Whether you’re a newbie to astronomy who’s thirsty for knowledge of our amazing universe or an amateur astronomer who’s planning on buying a telescope, you’re probably wondering what separates Schmidt Cassegrain from Newtonian telescopes.
Well, you’re in luck because we broke it down in simple terms for you.
In short, Newtonian telescopes, also called Newtonian reflectors, use mirrors to magnify objects, whereas Schmidt Cassegrain telescopes use mirrors and lenses.
The addition of the lens in the Cassegrain design allows this type of telescope to render sharper, higher-quality images than the Newtonian reflector, making it a great option for astrophotographers.
Not to be outdone, the Newtonian telescope also has its advantages. These include its affordable price and its ability to produce bright, clear images. This type of scope is great for observing deep-sky objects and wider regions of the night sky due to its wide field of view.
Read on for our in-depth Schmidt Cassegrain vs Newtonian telescope comparison and find out more about how these two types of telescopes work, their key features, and their main differences.
How Do These Telescopes Work?
Firstly, the Schmidt Cassegrain and Newtonian Telescopes use different optical principles, which means that they don’t work the same way.
The Newtonian telescope is a reflecting or reflector telescope and works based on the principle of reflection. This type of telescope uses mirrors to magnify objects.
The Schmidt Cassegrain telescope is catadioptric. It uses a combination of mirrors and lenses to render magnified images.
Let’s quickly dive a little deeper into how these two telescopes work.
How Do Mirrors Magnify Objects?
People began using mirrors in optical telescopes in the 16th century but they were not as popular as lenses, since mirrors in those days were made usually from metal and did not produce a clear reflected image.
The glass mirrors that we use today were not invented until the 18th century.
The main type of mirror used in optical telescopes is curved inward, like the front of a spoon.
This type of curved mirror is called a concave mirror and can be spherical or parabolic in shape. Parabolic mirrors produce better quality images and are more expensive than spherical ones.
In a telescope, the primary mirror is usually always concave and sits at the end of the telescope opposite the eyepiece, whereas the secondary mirror is much smaller and can be flat or curved.
Now that you have a better idea of how the mirrors are designed and placed, let’s explain how deep space objects look huge through a reflector telescope.
What Exactly Do Mirrors Do?
The primary mirror receives light rays through the aperture (opening) of the scope and reflects these rays onto the secondary mirror, which then reflects these rays toward the lens of the eyepiece.
Before they reach the lens, the rays all meet at a point (focal point), then start to spread out again. By the time the light rays reach the eyepiece lens, they have crossed over each other and produced an upside-down image.
Why Does The Image Look Bigger?
As the light rays pass through the eyepiece lens they become straight again, so when you place your eye to the eyepiece, the light rays spread out onto your retina (the part of the eye that receives light), tricking your brain into thinking that the image you’re looking at is closer than it is.
As we just mentioned, some telescopes render upside-down images of space. That’s not a big issue though, as many modern refracting telescopes come with a very useful part called a “star diagonal”, which flips the image’s right side up again.
Reflecting telescopes on the market today are more likely to render upside-down images. This is not a problem for many stargazers, as “there is no up or down in space”.
How Do Lenses Magnify Objects?
Lenses magnify objects similar to the way mirrors do.
The type of lenses used in optical telescopes are typically convex, and because of their shape, they cause light rays to bend inward.
Similar to light reflected off of concave mirrors, light passing through convex lenses meet at a point, then spread out again toward the lens of the eyepiece.
Schmidt Cassegrain telescopes use a parabolic primary mirror and a curved secondary mirror with a type of curved lens called ‘aspheric lens’, which is placed directly in front of it.
This special aspheric lens corrects a focus problem called spherical aberration, which occurs in reflector telescopes.
Why Are They Called “Schmidt Cassegrain” and “Newtonian”
These telescopes are named after the people who invented them.
While Isaac Newton was not the first to build a telescope, he is said to be the first to change the optical design of the refracting telescope using mirrors instead of lenses. This is how he created the first reflective telescope.
Laurent Cassegrain improved on that design a few years later, but instead of using a lens, his telescope used a curved secondary mirror to reflect light through a hole in the primary mirror at the back of the scope.
Almost 300 years later, Bernhard Schmidt added a lens to Cassegrain’s design to produce one of the most popular types of telescope on the market today.
Schmidt Cassegrain vs Newtonian: Key Features and Differences
As we mentioned earlier, the Schmidt-Cassegrain is a catadioptric telescope, which is a hybrid of refracting and reflecting telescopes.
The Schmidt-Cassegrain and the Newtonian are similar in appearance and design with a few exceptions:
- The Schmidt Cassegrain scope reflects light from the secondary (smaller) mirror through a hole in the primary mirror positioned at the back of the telescope tube, opposite the aperture. As you might have guessed, the eyepiece is also at the back of the tube.
- The Newtonian scope’s primary mirror is also at the end opposite the aperture, but it reflects light to the secondary mirror, which is closer to the aperture and the eyepiece.
The location of the Newtonian’s eyepiece on the tube can be inconvenient to stargazers if the telescope is large or positioned at an angle high off the ground.
- The Schmidt Cassegrain telescope typically has a short tube compared to other scopes, which makes it quite compact and easy to carry around. The size of its aperture is somewhat limited on amateur models compared to the Newtonian telescope models.
- The Newtonian scope, on the other hand, is usually heavier and less compact.
- Schmidt Cassegrain models tend to have a long focal length and a focal ratio of about f/10, while Newtonian models have an average focal ratio of f/6.
- Schmidt Cassegrain telescopes are known for producing sharp, high-quality images, which make them great for watching deep sky objects.
- Schmidt Cassegrains are also a great option for astrophotography.
- These types of telescopes also have a narrower field of view than their Newtonian counterparts, which makes them ideal for viewing planets and stars.
- Newtonian telescopes give a wider field of view, which makes them great for astronomy enthusiasts who are interested in viewing galaxies and constellations.
Spherical aberrations can occur in Newtonian telescopes that use spherical mirrors.
Another type of focus issue is called ‘comas’ and can occur in both Newtonian and Schmidt Cassegrain telescopes.
While a telescope’s magnifying capability has a lot to do with the eyepieces the viewer uses, it also largely depends on the scope’s focal length. The magnifying power of a telescope can be calculated using the focal lengths of the scope and the eyepiece.
Note that modern telescopes usually come with two or more eyepieces of different strengths, and the user can buy additional ones to suit his/her preference.
Both types of telescopes require regular collimation, which simply refers to the alignment of the mirrors.
The Newtonian scope’s unsealed optical tube allows dust to get inside the instrument, therefore the mirror/s need to be cleaned often.
In terms of price, the Schmidt Cassegrain model costs more than the Newtonian. This is no surprise given that it contains lenses, which cost more to produce than mirrors do, and the fact that it offers better image quality and portability.
Schmidt Cassegrain vs Newtonian Telescope: Recap
In our comparison of the Schmidt Cassegrain vs Newtonian telescope, we broke down the main features and differences between the two. Here’s a quick recap:
- Uses mirrors to magnify images
- Renders bright, good quality images
- Prone to spherical aberrations
- Has a shorter focal length; great for wider views of the night sky like galaxies
- Quite affordable
- Lens requires regular cleaning
- Not as portable as the Schmidt Cassegrain telescope
Schmidt Cassegrain Telescope
- Uses mirrors plus a lens to improve on the Newtonian design
- Produces high-quality images and reduces spherical aberrations
- Prone to coma aberrations
- Has a longer focal length; ideal for watching stars and planets
- Easier to maintain
- The compact design makes it easy to carry around
- Great for astrophotography
We hope this guide has helped you learn more about the two models of a telescope. Overall, each type of telescope has its pros and cons. Both telescopes can be used by beginners, but the Newtonian telescope is more commonly preferred by novices due to its affordability and great features.