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The major function of a telescope is to gather starlight, magnify it and focus it so that you are able see more than you can with only your naked eye. Telescopes have been used for hundreds of years and though stellar enhancements have been produced in the high quality of the telescope optics (and in how they work) and in the electronic devices and software available, the telescope optics have remained nearly the same over the years. There are actually only four principal issues to understand about the telescope optics. These are the aperture, the magnification, the focal length and the focal ratio.
Possibly the prime telescope optics feature is the aperture diameter. This is linked to the size of the lens or the mirror and provides you the ability to focus the starlight gathered. Ideally, you’d like to try to buy as much aperture as you can pay for as this definitely will give you the sharpest picture. Having said that, larger telescope optics mean a larger telescope so you do have to take into account just how big and heavy the telescope will be if you will need to transport it to a location with a lot less interference light. Normally a 3 inch (80 mm) aperture is regarded as good for the refractor telescope and a 4 to 8 inch (100 to 200 mm) aperture for the reflector telescope is about right.
Most individuals feel that magnification in the tens of thousands is necessary for telescope optics to work, but this is not true. All the magnification in the universe will not do you any good if your picture is not sharp and that depends on the volume of light you can gather and how you concentrate it. Normally it is necessary to to have approximately 40X to 60X magnification per inch of aperture. It is additionally nice to get a telescope that either has an adjustable eyepiece or one that has interchangeable eyepieces so you are able to tweak the magnification.
Focal length is described as the distance from the middle of a lens (or mirror) to its point of focus. Because focal length is a linear measurement, a typical telescope must be at minimum as long as its focal length. This is not the true in some compound telescopes as they have folded light paths and will be in a considerably smaller tube. Essentially, a telescope with a smaller focal length should be smaller and have a broader view field.
Focal ratio is the attribute of telescope optics that refers to the "speed", or the brightness and field of view of the telescope and is found by dividing the focal length by the aperture diameter. This is generally referred to as an “f-stop” and is expressed as f/#. A telescope with a focal length of 480 mm and an aperture of 80 mm would be an f/6 ratio. A small focal ratio means less magnification, wider field of view and a brighter picture. Fast ratios (f/6 and under) are ideal for deep space viewing and slower ratios (f/10 or higher) are best for viewing the moon’s features or planetary viewing. A good all-around focal ratio is about f/8.
These are the most significant aspects of telescope optics to consider when considering a residential telescope. Though it often appears that bigger is better, bear in mind that the telescope will need to be carted along with you and you want that to be an enjoyable experience - not a chore.