1. A refracting astronomical telescope makes use of two lenses. The lens in the front of the telescope, called the objective lens, produces an upside-down image of the object. The lens near the eye, called the eye lens, acts as an ordinary magnifying glass to magnify that upside-down image. Naturally, each of these two elements of the telescope could be made up of several lenses, to combat certain inherent limitations, or aberrations.
Standard-quality achromatic or apochromatic refractors have some advantages over other telescope designs. Firstly, refractors, by default, bear a entirely clear aperture. There is no central obstruction that leads light to be distributed from brighter to darker areas. So, the visitor can experience a better contrast in refractors. In these telescopes refractors often are adduced as the premier instruments for a planetary and double star observation.
Low maintenance is the second advantage of refractors as lenses do not need recoating like mirrors do. Furthermore, the optical tube assembly of a refractor generally does not depend upon collimation. Lens is placed into the tube and usually does not get misaligned, unless it is dag up to some major trauma.
As the refractor is a closed-tube assembly, it may demand a extended amount of time to cool to ambient temperature. Presently used thin-walled aluminum tubes have decreased this period significantly, but cool-down time still should be accept into account.
2. Newtonian reflecting telescopes consists of two mirrors - a large primary mirror at the bottom of the tube and a small, flat secondary mirror near the top of the tube. Front- lights arrive into the tube, fall on the primary mirror, get reflected to the secondary mirror, and then it is mirrored again into the eyepiece.
These reflecting telescopes does not ached any chromatic aberration. Included mirrors bear only one optical surface, whereas an apochromatic lens has between four and eight causing it less expensive to develop.
Secondary mirror forms what is named a 'central obstruction' which results in unnatural distribution of light and loss of contrast in the image. In case to resolve this problem, a few manufacturers have prepared so- called planetary Newtonians, having smaller central obstructions (some as small as 16% of the aperture).
Important things to keep in mind when investing a telescope :
a. Know everything you can about telescopes for instance manufacturers' ads, catalogs, and especially read telescope reviews with customer score on a particular product. The Astronomy magazine will be a useful resource for both advertise and telescope reviews.
b. If a telescope is included with low- quality, high- power eyepiece, high (albeit empty) magnifications can be attained. So, it is meaningless to claim 500x magnification for telescopes. To make change of the magnification of a telescope, you need to change the eyepiece.
c. If you are serious about buying a telescope, consult with your nearest astronomy club where you can get all the assistance on understanding all feature of a particular.
d. General review says that the bigger the telescope, the better to get view. But, if you need often use of it, then choose smaller one as it is easy to set up a small refractor on a standard tripod rather than a large Newtonian reflector on a heavy mount.
And finally, always remember: a telescope is a useful tool- but only if you know what to perform with it !