Telescopes

FAS Astronomers Blog, Volume 30, Number 14.

Have you ever walked out at night and looked up into the night sky? If you live away from a city, the sky is filled with stars. If you’re lucky you might see the Milky Way stretch across the sky. From time to time a brighter object or two comes into view. These are the “wanderers”, otherwise known as the planets.

There are many more objects such as nebulae, star clusters and even galaxies that can be found if you own a pair of binoculars or a telescope. In fact, binoculars are a great way to start exploring what’s up there. However, if you are like me, and find it difficult to hold binoculars steady, you might think about buying a telescope.

You would think this is an easy task. Go out to the local department store, check the telescopes on the shelf, decide what to spend and make the purchase. Many of us, including me, have done this. And the telescope ends up … in a closet.

Before purchasing a telescope, particularly your first scope, take some time and do a little research. I’ve listed a few articles below that might help you.

There are three primary types of telescopes.

  • A refractor is what most people think a telescope should look like. It has a lens that bends (refracts) light into the eyepiece.
  • A reflector is one with mirrors rather than a lens. It has a primary mirror that bends (reflects) light to a secondary mirror, which in turn, sends it to the eyepiece.
  • A compound telescope is more complex and has a combination of lenses and mirrors. The most common types are the Schmidt-Cassegrain and the Maksutov-Cassegrain.

Refractors and reflectors are good choices for manual telescopes. Reflectors typically provide more telescope for the money, although, they must be collimated from time to time. A compound telescope typically comes into play if you want a more automated scope.

No matter your choice, it is NOT all about magnification. The actual magnification is achieved by the eyepiece, although the telescope’s focal length does place a limit on the telescope’s magnification.

The longer the focal length, the more magnification can be achieved with a given eyepiece. Magnification is equal to the focal length of the telescope divided by the focal length of the eyepiece.

MAGNIFICATION = TELESCOPE’S FOCAL LENGTH / EYEPIECE’S FOCAL LENGTH

Many different eyepieces can be used no matter the size or type of telescope. In many cases there is a sweet spot where images are the clearest. Too much magnification results in a blurry image. Too little magnification limits the detail that can been seen.

Oh yes, eyepieces are designed to fit into the telescope’s focuser, which attaches to the telescope, holds the eyepiece, and allows you to focus the telescope to obtain a sharper image. Focusers and eyepieces come in two sizes: 1¼” and 2”. Almost every scope supports 1 ¼”, while larger scopes support 2”. The advantage of a 2” focuser, and the corresponding eyepieces, is that they provide a wider field of view. However, they are more expensive. Some focusers also have single or dual adjustment knobs, the latter for fine adjustment.

What is more important is that telescopes capture light, and the more light that is captured, the more detail that can be seen. The amount of light a telescope captures is determined by the size of the lens or primary mirror. Although, we usually talk about a telescope’s diameter, it is the area that is important. Okay, π comes into play here. A 6” diameter mirror will capture light based on its area (π r2 = 28.3 in2). A 12” mirror has an area of 113.1 in2 and captures four times, not twice, as much light.

The downside is that the larger the lens or mirror, the greater the weight of the telescope and the greater the cost. So, you need to trade off the ease/difficulty of movement and cost with the extra “seeing” power of the telescope. For many, a smaller scope is sometimes better, especially for beginners.

The quality of optics is also important. What many refer to as “department store” telescopes typically have poor optics. Just like binoculars, telescopes with higher quality lenses and mirrors will provide much better views. There are several companies that offer telescopes with high quality optics. Unless you live in a large city, you probably will have to order your scope online from one of these companies.

Holding a telescope in your hands doesn’t work too well. Telescopes require a mount. Make sure you purchase a stable mount – one that provides firm support for your telescope. A tripod is what usually comes to mind, but there is also something called a Dobsonian (“Dob”) mount, which can be used for reflectors. The advantage of the Dobsonian mount is that it sits on the ground, is simple to use, and is very sturdy. Dobs and many tripod mounts have “altazimuth” axes – they move up and down, right and left. These are simple and easy to use. Other mounts have equatorial axes. These are more complex and must be aligned but they follow the movement of the night sky as the Earth rotates.

It can be difficult to find objects in the night sky. A telescope’s field of view is very narrow. This makes it hard to locate objects by looking through the scope itself. So, you need a finder, which is a guide used to point the telescope in the right direction.

  • The traditional finder scope is a small scope with less magnification, but a wider field of view. Use it to locate an object for viewing, then view the object in more detail through your telescope.
  • Many people use “red dot” reflex finders. These display a red dot or some other illuminated object, which can be used to guide the positioning of your telescope. A favorite of many amateur astronomers is the Telrad finder. It is a bit bulky, but easy to align and use.
  • A third, and less common, option is a laser finder. These use a green laser pointer placed in a mounting tube with heating elements to keep it warm. Wires connect the laser to a switch, which allows one to turn the laser on and off.

For many of us a manual telescope will work just fine. You do need a map of the night sky or a smartphone app that will help you locate objects. Choose an object on the map, then use the finder scope to locate the object and have fun viewing the wonders of the universe.

For others, a more automated telescope might be the choice. These scopes have a motor and possibly a computer-based controller. A motor will automatically track the movement of the night sky. The controller will locate objects in the night sky, and, in this case, the motor will move the telescope to the desired object. The disadvantage is that most automated telescopes must be manually aligned by finding two to three stars. They are also more expensive, and you end up paying as much for the electronics as you do for the optics.

Technology keeps moving on, and today there are telescopes with built in GPS that will perform an automatic alignment. Various smart phone apps can also be linked to a telescope’s computer and used to guide and move the scope. As you can imagine, the cost and complexity for these options is also greater.

Once you purchase your telescope, you might want to invest in some eyepiece filters. Most are used to enhance the features on a planet or nebula, although they are more effective on larger telescopes. However, a 25% or 13% Moon filter is very useful for any telescope when observing a bright Moon.

Okay, now you ask, what should I do? Well, there isn’t a right and wrong answer. The best advice I’ve heard is that the right telescope is the one you will use and not leave down in the basement (or closet) to collect dust.

My suggestion, not necessarily the choice for everyone, is to keep it simple when you’re first starting out. Personally, I like the Dobsonian reflectors, provided you find one with quality optics. They are easy to set up and easy to use. The downside is that YOU must manually locate objects to view – the telescope won’t find them for you. A smartphone app or map of the night sky is a must. I have a small scope – I like the ease of transport and the low price. Although larger scopes do provide a better view, particularly of deep sky objects. There is also an advantage to the 2” focusers and eyepieces. They provide a wider field of view but are more expensive.

However, if you are more technically inclined, you might go for an equatorial mount or a computerized/motorized scope.

I will say it again, before you buy, DO SOME RESEARCH. Find your local astronomy club (FAS37.org) and talk to them. Find out what type of telescopes others USE, not what they buy. There are many online articles, several are listed below, which provide additional information and advice.

Once you purchase your telescope, read some of our previous articles about observing the night sky.

There are many astronomy apps available for smart phones and tablets. Most have augmented reality, which allows you to hold the phone or tablet up and the app will show you the portion of the sky you’re looking at. Sky Guide, SkySafari, and Star Chart are three that I use. You can find a few other suggestions listed in Kaleideum’s Astronomy Resources.

Astrophotography can be a great next step after you learn the basics of observing the night sky with a telescope. However, it does require much more knowledge and more sophisticated equipment. The night sky is dark and other than the Moon, capturing images requires long exposures. Well, the Earth moves (it rotates), so a scope with a motor that will track objects is a must. And an equatorial mount is usually helpful as it will follow an object in the direction of its apparent motion. Even then, most observers use computer software to stack multiple images and blend them into a single final photo. In other words, it’s not just point and click.

There are also solar telescopes and full aperture solar filters that allow you to safely observe the Sun. There is more to this and there are very important safety protocols that must be followed.

For more information, see the FAS webpages on Telescopes.

Selected Sources and Further Reading

Jerry Oltion. “Hobby Killers: What Telescopes Not to Buy.” Sky & Telescope. November 26, 2019. https://skyandtelescope.org/astronomy-equipment/choosing-astronomy-equipment/hobby-killers-what-telescopes-not-to-buy/

“Which is the Best Telescope for Me?” Astronomical League. (Accessed April 16, 2020). https://www.astroleague.org/files/outreach/Best%20TelescopeB.pdf

Editors of Sky & Telescope. “Choose the telescope that’s right for your observing interests, lifestyle, and budget.” Sky & Telescope. December 15, 2016.  https://skyandtelescope.org/astronomy-equipment/choosing-astronomy-equipment/telescopes/types-of-telescopes/

Adrian R. Ashford. “How to Choose a Telescope.” Sky & Telescope. January 25, 2021. https://skyandtelescope.org/astronomy-equipment/how-to-choose-a-telescope/

Tony Flanders. “What to Know before you Buy.” Sky & Telescope. 2010.  https://skyandtelescope.org/wp-content/uploads/AboutScopes.pdf

“How to Choose a Telescope.” Orion Telescopes. (Accessed October 16, 2022). https://www.telescope.com/How-To-Choose-a-Telescope/p/99798.uts

Phil Plait. “Telescopes: Crash Course Astronomy #6.” CrashCourse/YouTube. February 19, 2015. https://www.youtube.com/watch?v=mYhy7eaazIk

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