Dobsonian vs Cassegrain | Which should you choose?

If you’re just starting in the astronomy world, then it can be really difficult to know what the best telescope for your needs really is. Fortunately, it doesn’t have to be as difficult as some people make it out to be. With a little bit of research, you can ensure that you know what you’re talking about when considering telescopes. You’ll need to know some basics which can help you know the difference between different types of telescopes. Here, we’ll specifically look at the difference between Dobsonian and Cassegrain telescopes.

In the world of reflecting telescopes, Dobsonian and Cassegrain are the two dominant variations of reflector. For beginners, Dobsonians are considered to be the better option due to their low cost of products, whereas Cassegrain telescopes offer the same aperture in a more compact design – but there substantially more expensive.

Dobsonian vs Cassegrain

Firstly, we’ll sum up each of the different design of telescope and then we can take a look at what the main differences are between the two. If you’re thinking about purchasing a telescope, then you should read this before you make any decisions!

Dobsonian Telescopes

Dobsonian telescopes are currently the most popular telescope available for the amateur astronomer. Why? Well, one of the main reasons is because of their affordability. Their low cost makes them a great choice for people looking for a cheap telescope that isn’t ridiculously priced.

Dobsonian telescopes are named after their inventor, John Dobson. Well technically Dobson didn’t invent them – he would say this himself – but he was the first to put all the pieces of this type of telescope together. Before him, many reflector telescopes were out of the budget of your average, every day man. But after Dobson started to replace expensive parts of telescopes with cheaper, more affordable parts, he help revolutionize the telescope and make it affordable for amateurs as well as professionals.

Dobson was known for running his San Francisco Sidewalk Astronomy Club, which he started to try and encourage others into an interest in astronomy. There are some main aspects that really made a Dobsonian what it is and separated it from other reflector telescopes;

• Altazimuth mount – The main aspect of a Dobsonian is an altazimuth mount. This specific type of mount is very simple; it’s two perpendicular axes which allow the telescope easy rotation in a vertical and horizontal way. Nowadays, we consider this to be the defining difference between a Dobsonian and other telescopes.
• Thinner mirrors – The mirrors used in older reflector telescopes used to be thick and expensive. By using a different type of mount, this allowed Dobson the ability to use thinner and less expensive mirrors inside the telescope itself.
• Paper tubes – Another big change that Dobson made to the reflector that made it more affordable to the masses was to use paper tubes as opposed to fiberglass and aluminium. Although paper doesn’t sound strong, this generally refers to Sonotubes, the stuff used to hold heavy cement (meaning it’s more than strong enough for a telescope!

It’s pretty clear to see the impact that the Dobsonian has had on the astronomy industry, especially in the 70s where it helped allow anyone to venture into astronomy. Because of the way that the Dobsonian is designed, you really need to ensure that you have clear skies to use it – it won’t work well in the majority of inner cities. It’s best for seeing deep space objects, and it’s a useful telescope for seeing different galaxies.

A Dobsonian telescope is a type of reflecting telescope, which is one of the most common and generally one of the cheaper options if you’re looking for a telescope on the current market. There are other options available, but Dobsonians are the cheapest form of Newtonian – named after Isaac Newton, of course. A Newtonian Reflector telescope is designed to draw in light, and reflect that from one primary mirror onto a smaller, secondary mirror which is angled to produce an image through the eyepiece.

An alternative to Dobsonian telescopes is Cassegrain telescopes, which work in a completely different way to a Dobsonian.

Cassegrain Telescopes

Cassegrain telescopes have become some of the most popular instruments for amateur astronomers and professional observatories alike, due to their ability to be compact and easy to use. Cassegrain telescopes are a type of reflecting telescope which utilizes mirrors as well as lenses.

The Cassegrain design was invented by Henri Chrétien in the early 20th century, but did not gain popularity until the later part of the century. Cassegrain telescopes were used on lunar surface exploration during NASA’s Apollo program. A Cassegrain telescope is generally more compact than other reflectors due to its design utilizing mirrors instead of refractive lenses.

This also means that Cassegrain telescopes have longer focal distances than refracting telescopes with comparable aperture sizes.

Unlike two of the most common types of telescopes – reflecting and refracting – the majority of Cassegrains come under the category of catedioptric, though you will also find many Cassegrain reflectors on the market too.

Catedioptric telescopes are generally considered to be one of the better options, as they don’t have two very specific flaws that you’ll find in both reflecting and refracting telescopes. The two most popular form of catedioptric Cassegrans are the Maksutov (referred to as a Mak) and the Schmidt-Cassegrain (referred to as a SCT).

One of the most common issues that you will find with refracting telescopes is that sometimes you’ll have to deal with chromatic abberation (referred to as CA, a common problem in photography). This generally refers to colors on the edge of images that make the edges appear a different colors than they are, which happens when an image is incorrectly refracted and the colors that appear do not combine as they should.

With reflecting telescopes, you can have the issue of comatic aberration, commonly referred to as a coma. This can make stars appear to have some sort of tail, which in turn can lead to confusion and a distorted image. Catedioptric telescopes have neither of these problems, and combine the best of both worlds.

One of the main issues with Cassegrain telescopes is the high cost. They are made from higher quality materials, so it only makes sense that they are going to be more expensive than other telescopes with similar apertures. Cassegrain reflectors and catedioptrics work in different ways, so it’s important to understand how reflectors, refractors and catedioptrics work.

Dobsonian vs Cassegrain

These are two of the most popular variations of telescope. But, there are some differences between the two which may make one of them far more suitable for you than the other.

• Focal length – Dobsonians and reflector telescopes in general tend to have a short focal length, whereas Cassegrain telescopes have a long focal length. The longer the focus, the higher the magnification.
• Weight – Typically, a Cassegrain is going to be a little heavier than a Dobsonian, primarily due to the amount of glass that’s in each, with the Dob having less glass than a Cassegrain (typically).
• Image Brightness – In my experience, you’re going to get brighter images with more of a ‘pop’ with a Dobsonian telescope. This does not mean to say that a Cassegrain gives a bad image, but you’re likely to get a more vibrant colorful image with a Dob.

These are just a few of the different between these types of telescope, and you’ll likely notice more if you get the opportunity to use both types of telescope.

Conclusion

For beginners, a Dobsonian is still the undisputed champion when it comes to an easy to use telescope. This is primarily because you can get a Dobsonian even if you’re on a tight budget, but if you was a Cassegrain then you’ll need to up your budget a little.

Another reason that Dob’s tend to be a better option for complete beginners is that they have an easier mount for beginners to use. This kind of mount can make it difficult to focus on specific planets if you’re a complete beginner too, but Dob’s are generally great for seeing deep space objects like nebula and galaxies. This doesn’t mean to say that Cassegrain’s don’t have their place for the more experienced user, however.