The main function of a telescope is to gather light, magnify it and focus it so that you can see more than you can with your naked eye. Telescopes have been around for centuries and although great improvements have been made in the quality of the telescope optics (and in how they perform) and in the electronics and software available, the telescope optics have remained virtually the same over the years. There are really four main things to know about your telescope optics. These are the aperture, the magnification, the focal length and the focal ratio.
Probably the most important telescope optics feature is the aperture diameter. This is related to the size of the lens (for a refracting telescope) or the mirror (for a reflecting telescope) and gives you the ability to focus the light gathered. In general, you should try to buy as much aperture as you can afford as this will give you the sharpest image. However, bigger telescope optics mean a bigger telescope so you have to consider just how big and heavy the telescope will be if you will be needing to transport it to an area with less background light. Usually a 3 inch (80 mm) aperture is considered good for a refractor telescope and a 4 to 8 inch (100 to 200 mm) aperture for a reflector telescope is about right.
Most people think that magnification in the thousands is necessary for telescope optics, but this is not the case. All the magnification in the world will do you no good if your image is not sharp and that depends on the amount of light you can gather and how you focus it. Usually it is desirable to be able to have about 40X to 60X magnification per inch of aperture. It is also nice to get a telescope that either has an adjustable eyepiece or one that has interchangeable eyepieces so that you can change the magnification.
Focal length is defined as the distance from the optical center of a lens (or mirror) to its point of focus. Since focal length is a linear measurement, a conventional telescope must be at least as long as its focal length. This is not the case in some compound telescopes as they have folded light paths and can be in a much shorter tube. Basically, a telescope with a shorter focal length will be shorter and have a wider field of view. One with a longer focal length will be bigger and have a narrower field of view and will usually be lower in price.
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 found by dividing the focal length by the aperture size. This is usually called 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 image. Fast ratios (f/6 and below) are best for deep space viewing while slower ratios (f/10 or higher) are best for looking at lunar features or planetary viewing. A good all-around focal ratio is about f/8.
These are the most important telescope optics to consider when shopping for a home telescope or even one for the biggest observatory telescopes as well. Although it always seems that bigger is better, remember that your telescope will probably not be mounted permanently unless you live on the top of a mountain somewhere. You will have to cart it around with you and you want that to be a pleasant experience - not a chore. So you will have to make a few decisions about what is most important for your individual astronomical viewing experience. Whatever you decide we wish you happy viewing.