WARNING: This FAQ is a work in process. I intend it to be a combination education resource, and buyer’s guide for telescopes. Input in the form of comments or corrections will be gratefully accepted.
Considering your first foray into astronomy can be intimidating. The equipment can be expensive, delicate, and difficult to store. Should you jump in and get a telescope? Will a pair of binoculars work for you instead? Is a big, expensive piece of equipment really necessary? In these pages, I hope to give you enough information for you to make an informed decision, and avoid some of the common pitfalls.
Pitfall #1: Magnification vs Aperture; Magnification or “power” is usually the first thing a person considering the purchase of a telescope thinks of, but it turns out to be one of the least important factors to be considered. You don’t need a huge magnification in order to see objects. The ‘aperture’ or ‘light-gathering capability’ is far more important. Many inexpensive telescopes make outrageous claims of 500X or 600X magnification on a 2 or 3 inch refracting telescope. (As a side note, the maximum magnification available on the Challenger telescope at Fremont Peak with the eyepieces stored there is 633X, but it is a 30″ telescope! There is a ‘rule of thumb’ relating the maximum power of magnification to aperture, which is 50X per inch of aperture. That puts the maximum power of the Challenger at 1500X. I’ve looked through the telescope at 633X, and I’d say the rule of thumb needs to be revised downward ). While it’s true that magnification levels can be pushed sky high with certain eyepieces and optical appliances, the images you see will be fuzzy, diffuse, and useless. It’s far better to view with a telescope of larger aperture and moderate magnification than to push a smaller scope to ultra-high magnification levels. Image size will be a bit smaller with less magnification. But because the image you do see will be of far better quality, you’ll actually see a lot more! Some manufacturers of “department-store-brand” telescopes take advantage of this number-one misconception among first-time telescope buyers – that magnification is everything. It isn’t. So beware!
Pitfall #2: Aperture – Bigger isn’t always better; The most important attribute of a telescope is its aperture size. This is the diameter of the telescope’s light-gathering lens or mirror – often referred to as the telescope’s objective. It’s usually measured in millimeters or inches. Why is aperture size important? Because it’s like having a bigger eyeball. The size of the ‘light bucket’ determines what you can see. A bigger aperture will bring out greater detail, dimmer objects, and allow you to see color in some cases. Aperture is also the second most frequently over-hyped number (second only to magnification, above) that telescope manufacturers use to sell their products. The downside of aperture is size: you don’t want to forget about portability when making your purchase decision. If you spend thousands of dollars on a monster telescope, and you don’t use it, you’ve wasted your money. Ask yourself “Where will I want to use my new telescope, and how will I get it there?”. If you will want to take your telescope to a dark location away from city lights in order to get good viewing, you need to be concerned about size, weight and ease of setup/teardown.
Pitfall #3: Whiz-bang technology that doesn’t always work – I own a telescope that has a built-in GOTO computer: A hand controller will allow me to go to any object in the sky… sometimes. The problem is that the computer has to be aligned in order to work properly, and that alignment procedure can’t happen until full dark, when certain bright stars the computer uses as index stars appear in the sky. Until that point, the scope does not properly track, and it is difficult to impossible to use up until that point. Once, up at Fremont Peak, I watched an experienced amateur astronomer attempt to align a new, out-of-the-box computer-controlled scope for over two hours. This was a beautiful 16″ Schmidt-Cassegrian that I would have loved to look through, but after fighting with the computer for over two hours, he finally gave up and packed it away. My recommendation is to stay away from computerized scopes until you are really ‘into’ astronomy. You’ll be better off spending the money on other things rather than a computer controller.
Pitfall #4: f-number or “speed”: Reflecting telescopes in particular are subject to yet another manufacturer ‘trick’ and that is to reduce the overall length of a reflecting telescope (especially Newtonian reflectors) by increasing the curvature of the mirror. The ‘f/number’ of a reflecting telescope represents the ratio between the focal length of the primary mirror over the diameter of the mirror. This is also called the ‘speed of the mirror’. The Challenger telescope is a f/4.8 30″. This means that the focal length of the primary is 144 inches, and the diameter of the primary is 30 inches ( 144 / 30 = 4.8 ). This is in the ‘sweet spot’ of focal lengths for Newtonians (f/4 – f/8). An increase in the size of the aperture results in a geometric increase in the focal length, for the same f/number. My usual observing scope is a f/8 114mm (about 4.5 inches). An increase of aperture to 6 inches would result in a 48″ focal length, or about a 4 foot long telescope. Since the eyepiece on the Newtonian design is at the top of the scope, this begins to become unwieldy. The usual practice is to reduce the f/number of the primary. The trade-off is that every optical system contains some aberrations. In the case of a Newtonian reflector, the primary aberration is ‘coma’. In Newtonians under f/4, coma becomes pronounced and the observer will begin to see wedge-shaped flares on stars that are not in the exact center of the field of view.
Pitfall #5: Complexity; This pitfall relates to #3 above, but extends to other factors. There are a lot of beautiful telescopes available. Many of these telescopes have advanced features that are useful to dedicated amateurs. But, unless you are a dedicated amateur, or a professional, many of these advanced features simply add complexity, and money to your purchase, without adding much to the usefulness of the telescope. One example is the mount: A motorized German equatorial mount can cost as much as a good telescope, and it adds significant value if you are going to be doing long-exposure astro-photography. Most amateur astronomers could get by with a very simple mount such as a Dobsonian, which is not computerized, not motorized, and very simple to set up, use, and take down.
On to the scopes! Now that we’ve covered some of the basic pitfalls, lets discuss the telescopes themselves: There are three basic types of telescopes: refractors, reflectors, and catadioptrics. All three designs have the same purpose, to collect light and bring it to a point of focus so it can be magnified with an eyepiece for examination by the eye. Each does this work differently, and each has advantages, as well as disadvantages.