Telescopes have the ability to attach filters for many different purposes – some of the most common filters are for looking at the Moon, Planets and Sun. Eyepiece filters are an invaluable aid in lunar and planetary observing. They reduce glare and light scattering, increase contrast through selective filtration, increase definition and resolution, reduce irradiation and lessen eye fatigue.
A Moon Filter will thread directly onto the bottom of your eyepiece. Nearly all eyepieces are threaded for filters. Think of a Moon Filter like sunglasses for your telescope. Moon Filters will cut down glare and bring out much more surface detail and give you better contrast.
Astronomical filters work by blocking out certain colours in the visible spectrum of light. A red filter, for example, will block out all but the red wavelength of light. If you look at an object that is primarily red while using a red filter, the object will appear very bright. Areas which are not red will appear more clearly because they contrast with the wavelength of light which is being passed by the filter.
There are two types of solar filters: white light and hydrogen-alpha. White light solar filters are simply very dark neutral density filters. These allow you to see sunspots on the surface of the sun and are ideal for viewing solar eclipses and transits of Mercury or Venus. Hydrogen-alpha filters are narrowband filters. These allow you to view solar flares, prominences, and other associated activity. Unlike other types of filters for astronomical observing, solar filters fit over the front of the telescope, completely covering the aperture. This is to protect not only the observer’s eyes but also the optics themselves. The telescope’s optics would become very rapidly heated and potentially damaged without the filter in place.