It’s wise to brush up on your telescope types if you’re adding variation to your collection or if you’re looking to buy your very first one.
You may know the terms, but how do you tell the differences between them?
This is a good question seeing as there are many types of telescopes and variants between them.
For example, The Hubble Telescope is a Ritchey-Chretien and then you have the largest single-aperture telescope in the world the Gran Telescopio Canarias that is a reflector.
To get a rundown on the most common types that you’ll come across in the amateur market, here’s your quick guide to get started.
The three most common types you’ll encounter are refractors, reflectors, and catadioptrics.
They each their own pros and cons and have their various strengths in different types of astronomical observation and imaging.
To get started, in this context we are only discussing the OTA (Optical Telescope Assembly/Assemblies).
The OTA is the metal tube that comes to mind when you think of telescopes. It houses the optical components needed to see an image with magnification.
The optical components consist of glass lenses, mirrors, or both. This is a fast way to determine the type of telescope you’re considering. Refractors use lenses to gather light, reflectors use mirrors, and catadioptrics use both.
With a little prep knowledge under your belt, let’s get into the meat of it.
The early 1600’s marks the popularization of refracting scopes by Galileo. Yes, they’ve been around for that long.
Why mess with something that works?
The Galilean telescope had flaws that multiple inventors improved upon a little at a time over a long period of time. Today, we have some very high-quality refracting telescopes that are not perfect but are excellent for their purpose.
How do Refractor Telescopes Work?
Refracting telescopes refract or bend light. It’s why their design type is named appropriately. You have glass that makes up the objective lens – the large, round plate of glass at the front end of the telescope.
This is where we measure the aperture or the size of the diameter of the objective lens. It’s responsible for light-grasp which means how much light can be collected within the tube.
Light enters through the objective lens and are split into wavelengths or what we know as colors. They travel through the tube and meet at a focal point where a telescope component, usually a diagonal, allows for that light to be focused to the eyepiece.
The result is a formed, magnified image.
Today, there are multiple types of improved versions of the original refractors of yesterday. The most basic type of refracting telescope is the single element lens refractor. This is what you will see in the entry-level market where image quality is sub-par and there is a lot of chromatic aberration (CA).
Achromat doublets would be the standard of basic quality for today and is considered entry-level. There are two elements that make up the objective lens assembly. It helps to correct for some CA, but it will still be present.
Semi-APO (apochromatic) refractors typically use two elements (doublet) but they may incorporate ED (extra-low dispersion) glass. Note, this is “extra” as doublets already provide low dispersion benefits. Semi-APOs can be moderately expensive to expensive.
The most expensive type of refractor is the APO. It can use a minimum of three elements but can incorporate more which increases weight and cost. However, it corrects for multiple aberrations such as CA.
These types of refractors are used for astrophotography purposes due to its excellent CA correction and overall imaging quality.
- Easy to use
- Collimation rarely necessary
- Makes full use of aperture
- No obstruction
- Fast cooldown
- Can be used for terrestrial observation
- No secondary obstruction
- Small apertures
- Expensive with larger apertures
- Chromatic aberration
- Refractors come in small aperture sizes but utilize their entire aperture for light-grasp.
- Chromatic aberration is the result of wavelengths unable to come to the same focal point for color fidelity and sharpness. This looks like color bleeding or false color around objects.
- Refractors are usually lightweight, portable, and do not suffer from thermal changes.
- Refractors can be used with the appropriate diagonal for terrestrial/land-based observation.
Reflecting telescopes came about years after the refractor design as an alternative with mirrors instead of lenses. There has been experimentation with the shape of the mirrors and mirror coatings in a reflecting telescope.
Sir Isaac Newton was credited with the design in 1668 with the spherical primary mirror come to be known as the Newtonian.
As such, reflecting telescopes are known as the reflector, Newtonian, and Newtonian reflector.
How do Reflecting Telescopes Work?
Reflectors use mirrors, and so it’s obvious to assume that the optical path within the tube depends on reflections. This assumption would be correct.
First off, you have a primary mirror which is situated at the rear or bottom end of the telescope. At the top end of the telescope is a secondary mirror and to the side will be the eyepiece and focuser.
The sizes of the mirrors are important as reflectors suffer from obstruction. Light enters the open tube, reflects off the primary mirror towards the secondary mirror. The secondary mirror directs that light towards the eyepiece.
Because of where the mirrors are placed, the secondary mirror blocks some of that incoming light from reaching the primary mirror.
In this case, it’s fair to say that refractors can collect more light than a reflector, but a reflector can be made larger in aperture much easier and cheaper than a refractor.
So, reflectors still have an advantage as they can be made bigger than a refractor even though obstruction is an inherent design flaw.
The most common type of reflector telescope is a Newtonian and then there is the Gregorian. While there are large Gregorian telescopes around, it’s the Newtonian that you will mostly likely encounter in the amateur market.
A type of Newtonian that has been popularized is the Dobsonian. Briefly, it consists of a Newtonian OTA with inexpensive materials used to construct telescope movement with a base.
The base is made with a laminate-covered piece of plywood with sideboards. Teflon or plastic materials are used as strips to provide smooth movement along one or both axes. Variation in materials or designs are expected between larger Dobs and manufacturers.
Dobsonians are known for their simplicity and mount/base ease-of-use. They are available with very large apertures offered at cheaper price points versus refractors and reflectors on tripods.
- Value per inch in aperture
- Large apertures
- Good contrast
- No chromatic aberration
- Easy to use
- Basic mount
- Relatively inexpensive
- Popular design for amateur building
- Dobsonian variant
- Open tube/exposed optics
- Requires collimation
- Longer cooldown time
- Secondary mirror obstruction
- Other possible optical aberrations
- Spherical aberration is noticeable with spherical primary mirrors at focal ratios of f/8 and lower.
- A parabolic mirror is usually used on a Newtonian with a focal ratio of f/5 and higher to correct for spherical aberration.
- Provides excellent value in aperture per inch versus refractor.
- Short focal length fast Newtonians can suffer from optical aberration known as coma.
- Dobsonians are popular for DIY builds and offer excellent optical quality with an inexpensive, basic mount.
- Requires collimation procedure to be performed.
- Produces an image that is upside down and reversed. Not suitable for terrestrial/land-based observation.
The most common types of catadioptric telescopes in the amateur market are the SCT (Schmidt-Cassegrain) and Mak-Cass (Maksutov-Cassegrain) designs. They incorporate both lenses and mirrors into the optical design.
How does a Catadioptric Telescope Work?
The SCT telescope has a spherical primary mirror with a perforation, a convex secondary mirror, and a thin corrector lens behind the secondary mirror. The primary mirror is located at the rear end of the telescope where the eyepiece/visual back assembly is mounted behind it.
The secondary and corrector lens is situated at the top end of the telescope.
Light enters the telescope and reflects off the primary mirror towards the secondary and corrector lens. It’s then reflected back to create a narrower light cone that enters through the perforation in the primary mirror to come to focus behind it.
They typically come with apertures from 4” to 16”.
The Mak-Cass or MAK also has a spherical primary mirror with a perforation. It differs from an SCT only slightly since it has a thicker, highly-spherical corrector called a meniscus lens.
Instead of a secondary mirror, an aluminized spot on the meniscus lens acts as the secondary mirror.
The light path is the same as an SCT, but where reflected light from the primary would normally hit the secondary mirror, in the MAK it hits the aluminized spot to reflect the light cone through the perforation in the primary.
Since the meniscus lens is considerably thicker which means more weight, expense, and expertise to manufacture, they typically come with apertures from 4” to 7”.
- Small to medium aperture
- Practical to use
- Good correction of optical aberrations
- Comfortable viewing
- Closed tube
- Short tube length
- Long focal length
- Good for planets
- Moderately expensive starting price
- Almost always on GoTo mounts
- Secondary mirror obstruction
- Small fields of view
- SCT and MAK telescopes are part of the catadioptric family.
- They suffer from secondary mirror obstruction like Newtonians.
- They have long focal lengths of around 1500 mm with narrow fields of view.
- They have slow focal ratios of around f/10 and slower which is good for viewing and imaging planets.
- They are very lightweight and portable due to their shorter tube lengths.
- They often come on GoTo fork-arm mounts.
Which Type of Telescope is Best for You?
It’s easier to narrow down the right type of telescope by way of interest.
What are you interested in seeing?
Do you have any astronomical goals?
Will you be imaging?
These questions are very valid and being able to answer them will help in the long-run in choosing the right type of telescope for the job.
No one telescope does it all. No one telescope type is perfect. The best telescope is the one that best fits your needs.
Sorry to be the bearer of bad news, but as soon as you understand the attractive features and flaws between them, you’ll be able to make an informed decision.
If all you can think about is aperture, you may be cutting yourself short. Larger apertures are better for maximum light-gathering, but aperture is not the full story. Aperture alone can be an inadequate measure of telescope performance.
Take The Hubble Telescope for an example. It does not have the largest aperture, but it orbits above the Earth’s atmosphere to achieve excellent resolution, make use of excellent light-gathering capability, and take long-exposure photographs to produce exceptional and breathtaking images that has made it famous.
With refractors, you have small apertures generally from about 2” (50 mm) to 5” (127 mm). While small, they make use of the full aperture to gather light.
Reflectors have large apertures for the best prices. They range from 4” to 16”+. Dobsonians are known as the value reflectors as they offer large sizes with very simple mounts. However, reflectors suffer with an inherent design flaw – secondary mirror obstruction. This must be considered if you’re all about aperture.
Catadioptrics usually range between 4” to 16” with SCTs and MAK telescopes are small with sizes between 4” to 7”.
All types of telescopes do what they’re designed to do – gather light. They’re all good for observing bright objects like the moon, sun, planets, and the brightest DSOs (Deep-sky Objects).
Knowing this, you can find a telescope with a large aperture that also has optical quality behind it.
Most telescopes will be able to find the planets. How you see them and how much detail you can see will depend on the type. Catadioptric telescopes are excellent for planetary observation and imaging as they provide a slow focal ratio that is necessary to see more planetary details within the narrow field of view at higher magnification.
Focal ratios slower than f/10 will be better suited to seeing planets.
If you’re wanting to see more star clusters, nebulae, and galaxies, then you will be better off with a telescope that has a large aperture as you need more collected light to see faint objects at higher limiting magnitudes.
A fast focal ratio is also helpful as it provides a wide field of view that allows you to use low to medium powers while fitting in the entire object.
Reflectors, particularly Dobsonians, match the specs you need to see deeper into space to see DSOs and its faint structures.
If you’re a traveler or you intend to travel to dark skies often with your telescope, you’ll need a portable and travel-worthy model. A refractor telescope is a great travel scope to consider as they’re smaller in aperture, but semi-APO and APO models are capable of delivering excellent optical quality.
Carbon fiber tubes offer thermal stability and some slight weight-reduction benefits. Tabletop reflectors are good for travel as they’re small, lightweight, and can be transported fully assembled.
Traveling in a vehicle allows for more constant security checks and proper handling. Traveling by plane would require you to be very specific about its dimensions so that you can fit at least the OTA in a carry-on bag/case.
Don’t be a Telescope Snob
You will find that many types of amateurs and advanced users will eventually own different types of telescopes. You may have your favorite type, but don’t fall into the trap of becoming a telescope snob.
Instead, understand that they each have their own strengths and weaknesses and can be good for different types of observation and imaging.
With that in mind, why not buy a small refractor for travel and eventually buy a Dobsonian for deep-sky observation?
You may have specific tastes about each type of telescope, but if love them all, you can maximize performance and user satisfaction for all circumstances.
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