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Visual and photographic filters play a big role in how we see or capture the objects in the night sky. There is a filter to enhance every situation. Photographic color filters are an essential accessory to the astrophotographer, while visual color filters help to bring out amazing detail to the human eye. There are broadband filters for boosting contrast or reducing the effects of light pollution and narrowband filters for concentrating on very specific wavelengths of light, including those only our cameras can see. There is also a wide range of sizes available. The size you'll need is usually determined either by the size of your eyepiece barrel (typically 1.25" or 2") or the size required by your filter wheel, carousel, or slider. There are also specialized filters sizes that are made to clip into the body of your DSLR or attach directly to the back of your SCT! OPT has divided these filters among several categories by type and other search options to make them easier for you to find exactly what you need.
Solar Filters are a great way to view the sun without investing too much in a specialized telescope; you can simply find the correct solar filter for the telescope you already have, and keep using that scope you're comfortable with to view the sun. Solar Filters come in a number of varieties, each type resulting in a different solar astronomy experience. Solar filters also come in the same varieties as other filters; imaging and visual.
Many filters are specialized for imaging use, which ranges from narrowband filters meant for specific image types to LRGB filter sets for turning a monochrome image into a full color one. There is a very wide variety of different sizes and specialties among imaging specialized filters, ranging from various colors to high level research filters to simple light pollution negating filters. The choice is yours.
Some filters are meant purely for enhancing your visual astronomy experience, whether the goal is to minimize light pollution for a clearer view, or to add a color to your view. These are great for observers who want to see new and interesting things in the night sky, or who want to enhance their clarity by piercing light pollution. Just like imaging filters, there are a wide range of options.