Newcomers to astronomy commonly ask, “What telescope should I buy?”. First, the easy answer: Buy a six inch newtonian reflector, with two or three eyepieces, and be sure to not buy it from any place that doesn’t specialize in telescopes (although you might be safe with a large photographic shop, if they have staff who have actually used telescopes to look at the stars).
The problem is that telescopes are currently available in a mind boggling range of prices, designs and sizes, and the perfect choice depends as much on what your astronomical focus is as on how much money you have. Astrophotography or visual, deep sky or planetary, all are best served by different types of telescopes. And if you need to ask what sort of telescope to buy then the chances are that you don’t know which of these specialties will be more interesting to you. So buy the six inch newtonian reflector. It’s simple enough that a child can operate it, it’s cheap enough that if you realize this astronomy business isn’t for you then you haven’t wasted a lot of money, and it’s big enough to glimpse most deep sky objects while still being small enough to easily move around.
That said, it’s still worth knowing what the other options are. Personally I have never bought a telescope. I have made do for the last few years with a pair of 10×50 binoculars, and am halfway through making my own telescope from scratch. It is, of course, going to be a six inch newtonian reflector. But I have had access to a range of other telescopes, and the first (given to me as a teenager) was an old-looking 60mm refractor. Refractors are what most people think of when they imagine a telescope – it’s a long skinny tube, with a big lens on one end. This lens (called the Objective) is the only working part of the telescope. Everything else is structural. The objective lens gathers up the light and brings it to a focus near the opposite end of the tube, creating a tiny little image. In order to see the image, we plug an eyepiece into the open end which is optically similar to a jewellers loupe. You put your eye to it so that it will magnify the image and allow you to see whatever the telescope is pointed at. The two most important structural parts are the focuser (which moves the eyepiece back and forth to bring the picture into focus) and the mount (which holds the telescope firmly in whatever position you move it to). Both parts are actually critical to the comfortable operation of a telescope, but neither has any bearing at all on the optical quality. Refractors have something of a cult following, as they always have an unobstructed light path which allows for an exceptionally high contrast picture. The second refractor I owned was fairly large (4 inches aperture) and had a very high quality lens. I saw more objects through that instrument, with more clarity, than any other telescope I have ever looked through. The downside is that they are prone to false colour: the lens acts like a prism, casting a rainbow fringe around every single object in the field of view. Certain clever designs can minimise this (and occasionally completely eliminate it), but such lenses are hard to make. As a result, refractors are expensive, and are sold in a much smaller range of sizes than even entry level reflectors.
Reflectors don’t have a lens at all. Instead, they use a curved mirror which focuses the light in the same way as a lens, except that the image is cast in front of the mirror and not behind it. A small secondary mirror is arranged in front of the objective mirror to reflect the image to somewhere that it can be seen. In a Newtonian design (the one I keep recommending), this mirror is angled at 45 degrees so that you see the image by looking into the side of the tube. In a Cassegrain design, the secondary mirror is not angled at all, but reflects the image back through a small hole in the centre of the objective. There are many variations on these designs, but all follow one of these two layouts. They have the advantage of being much easier to make, which means they are available in much larger sizes, at lower cost. The Newtonian is especially simple to construct, which is why they are both the cheapest to buy and the most popular for Amateur Telescope Makers to build. Just like refractors, a reflector needs an eyepiece to magnify the image, a focuser to position the eyepiece, and a mount to hold the telescope steady.
Edit: Since I originally wrote this, I’ve come into posession of an 8 inch Schmidt-Cassegrain telescope on an equatorial mount. It is a lovely telescope, and immensely capable, but has only one feature that recommends it to beginners: It’s folded optical arrangement makes it very compact and relatively easy to transport. But they are expensive, have very long focal lengths, which leads to a narrow field of view and much higher magnifications. This makes it much harder to find your targets, and causes even the slightest vibration in your mount to be amplified hugely. The fork mount also often puts the eyepiece in very awkward positions. It’s a great telescope for more experienced observers, and I love it, but if you’re just starting out you won’t want those frustrations, so my advice stands: buy a six inch Newtonian.
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