Telescopes
If you have ever looked up at a crystal clear night sky and pondered about what lies beyond the clouds or where exactly Caseopeia is, or even how many rings there are around Saturn, then you probably would be interested in having a telescope.  To learn how telescopes work, go to howstuffworks.com for a tutorial.
Tips on Buying Telescopes - Telescopes are ever more impressive these days as the technology keeps improving.
- Don't get suckered into buying a scope because it looks fancy or because it boasts a really powerful magnification (see the details below to find out why).
- Get the model with the functions you need. If it looks really complicated, then it probably is.
- Size is important, especially if this is to be a gift for a child. It needs to be lightweight and compact to move around without a problem.
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Telescope Type in Depth
Major Brands and Models
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Refractors
Refractor telescopes by AstroPhysics, Televue, and Takahashi usually make up the bulk of the entry-level telescope products on the market. Other models, unless they are quite expensive, tend to produce visual aberrations, including chromatic aberrations, which is when colored rings show up around objects in the viewfinder. To counteract this effect, look for extra-low dispersion glass. - Non-achromatic: The least expensive. Not the best color rendition or view. Not recommended unless it's for a child perhaps.
- Achromatic: Colors of light focus at nearly the same point making them sharp and contrast well. The negative is that they are somewhat expensive relative to the aperture they offer (3" for $500). They are fine for viewing planets, but not deep-space.
- No need for recollimation.
- Can have some chromatic aberration.
- Apertures are typically small.
- Apochromatic (Apo): Very accurate color rendition (no chromatic aberration) but it will cost you ($500+) because of the increased number of lens elements that this type of telescope uses. Extra sharp images, portability, and the fact that it doesn't need recollimation are some of the high points.
Reflectors
Reflector telescopes are Newtonians reflectors that use a concave parabolic primary mirror to collect and focus incoming light onto a flat secondary (diagonal) mirror that in turn reflects the image out of an opening at the side of the main tube and into the eyepiece. In layman's terms, a reflector telescope uses a large mirror at the bottom of the telescope to focus light rays on a smaller mirror near the opening which then reflects the light into an eyepiece located at the top of the telescope. The Newtonian reflector is the oldest reflector design in use and still the most popular reflector telescope. - Newtonian
- This is the layman's term for "reflector telescope."
- Produces much better quality images because the color reproduction is more accurate and there is often little or no chromatic aberration.
- Because of their design, with the focuser on the front, they are easier to use than refracter telescopes.
- These are good telescopes for the money. The quality is as good as with an achromatic telescope. Basically, they are a good all-purpose telescope.
- They can collect dust and dirt, and they need periodic recollimation.
- You may also be interested in Maksutov Newtonian Telescopes.
- Catadrioptic
- Schmitt Cassegrain : This is a dual-action telescope using both a reflector and a refractor in one corrector lens. It makes for a portable, compact, and easy to use telescope. They don't have the best contrast, but they are reasonably priced and offer high power. Make sure that you save enough money to get another mount though because the Schmitt mounts are not the most stable.
- May need a dewsheild or a dew heater.
- Not as bright because the corrector it uses diminishes the light.
- Not that easy to focus.
- Maksutov Cassegrain by Questar. These are like the Schmitt but better as far as contrast goes. Plus they have big apertures and are reasonably priced for what they offer, including sharp images, portability, and larger eyepieces and focal lengths.
- Expensive compared to the aperture offered.
- Wide field of view is restricted because of the large focal length.
- Subject to dew and moisture like the Schmitt Cassegrain.
The Details About the Features
Aperture - This is such an important element to consider when buying a telescope. So what is aperture, you ask? It is much like camera aperture. It is the opening in the lens which allows for different amounts of light to enter into the barrel. The more light that comes in, the better the image you will see.
- You will see aperture labeled in millimeters and rarely in inches, and the rule of thumb is bigger is better.
- Optical quality and mounting is of prime importance as well.
- Magnification, which often is an improper indicator of a telescopes power, should not be mistaken for aperture. A way to compare telescopes without making this mistake is to multiply the aperture by 50x. That will give you an adequate idea of the actual magnification you will achieve.
Focal Ratio- Focal length has a lot to do with how much magnification power you have.
- By dividing the lens diameter of the eye piece into the focal length you will get the magnification power.
Mounts- Stability is important. You don't want anything that shakes or jiggles. Preferably, the tripod should be a strong altazimuth mount, instead of an equatorial mount.
- Dobsonian: Dobsonian mounts are made for telescopes with Newtonian lenses. They are inexpensive, made with wood and nylon and Teflon bearings, and can even be made at home. Because they are made to sit low to the ground, they are very stable as they sit on a swiveling base.
- Altazimuth: Altazimuths are fairly standard mounts for the majority of refractor telescopes. Their stand-out feature is that they allow for both horizontal and vertical movement. They aren't quite as stable as the Dobsonians because they rest on top of a tripod.
- Equatorial: The equatorial mount design is slightly more cumbersome to use because it only moves along the polar axis, ie, horizontally. Any vertical adjustments then need to be made by hand.
Eye Pieces- Lenses, or eye pieces, are made by Kellner, Barlow, and Plossl.
- You will often see the degree-field and the number of elements an eye piece has. Again, more is better.
- Solar and nebula filters can be purchased to observe deep-sky contrast as well as the sun.
- Eye pieces should be 1.25" or 2" in diameter, which is not only standard, but also compatible with the best quality replacement lenses.
Finderscopes- These are used like mini-scopes. They sit on top or on the side of the telescope and are used to view large portions of the sky at a lower magnitude to help you locate where it is that you want to zoom in on.
- Some telescopes now feature "auto-finders" that have a stored database of where things are located so that it saves you time looking for them. Autostar is one such series that features this. Some telescopes come with them and some don't. Either way you can purchase one for about $50.
- Refractor Finders : These are very useful and work quite well. However, they display the image upside-down.
- Reflex Finders : For the average stargazer these will probably be the preferred type of finderscope since it displays the images right-side-up.
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