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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 the secondary mirror. Schmidt has a thin complex-shaped corrector lens with a secondary mirror, and Maksutov has a thick spherical corrector lens and a 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.
Cassegrain design
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 a 45-degree 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 a convex secondary mirror at the front. However, the primary mirror has a hole in it, and the 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 the eyepiece is. The focuser with eyepiece holder is mounted at the back of the optical tube like on refracting telescope.
Almost all Cassegrain telescopes use spherical primary mirrors. Spherical mirrors are cheaper to make compared to parabolic mirrors, but they suffer from spherical aberration, and that’s why most Newtonian reflectors use 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 corrects 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 a secondary mirror spider mount.
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Cassegrain focusing: How it works
Another huge difference compared to a standard reflector or refractor is the focus.
The focuser on the refractor or reflector is built 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 a 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 for a pretty short tube with a 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 that they can be seen with this type of telescope, but typically, the Cassegrain is mainly for viewing planets and the Moon in high detail.
Because of the small tube, even the big aperture weighs much less than the equivalent of the same aperture of a reflector or refractor. Thanks to that, they are very portable and easy to transport. You also don’t need a heavy-duty mount for a big aperture like a standard telescope. You can mostly see them used on the altazimuth fork-type mounts.
Schmidt cassegrain
The corrector plate of the Schmidt Cassegrain is an aspheric lens. This shape corrects 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 the outside temperature.
The MAKs correcting plate is much thicker, with 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 inside the correcting lens.
That’s why it’s free of diffraction spikes. The secondary mirror has three collimation screws to be collimated like on the reflector telescope. There are also collimation screws for the primary mirror at the back.
Still, the collimation is usually solid, and there is no need for doing the collimation very often because it is a closed system.
An example of the Schmidt Cassegrain telescope is Celestron NexStar 8 SE.
Maksutov cassegrain
The Maksutov corrector plate is a meniscus corrector with a highly curved spherical lens. This corrector lens corrects a coma found in all reflecting telescopes while also correcting spherical aberration.
The corrector is much thicker compared to the SCT corrector, so that’s why MAKs are usually made with a small aperture.
Meade Instruments Lx65 6″ Maksutov-Cassegrain
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 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 a longer focal length with a higher focal ratio, many times around F/14. This makes them suitable for planetary observing but not good for faint deep-sky objects.
An example of the Maksutov-Cassegrain telescope is Celestron NexStar 127SLT.
The great telescope debate: Schmidt-Cassegrain or Maksutov-Cassegrain?
In summary, the Cassegrain telescope stands out for its versatility and proficiency, particularly in the catadioptric designs favored by observatories worldwide. This design, exemplified by both Schmidt-Cassegrain and Maksutov-Cassegrain models, excels in achieving long focal lengths while maintaining a compact optical tube.
These telescopes are renowned for their exceptional planetary observation capabilities and their adaptability to astrophotography. Their design allows for easy attachment of various cameras, facilitated by the movement of the primary mirror to achieve focus. This feature simplifies prime focus astrophotography, making these telescopes highly compatible with a range of cameras.
Additionally, the option to use a focal reducer to decrease the focal length broadens the field of view, enhancing their utility. Whether for casual stargazing or serious astronomical pursuits, a Cassegrain telescope is a valuable and multifaceted tool for any astronomy enthusiast.
Ultimately, the decision between a Schmidt-Cassegrain and a Maksutov-Cassegrain telescope depends on the specific requirements, interests, and budget of the observer. Each design has its unique advantages, making them both excellent choices for different aspects of amateur astronomy.
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