Choosing the right telescope


The only think better than the view of a dark night sky from the cottage or wilderness is the same view through a telescope.

But which one to get? Here’s a quick look at the differences between kinds of telescopes and what each is good for…


There are two main types of telescopes…


Lens-based “refractors” – like the tripod-based one pictured above, at right – look like what you probably think of when you think of a typical telescope.


They magnify by bending light through a curved lens to be focused at an eyepiece.

Refractors are exceptional for their crisp, high-contrast views of objects like the Moon and planets.

However, they are expensive to produce in large sizes.


Mirror-based “reflectors” are basically the same design as Isaac Newton’s original telescope as well as most large optical telescopes in observatories, and similar to the Hubble Space Telescope.

testing_new_telescopes_efstonscience_skywatcher_dobThey use a curved primary mirror to bounce light to a smaller secondary mirror at a 45-degree angle that, in-turn, bounces light into the telescope’s eyepiece at the front of the scope (the part of the tube that would typically point to the sky.)

While reflectors give less-“contrasty” views because of the obstruction of the secondary mirror (as you can see in the image at right) they can be cheaply made in much larger sizes: While refractors can only be made with lenses up to 6 or 7 inches in diameter at consumer costs, reflector mirrors can reach sizes of 20, 24, 28, even 30 inches for a similar price.



crappy_telescopeContrary to popular-belief, the most important factor to consider when buying a telescope is NOT magnifying power. DON’T FALL FOR THE “MAGNIFICATION SCAM”!!!

For larger, fainter objects such as some star clusters, nebulas, and galaxies (some of which don’t even fit into the field of view of some telescopes) you actually want less power, in order to see their huge-but-faint details.

refractor_aperture_qualityDepending on your needs, the most important factor to consider when shopping for a good telescope is light-gathering ability (width of the main optics) and optical-quality.

The ability to magnify an object is determined by the eyepiece. Thus, any telescope (even a crappy plastic one from a big-box store!) can magnify 900x with the right eyepiece.)

However, every time you magnify an object it space, it gets dimmer and fuzzier. (Even a large, expensive telescope can’t magnify x900. So imagine the view you’d get through that department store telescope trying at such power!)

deep_space_fuzz_andromeda_galaxyThe larger a telescope’s optics, the easier it will be for that telescope to focus a magnified image into a crisp, clear image.

The general rule of thumb is that a telescope can effectively magnify x50 per inch or aperture. So a 3″ (80 mm) diameter telescope can magnify objects in space up to x150 before the image starts to degrade. A 5″ telescope can magnify x250, and so-on.

Even so, with a huge lens or mirror of exceptional quality, the practical upper-limit for telescope magnification is about x300.



telescopes_equitorial_mountFor telescopes on equatorial mounts

If your telescope has an equatorial-style mount, like the one pictured at right, congratulations! (and my condolences.)  Such mounts allow you to line your telescope up with the north star so manual controls or motors can let your scope “track” the sky as it appears to move due to the Earth’s rotation.
These mounts are highly convenient but are among the most expensive and complex to operate.


To aim a telescopes on an equatorial mount:


1. Unlock one axis and move the tube with your hands until close to the object you’d like to see, then lock that axis

2. Unlock the second axis and repeat. (Once you get good at this, you can partially loosen both locks and move the scope on both axis at once.)

3. Finally, you can use the slow motion control cables to fine-tune your aim until the object you’re looking to view is in the crosshairs of the telescope’s finderscope.

For telescopes on 
Alt-AZ mounts

alt-az_mountFor simpler “up & down” Alt-AZ (short for altitude and altazimuth – i.e. right/left) mounts, like the one at right, or “dead-simple” particle-board Dobsonian mounts such as the large one in the image below, left:

1. Just untighten any locks, nudge one way, then the other and…

2. Look into the finder scope to refine your aim.

3. Done!

While such mounts don’t have the convenience of fine controls or being able to automatically keep what you’re looking at from drifting out of view, they lower the cost of a telescope substantially and make aiming much easier for novices.


telescope_use_simplifiedSELECTING THE RIGHT TELESCOPE

Today’s telescopes are better-made, easier-to-find, and less costly than ever before.

Here are three recommended scopes – one refractor, one reflector, and one portable model: