Buying a Telescope
This article originally appeared as a series of blog posts in November 2014.
Back in the planetarium days I was frequently asked about buying a telescope, especially around Christmas. Over the decades since then people still ask me from time to time and still ask more often around Christmas. I had a pretty well-formulated answer eventually and I’ve written it up a number of times. (I’m not sure if I know where any of those old write ups are).
We’ll start with the basics, what’s important when it comes to telescopes: aperture, mounting and portability.
Note that magnification, computers and electronics, and accessories aren’t on the list.
What’s Important
Aperture
When it comes to telescopes, aperture is more important than anything else. Like a bucket in rain, a telescope’s most important job is capturing light raining down from stars, galaxies, planets, etc. Light carries information and the more light that’s gathered the more you can see. This is why the next big observatory is always about how large the aperture of the next telescope is going to be. In my early years, the 200-inch Hale Telescope on Mount Palomar was the largest. These days we call it the five-meter telescope. Now we have the E-ELT European Extremely Large Telescope in the queue at 39 meters.
The aperture is simply the diameter of the main lens (on the front) or the main mirror (in the back) of the telescope.
In the past, most amateur astronomers might have a six- or massive eight-inch telescope. A 12- or 16-inch was truly gigantic. Now, it’s not uncommon for an amateur to have a 24- or 30-inch telescope. The 10- and 12-inch apertures are not uncommon.
Basically you should get the largest aperture you can afford, that has a sturdy, stable mounting and is portable enough to use. That last part is the trick.
Mounting
After the aperture, the next important feature is the mounting. A good mounting should be sturdy and solid. If a telescope shakes, even a tiny, barely perceptable amount, it degrades the image.
I know of no better example of this than binoculars. It’s a great joy to take a nice pair of binoculars out and look at the sky. But put them on a camera tripod and it’s like you suddenly switched to HD. More details suddenly become visible. It’s because our hand-holding always shakes at least a tiny amount and it’s enough to make a difference.
A good mounting should also be easy to set up. Some equatorial mountings seem like they take 30 mintues to assemble. Some Dobsonian mounts take about 10 seconds.
The mounting should also make it easy to move the telescope smoothly to point it at the objects you want to observe. It should easily stay where you point it.
Portability
I can’t tell you how many times I’ve given all of this information to people, answered their questions, and hear from them that they finally got their new telescope! Then later I find out they’ve only used it one or two times and then not since. Often never.
Many, maybe even most telescopes are bought and only used once or twice and then abandoned. One implication of this is that there’s a huge supply of nearly new, “used” telescopes if one could only find them.
I’ll address another aspect of this try-and-abandon problem later, but a huge factor is portability. The basic rule is, the less a telescope weighs the more it gets used. Smaller is better in this sense, exactly the opposite of aperture. Having the nicest, big telescope doesn’t do anything for your observing hobby if you never want to take it out and set it up. On the other hand, a smaller telescope that lets you grab and go is an instrument you’ll use. That makes it more enjoyable regardless of what it’s aperture is.
For this reason, it’s a good idea to try before you buy.
I truly love my 10-inch Dobsonian telescope. It will always be my favorite. However, sometimes I find that I hesitate to take it with me. I find myself thinking I wouldn’t mind something more like a six-inch Dobsonian, purely for portability, convenience, and my back.
Recommendations
Orion
Since the planetarium days, I’ve favored Orion Telescopes as an excellent and reliable source of reasonably priced and good quality telescopes. Their telescopes are well-made, of outstanding quality, and not the absurdly overpriced telescopes you find at your nearest big store.
What we used to call those “department store” telescopes, which typically were of poor quality and vastly overpriced. The classic starter telescope is a 2.4-inch (60-mm) refractor on an altazimuth mount. I’ve seen the “department store” versions of these for $250, even $400. The are often advertised as 400X or 300X magnification (which is absurd and meaningless). That same telescope, of higher quality, outstanding and appropriate accessories at Orion is $100.
I’m not going to cover all of the different types of telescopes but just refer you to the web for that. You should read about refractors and reflectors. Equatorial mounts and altazimuth. Dobsonian, Newtonian and Schmidt-Cassegrain, Maksutov and catadioptric telescopes in general. It sounds like a lot, but the basic ideas are not complicated.
Binoculars
The first recommendation is always binoculars. They are two telescopes instead of one. They are the most portable telescopes you can get and you get two, one for each eye.
The investment is around $100. It’s a good investment because, regardless of how serious you become as an astronomer, you always have a need for binoculars. There are some objects that don’t look as good in a telescope. A total lunar eclipse is an example. When I was a serious deep sky observer, I often found objects or at least the stars surrounding their location using binoculars.
Finally, if you decide you don’t enjoy astronomy that much, instead of binoculars getting forgotten in a closet, they are always useful for things like concerts, sporting events, and nature viewing.
The ideal binocular size for astronomy is 7x50 or 10x50. That means the magnification is 7x or 10x and the aperture is 50-mm. The aperture divided by the magnification is the diameter of the exit pupil and having an exit pupil near the size of the dilated pupils of your dark-adapted eyes is the goal. The 7mm to 5mm exit pupil sizes fit the dilated pupil size of young and middle-aged adults fairly well. Our eyes dilate less as we age.
The binocular should also have a tripod mounting point, usually a hole in the middle tapped for a ¼-20 screw. You can then buy a binocular tripod adapter to attach them to a camera tripod. You can make an adapter for the cost of a few cents at a hardware store with a ¼-20 bolt and nut or two, and a small angle bracket.
I’ve had a pair of Orion Scenix 7x50 binoculars for quite a few years and I’ve been quite happy with them. They aren’t the highest precision or quality binoculars but they are well-made and do the job pretty well for my own observing.
The 2.4-inch Refractor
I always recommend binoculars as a good way to start observing. However, if you’re a kid who wants a telescope, binoculars might be a disappointment. Many astronomers would scoff at the old 2.4-inch standard but I think this is a fine telescope to start with. It’s the lowest cost beginner scope and you can use it to see the beginner objects.
My first telescope was a 2.4-inch (60-mm) refractor from Sears with a 700 mm focal length making it an almost an f/12 focal ratio. Mine had a 25-mm eyepiece for 28x, a 12.5-mm for 56x (still okay) and an absurd little 4-mm eyepiece for an unusable 175x. These were inexpensive 0.965-in “toy” eyepieces.
Orion has the nicest 2.4-inch (60-mm) I’ve seen with serious accessories and a very reasonable price, $100.
The telescope takes serious, full-sized 1.25-in eyepieces and comes with the typical 10-mm and 25-mm giving very nice 28x and 70x magnifications. It has a solid wood tripod. If you’re on a budget, this is probably the best bet and I’m sure it’s better quality and less expensive than a similar telescope you could walk into a store down the road and buy.
The Dobsonian 4.5
I think the Dobsonian is the ideal amateur, visual observing telescope. Invented by amateur telescope maker John Dobson, and achieving hobby-wide popularity around 1980, the Dobsonian changed everything. Suddenly, large-aperture amateur telescopes became affordable.
The Orion SkyQuest XT4.5 Classic 4.5-inch Dobsonian which is, in my opinion, the perfect beginner telescope. It’s portable, even for kids, a super-sturday mounting, and has a nice, large aperture that will allow you to see anything the typical amateur would look for. Beginner telescopes are usually 2.4 to three or four inches. This is 4.5! It has a very nice 6x26 finder scope and two perfect eyepieces, a 25-mm and 10-mm giving magnifications of 36x and 91x. The focal ratio is about f/8 which is classic for a Newtonian reflector like this. At the time I’m writing this the price is $240 including standard shipping.
Dobsonians are easy to maneuver, stay where you point them, and they’re just a joy to use. The only thing this telescope isn’t really ideal for is any type of astrophotography. There’s not a clock drive so the telescope won’t follow the objects in the sky as the earth rotates. For visual observing (looking through the telescope) this isn’t a problem.
If I was going to recommended just one telescope, this would be it.
The 8-inch Schmidt-Cassegrain
If you have a significant budget and want a telescope that will do it all but is still portable, I recommend the 8-inch Schmidt-Cassegrain telescope (SCT). These telescopes have fine optics, perform well, and typically make an excellent platform for astro-imaging. The 8-inch is still not too heavy so it’s still reasonably portable.
I grew up with the Celestron C8 and I have a used one now with the most-solid, locked-triangle tripod from the 70s and 80s. I have to confess that I’m not up to date on what the current crop of 8-inch SCTs are. Celestron and Meade are the famous names in this arena. There may be others now.
One nice thing about most of these telescopes is how the the eyepiece tailpiece is mounted solidly to the back of the telescope. That means any camera or equipment you attach will be similarly solid. The telescope focuses by moving the whole primary mirror forward and backward along the baffle tube. It’s a clever design and makes it all very solid and stable.
These telescopes range in price from $1000 to $2000 roughly, and probably beyond.
Considerations
Magnification
There’s a common misconception that telescopes are all about magnification. One sure sign of a questionable telescope is one that puts its magnification up front. Worse is one that advertises 200x, 300x, and more.
It works like this. The main mirror or lens, called the “objective,” collects light, information, and produces a real image somewhere in the back of the telescope. It’s usually inside the eyepiece holder. It’s called a real image because it acts like a little picture floating there in space. If you point a telescope at something and look at the back of it, with no eyepiece, you can see the real image.
The quality of the image, including it’s resolution, depends on the amount of information collected by the objective, generally the larger the aperture, the more detail is in the image. The eyepiece is a little magnifying lens you use to look at the image, just like you use a magnifier to look at a small object like jewelry or a photograph.
If you imagine taking something with a picture like a newspaper, magazine or a book (if you can even find one of those), and look at it with say a 10x or 20x magnifier, you can probably see more details in the picture. You might be able to make out something that you missed without the magnifier.
However, if you then find a 100x or 200x magnifier and look at the picture, what do you think you’ll see? You’ll probably see the dots the picture is made of, or just a fuzzy blur but you won’t see any more details in the original picture. There’s not enough detail, resolution, information in the picture for 200x to work.
That’s the way a telescope real image works. Even if the manufacturer includes an eyepiece that gives 200x magnification, you’ll just see an unimpressive blur, if you could ever find the object at that magnification.
Worse, you’re also magnifying the speed at which an object in the telescope appears to move (as the earth turns). At 30x to 50x this isn’t that much a problem. At 100x it’s moving but you can keep up. At higher magnifications, unless you have a mounting with a clock drive, the object goes screaming by.
The basic rule of thumb is 20x to 30x per inch of aperture for normal observing, and you might double it to get a closer look. For a 2.4-inch telescope that means 40x to 80x. Note in the previous post that the Orion 2.4-inch has eyepieces for 28x and 70x. That’s perfect! 200x or 400x for a 2.4 is absurd.
For the 4.5-inch telescope, 90x to 180x in theory are good. It’s eyepieces at 36x and 91x are okay, but you could add an accessory to take the 91x up to 150x to 200x but I’d expect you to use those magnifications rarely.
Back in the old observatory where we had a 14-inch Celestron Schmidt-Cassegrain on a permanent pier mounting. It was an f/11 telescope meaning the focal length was 154-inches (14 x 11) which equals 3912 mm. Our typical eyepieces when public observing were a 32-mm and 10-mm. The 32-mm gave 122x and 24-mm at 162x. (Magnification is just the telescope focal length divided by the eyepiece focal length). We might use a 10-mm or so for 391x, but usually not for the general public. At some point, the atmosphere itself limits the detail in the image, regardless of how large the telescope is. Astronomers call that effect the “seeing.”
Clock Drives and Tracking
As discussed above, we’re standing on a moving platform, the earth. For that reason, the whole sky is constantly turning as if we’re on a merry-go-round. The sun, moon, stars, everything in the sky rises in the east and sets in the west for this reason. Some telescopes have an equatorial mounting and a clock drive or computer driven tracking system to follow the stars across the sky. If you’re just doing visual observing, and you have a good mounting, you can easily keep the object you’re observing in the field of view. There’s great joy in visual observing this way.
If you do become interested in some types of visual imaging or more serious observing of certain types, then you may want to look at a telescope with a tracking system like this, but I wouldn’t start with one.
Try Before You Buy
It’s a great idea to see what it’s like to use a telescope. Visit your local planetarium or observatory on an observing night or find a local astronomy club. Maybe go to a meeting or two, one where they offer some presentation or have a speaker on some topic and then, either at the same meeting or a separate one, where members bring their telescopes which they’ll happily show you along with views of a variety of objects through them.
Use the Internet
I’ve skipped explaining a lot of terms and concepts that surround amateur astronomy and telescopes. Excellent explanations are readily available on the Internet. Orion has nice tutorials and beginner guides on their site. Of course Wikipedia has articles on many topics surrounding amateur telescopes. And you’ll find other sources.
You should also download the free software for your computer, Stellarium. This is a beautiful “planetarium program” that shows you the current night sky or the sky from any location, time and date on earth. You can use it to learn constellations, what planets are visible, and every other object in the sky an amateur would be interested in. You can also print star maps to take outside.
Finally and Most Importantly
Now I’ll tell you the most important secret about telescopes and observing, the thing that’s more important than the aperture, more important than how sturdy the mounting is, and more important than portability.
That thing is experience. Here are three examples.
Jupiter
With my 2.4-inch I independently discovered the planet Jupiter. I pointed my telescope at a bright “star” rising in the East and found it wasn’t a tiny, sparkling (due to the atmosphere) point like stars but a little, pale, white oval. Also there were tiny “stars” on either side in a straight line. They were the Galilean satellites discovered by Galileo, Io and Europa (the size of our moon) and Ganymede and Callisto (about the size of the planet Mercury). Jupiter itself was the white oval in the middle.
Later, I visited the planetarium and saw Jupiter for the first time in a 10-inch telescope. I could see the dark belts on the cloud surface! Now here’s the magic. After that, I could always see the North and South Equatorial belts on Jupiter in my telescope. As my mentor would later say, my eye had become educated.
M33
In my super-observing 20s I’d seen many galaxies but had never observed the face-on galaxy in our Local Group, M33. I’d tried many times and could point a telescope right there but couldn’t see it. Galaxies are very dim with a very low surface brightness. After all, notice how hard it is to see our own Milky Way and we’re inside of it! And M33 was not even at an angle but flat, face-on.
Back then there was this precocious, teenage amateur in the astronomy club who could find the galaxy without any trouble. At one gathering in the observatory, he put the 14-inch Celestron on the galaxy and for the first time I was able to see it! From that time on, I had no trouble finding and seeing M33.
The 2.4
One night, at my parents’ house, when I was in my 20s, I took my old 2.4-inch telescope out to observe. By that time, I was an experienced, star-hopping, deep-sky observer. The finder didn’t work and there was no diagonal and I had to crane my neck and look up through the telelscope while sitting on the ground. I had my old Norton’s Star Atlas I’d bought as a teenager. With that little, old, $30 Sears telescope I was able to find M81 and M82 galaxies in Ursa Major, M51 the Whirlpool Galaxy, M1 the Crab Nebula, M27 the Ring Nebula, and more!
The conclusion
What you see through a telescope has more to do with your personal experience than the telescope itself. That means, to see things, you have to use the telescope! The more you look, the more you’ll see. That’s true in a single night and it’s true in a lifetime.