Telescope filters most commonly consist of a circular glass threaded ring that attaches to the bottom of the eyepiece, the glass filter can be coated with a large variety of options from colour dyes in gelatine to exotic metal oxides. Each type of coating modifies the behaviour of the glass and offers a variety of optical improvements and enhancements.

The filter model numbers / codes are called the Wratten Number, which are named after the founder of the first photography company and British inventor Frederic Wratten. The numbers are just a reference number however the letter indicates an increase in filter strength.

Below we have listed a few of the most popular filters used for astronomy viewing and photography.
Not all the Wratten numbers are shown.