Two Telescopes In One
The right combination of eyepieces can transform a single telescope into two.
When beginners think of telescopes, they generally think of long, sleek tubes with fairly high power. In fact, that's a pretty good description of a long focus, high power telescope; the longer the scope, the longer the focal length and therefore the higher the magnification. Once you begin observing, however, you might find that high power isn't always the way to go. Sometimes, going low power and sweeping up larger arcs of sky can be very satisfying. This is the best way to observe some of the best the sky has to offer; the Milky Way, the Pleiades, comets, etc. Long focal length scopes are best suited for planetary work, deep sky, binary and multiple stars and close up lunar. There is only so low they can go. The best solution is to purchase a short tube telescope. This gives you the best of both worlds.
The Right Instrument For The Right Job
Short tube telescopes are known as RFT, or rich field telescopes, due to their ability to sweep up large portions of sky at one time. They have a number of advantages immediately over their longer kin. First is portability. It is easier to carry a short tube Newtonian than a long one, for instance. As a result, they are also lighter, making them easier to carry over distances. Rich field telescopes have to be made to more exacting standards as well. Concentrating light over such a short distance is problematic; if the optics are not figured properly, you can get severe chromatic aberration, the colors breaking down into a minor prismatic effect. For this reason, some of the higher quality instruments cost more.
Not all short tube telescopes are true RFTs, however. Obviously, catadioptrics (Schmidt-Cassegrains, Maksutovs) are not short focus telescopes at all, they just happen to be short due to their compound nature. Some companies sell short tube instruments that use lenses to shorten the focal length as well (and sometimes these are to be avoided). In their purest essence, RFTs are very basic telescopes, just shorter versions.
My Two Rich Field Telescopes
I own two commercial rich field telescopes, an Apogee Instruments Widestar 80mm refractor ("Benjamin") and a rebuilt C-Star 114mm Newtonian ("Felix"). I own a third RFT, a 57mm scratch built instrument, "Lil' Bernie", but it really falls outside of this discussion.
On the left is the 80mm Widestar Refractor, the right the 114mm C-Star Newtonian
The 80mm refractor has a focal length of 400mm (focal ratio f/5) while the 114mm Newtonian has a focal length of 500mm (f/4.38).
(Before I venture any further, though, it's necessary to explain how the 114mm, "Felix", came into my possession. I came across this scope at a local Goodwill. It really wasn't that old, but obviously had a rough life; the tripod was a mess, the finder scope was damaged and there wasn't an eyepiece to be found. The mirror, though, wasn't bad, so I plunked down my $30 USD and took it home. As luck would have it, I had legs to a very similar tripod, though wooden and not aluminum. After modifying the focuser to handle 1.25" eyepieces and investing roughly 30 minutes of my time, I ended up with a nice little scope.)
Both of these scopes represent fairly common designs that you are bound to encounter from other manufacturers such as Meade and Orion, and are not terribly expensive, ranging from $100 to $250 USD. There are also a number of other manufacturers who sell similar instruments, so do look around. If you do purchase an RFT, though, it is very important to consider the eyepieces, since poor designs will lead to poor performance. And this is where our single scope becomes two.
How To Lengthen Your Telescope With Minimal Muss And Fuss
Most good manufacturers include at least two eyepieces. If the eyepiece has less than a 10mm focal length, you need to be wary of its performance. Short focal length eyepieces usually mean poor eye relief, which can make viewing difficult at best (and a headache at worst). It is best to go with at least two decent eyepieces, a 10 to 16mm for higher magnifications and a 25 to 40mm for lower magnifications (Sir Patrick Moore and Rev. T.W. Webb recommended a minimum of three; low power for locating, medium for general purpose work and high for closing in). Eyepieces have gone down substantially in price in the past decade, so if your telescope has eyepieces of questionable quality, simply replace them.
A 20mm Kellner & "Ben"
The trick that we use to increase the telescope's apparent focal length involves using a Barlow lens. A Barlow is a concave lens arrangement that increases the lights focal cone. Not all Barlow's are made equally, however. All of the major manufacturers produce good quality Barlow's, and they can run from $29 USD up. By simply inserting a Barlow in ahead of the eyepiece, you can increase your telescope's focal length up to 2x (which is really the limit).
"Ben" and "Felix" Sporting A Barlow And A 20mm Eyepiece
With this simple trick, you improve the flexibility of your telescope by a factor of two, It might not be unusual to accomplish this feat for less than $200 USD, which is considerably less than the costs of two telescopes. While not perfect (again, if the optics are not very good), it does greatly improve your capabilities for a marginal expense. The most important thing to remember here is do not purchase the cheapest Barlows you come across, always go with a known manufacturer. Follow these simple steps, and you will effectively double your pleasure.
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