. The corrector is much thicker compared to SCT corrector, so that’s why MAKs are usually made with a small aperture.
Another disadvantage is that most commercially available MAKs have only a small aluminized spot on the corrector lens instead of the proper secondary mirror. It is much cheaper and more convenient, but there is no issue with this in small apertures. On the other hand, the secondary mirror is smaller than in SCT, so there is a less obstruction in the light path. This results in a sharper and brighter image in MAK. Despite the secondary mirror design, the Maksutov Cassegrain has a much superior image than the SCT. It is also because the corrector lens can be shaped and polished more precisely than the complex SCT corrector plate.
Maksutov is manufactured with longer focal length with a higher focal ratio, many times around F/14. This is making them suitable for planetary observing but not good for faint deep sky objects. The example of the Maksutov Casagrain telescope is Celestron NexStar 127SLT.
The catadioptric design is commonly used around the world in the big telescopes used by observatories. It is a perfect design for achieving long focal length and keeping the optical tube small. The Cassegrain telescopes are great, and if you stick with this hobby, you will eventually want to own one.
They are the ultimate planet hunters and also suitable for astrophotography. You can attach any camera to this type of scope because you are moving the primary mirror to achieve the focus. So, prime focus astrophotography is no trouble with any camera here. And you can always add some focal reducer to reduce the focal length and get a wider field of view. All this is making Cassegrain telescope extremely versatile instrument for astronomy.