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A task that daunts every new amateur astronomer is choosing their first telescope. Often, many first time astronomers don’t know the right questions to ask when comparing different telescopes. There are several things an amateur astronomer needs to know about telescopes before he can make an informed purchase.
Every amateur astronomer needs to know is that telescope size has nothing to do with magnification. The telescopes objective simply collects starlight and directs that light toward the eyepiece. The more starlight that is collected, the fainter the observed object can be. Therefore, aperture size will limit the ability to view deep sky objects, such as nebulas and globular clusters. The eyepiece is what provides any and all magnification. Every telescope, whether it is a refractor or reflector, is capable of utilizing any number of different eyepieces with different magnifications. While it is possible to put a high magnification eyepiece on a telescope with a small aperture, this is usually not recommended since the image quality will be significantly degraded.
The most important rule when choosing your first telescope is don’t run out and purchase one of those department store telescopes that are advertised by their magnification power. These are often flawed and found with overall poor optical quality. Ideally, you want a telescope with good quality optics and a smooth mount. There are two main types of telescopes, refractors and reflectors.
Refracting telescopes use only lenses to collect starlight and to bring the image into eyepiece. A good refractor telescope for a beginner has a 60mm aperture, which in turn provides a 60mm viewable image. The aperture number is the diameter of the telescope’s main lens or mirror, which is called the objective. Refracting telescopes are also ideal for terrestrial daytime viewing. The main disadvantage with refractors is that with every increase in objective size the price jumps exponentially. Refractors are able to provide very sharp images, but are often budget-limited in aperture size. The Yerkes Observatory is home to the largest refracting telescope ever built, which has only a 40in aperture.
Reflecting telescopes use mirrors to collect starlight and to bring the image into the eyepiece. A beginner should start with either a 6in or 8in aperture. Due to the fact that mirrors are substantially cheaper to create, reflectors with large apertures are much more affordable. Reflectors are highly utilized for astronomical photography, since they are commonly available with larger apertures and therefore able to collect more starlight. The main disadvantage with reflectors is that they more maintenance. Reflectors are susceptible to mirror defects and mirror misalignment, and therefore require periodic cleaning and polishing, followed by realignment. Since reflectors are not as budget-inhibited as refractors, professional observatory telescopes have gotten huge over the years. The Keck Observatory in Hawaii has two telescopes, each with a 10-meter aperture.
Often, purchasing a good mount can be as important as purchasing a good telescope. Since the Earth is constantly rotating, it is necessary to manually adjust the telescope every 5-15 seconds to keep an object in the field of view. A good, smooth turning mount is essential to perform manual tracking easily and correctly. If you have the funds in your budget, I would recommend purchasing a tracking drive. This device will track any object across the sky, moving with the rotation of the Earth. It is therefore possible to leave a telescope locked onto a particular object for hours.
After you have some experience, you may want to upgrade to a telescope with a specific f/ ratio (focal length), certain type of mount, or even a telescope that can be used in conjunction with a CCD camera devise. For now, by purchasing a good beginner telescope you will be able to begin to explore all the night sky has to offer.
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