Many children receive their first telescope as a Christmas present. You can make sure it’s a pleasant experience and one that will last for years by avoiding some of the common telescope pitfalls. The major pitfalls are poor quality optics, rickety telescope mounts, small view finders, and bad eyepieces.
Avoid telescopes that advertise their magnification as their most important feature. In reality, magnification power is not the most important feature. In fact, it’s not even the second most important feature. Astronomical objects tend to be faint first, and small second (many planetary nebulas are an exception). For example, I can see the Andromeda Galaxy and Orion Nebula well with a pair of binoculars which magnify seven power. So the first consideration for a telescope should the aperture, or diameter, of its objective.
An objective is the main lens of a refracting telescope or main mirror of a reflecting telescope. The larger the telescope’s objective, the more light it gathers and the fainter the objects that you can see. An aperture of at least three inches is required to see the most popular objects in the heavens, but an aperture of six or eight inches makes a better first telescope. Telescopes larger than 8 inches tend to be heavy, and as a result, it’s difficult to carry them outside or load them into the car. So unless you’re really gung ho on astronomy, a big telescope will not get much use.
After looking at the size of a telescope’s objective, look at its focal length or f-ratio. Focal length is the distance light rays must travel to be brought to a focus. As a result, the focal length indicates the length of the telescope’s tube. The f-ratio is the telescope’s focal length, but given in terms of its objective diameter. F-ratio is calculated by dividing the telescope’s focal length by the diameter of its objective (be sure to use the same units for the focal length and objective diameter). Popular telescopes have short f-ratios, like F/4.5 or F/6. Telescopes having larger f-ratios are longer in length and therefore heavier in weight. They also have narrower fields of view, making it more difficult to locate objects through the telescope. So pick a telescope with a shorter focal length.
A wobbly mount is the second curse of the Christmas telescope. When the slightest breeze makes the image in the eyepiece dance, you can’t detect any details. A solid mount allows steadier views and makes using the telescope much more pleasant. It’s not necessary to get a fancy equatorial mount with clock drive for a first telescope. Doing so is sort of like buying a Mazerati for a first car. A telescope that tracks the stars is of no use if you can’t first locate the object you want to look at. Besides, a high quality equatorial mount is heavier and more expensive than a simple Dobson mount (AKA Dob). The Dob is a simple and rock steady mount designed for reflecting telescopes. It’s easier to use than the equatorial; you just push the telescope where you want to look. Since Dob mounts are less expensive than equatorials, a Dob mount allows you to purchase a telescope with a larger objective.
No matter how good the objective of the telescope is, it can’t do its job if the eyepiece is poor. Inexpensive telescopes tend to have eyepieces with plastic bodies and plastic lens. It doesn’t take much to scratch a plastic eyepiece, at which point it become less than useless. High quality eyepieces are made with a metal barrel and glass lenses.
There are two standard size eyepieces. The most popular has a barrel 1-1/4 inches in diameter and the other is 2 inches in diameter. There is a less popular size eyepiece that you probably won’t find any more, that’s the Japanese Standard eyepiece with a barrel diameter of 0.965 inches. Note that the diameter of the eyepiece barrel is not related to the magnification created by the eyepiece.
The magnification created by an eyepiece is determined by dividing the focal length of the objective by the focal length of the eyepiece. For example, using a 25 mm focal length eyepiece in a telescope with a 1000 mm focal length objective yields a magnification of 40 power (1000 mm divided by 25 mm). The shorter the focal length of the eyepiece, the greater the magnification it creates. An inexpensive way to increase the magnification of an eyepiece is to insert it into a Barlow lens before inserting it into the telescope. The typical Barlow doubles the magnification of the eyepiece.
Telescopes need several eyepieces so you can change the telescope’s magnification. The maximum useable magnification of a telescope depends on the quality of its optics. For a high quality telescope, a maximum magnification of 60 times per inch aperture is a good rule of thumb. So expect a good quality six-inch telescope to begin creating poor images if used above a magnification of 360 times. Be very suspicious when you see a two-inch refractor advertising a magnification of 300 power! The really tragic thing is that I have seen telescopes like this.
There’s also a lowest useful magnification that a telescope can create. If the magnification is too low, not all the light gathered by the telescope can enter your eye. When you use too low of a magnification, it’s as if you are using a telescope with a smaller objective. The lowest useable power of a telescope is calculated by dividing the diameter of the telescope (in millimeters) by 7 for young people, and by 5 or 6 for older people.
A telescope’s finder scope or viewfinder helps you point the telescope at the object you want to observe. A viewfinder is a secondary telescope of low magnification and wide field of view. Its wide field of view is centered on the narrow field of view of the main telescope. It’s easier to find an object in the viewfinder than it is in the telescope, especially as the magnification of the telescope is increased.
A good viewfinder has a diameter of two inches. A smaller diameter viewfinder doesn’t gather enough light to make enough guide stars visible. And that makes it difficult to locate faint astronomical objects. Larger diameter viewfinders are useful on large diameter telescopes, but for a first telescope, the two inch viewfinder is just what the doctor ordered.
A viewfinder that is not aligned, or doesn’t remain aligned, with the main telescope is a major source of frustration. When the viewfinder is not properly aligned or won’t stay aligned, you’ll end up spending several minutes sweeping the sky with the main telescope trying to locate your astronomical object of interest, instead of seeing the object in your telescope when you first look through it. So using a poorly aligned viewfinder means that you’ll miss the object of your interest and will have to start all over again. Do this enough times and you will give up for observing for the night. Every time you take a telescope outside, you should point it towards a distant street light and align the finder. This will only take a minute once your get some practice.
Good places to find a telescope are the major advertisers in magazines like Sky and Telescope and Astronomy. Two examples of telescope manufacturers you’ll find advertising in these magazines are Meade and Orion.
So here’s my recommendation for a good first telescope. It should be a Newtonian reflector with an objective of six or eight inches and an f-ratio between f/4.5 and f/6. Its mount should be a Dobsonian. The telescope should have one or two plossol eyepieces (a popular wide-angle design) with focal lengths between 12 mm and 40 mm.
Good luck with your new telescope.
