The most important function of a telescope is to collect starlight, magnify it and concentrate it so that you can see better than it is possible to using only the naked eye. Telescopes have been around for hundreds of years and although good improvements have been produced in the high quality of the telescope optics (and in how they perform) and in the electronics and software available, the telescope optics have remained nearly the same since the 1600’s. There are actually only 4 major issues to understand regarding your telescope optics. These are the aperture, the magnification, the focal length and the focal ratio.

Aperture Diameter

In all probability the primary telescope optics characteristic is the aperture diameter. This is related to the size of the lens or the mirror and offers you the potential to concentrate the starlight gathered. Ideally, you should try to buy as much aperture as you can manage as this definitely will give you the sharpest picture. Even so, larger telescope optics mean a larger telescope so you do have to consider just how big and hefty the telescope will be if you will have to carry it to a location with less interference light. Generally a 3 inch (80 mm) aperture is thought ofas good for the refractor telescope and a 4 to 8 inch (100 to 200 mm) aperture for the reflector telescope is just about right.

Magnification

A lot of people think that magnification in the tens of thousands is vital for telescope optics to work, but this is not the case. All the magnification in the universe will not benefit you if the image is not sharp and that depends on the overall amount of starlight one can collect and how you concentrate it. Usually it is necessary to to have approximately 40X to 60X magnification per inch of aperture diameter. It is additionally nice to get a telescope that either has an adjustable eyepiece or one that has interchangeable eyepieces so that you are able to tweak the magnification.

Focal Length

Focal length is defined as the distance from the optical center of a lens (or mirror) to its point of focus. Because focal length is a linear measurement, a standard telescope has to be at minimum as long as its focal length. This is not the case in the compound telescopes as they have folded light paths and may be in a considerably shorter tube. Basically, a telescope with a reduced focal length will be shorter and have a bigger field of view.

Focal Ratio

Focal ratio is the feature of telescope optics that refers to the “speed”, or the brightness and field of view of the telescope and is identified by dividing the focal length by the aperture size. This is commonly referred to as an “f-stop” and is expressed as f/#. A telescope using a focal length of 480 mm and an aperture of 80 mm would be an f/6 ratio. A smaller focal ratio means less magnification, bigger field of view and a brighter image. Fast ratios (f/6 and under) are ideal for deep space viewing while slower ratios (f/10 or better) are best for looking at the moon’s features or planetary viewing. A really good all-around focal ratio is right about f/8.

These are the prime aspects of  telescope optics to consider when considering a home telescope. Although it often seems that bigger is better, bear in mind that the telescope will need to be carried along with you and you want that to be an enjoyable experience – not a pain.

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