An astronomical filter is a telescope accessory used by amateur astronomers to simply enhance the details of celestial objects (much as with amateur photography). By contrast professional astronomers rigorously use filters on telescopes in order to understand the astrophysics (such as stellar classification and placement of a celestial body on its Wien Curve), occurring for the object in a given bandpass via photometry.
Most astronomical filters work by blocking a specific part of the color spectrum above and below a bandpass, significantly increasing the signal to noise of the interesting wavelengths, and so making the object gain detail and contrast. While the color filters transmit certain colors from the spectrum and are usually used for observation of the planets and the Moon, the polarizing filters work by adjusting the brightness, and are usually used for the Moon. The broadband and narrowband filters transmit the wavelengths that are emitted by the nebulae (by the Hydrogen and Oxygen atoms), and are frequently used for reducing light pollution.
Solar filters block most of the sunlight to avoid any damage to the eyes. Proper filters are usually made from a durable glass or polymer film that transmits only 0.00001% of the light. For safety, solar filters must be securely fitted over the objective of a refracting telescope or aperture of a reflecting telescope so that the body does not heat up significantly.
Small solar filters threaded behind eyepieces do not block the radiation entering the scope body, causing the telescope to heat up greatly and not being unknown that they may shatter from thermal shock. Therefore, most experts do not recommend such solar filters for eyepieces, and some stockists refuse to sell them or remove them from telescope packages. According to NASA: "Solar filters designed to thread into eyepieces that are often provided with inexpensive telescopes are also unsafe. These glass filters can crack unexpectedly from overheating when the telescope is pointed at the Sun, and retinal damage can occur faster than the observer can move the eye from the eyepiece."
Solar filters are used to safely observe and photograph the Sun, which often appears as a yellow-orange disk. A telescope with these filters attached can directly and properly view details of solar features, especially sunspots and granulation on the surface, as well as solar eclipses and transits of the inferior planets Mercury and Venus across the solar disk. The Herschel Wedge is a prism-based device combined with a neutral-density filter that directs most of the heat and ultraviolet rays out of the telescope, generally giving better results than most filter types. The H-alpha filter transmits the H-alpha spectral line for viewing solar flares and prominences invisible through common filters.
Color filters work by absorption/transmission, and can tell which part of the spectrum they are reflecting and transmitting. Filters can be used to increase contrast and enhance the details of the Moon and planets. All of the visible spectrum colors each have a filter, and every color filter is used to bring a certain lunar and planetary feature; for example, the #8 yellow filter is used to show Mars's maria and Jupiter's belts. The Wratten system is the standard number system used to refer to the color filter types. It was first manufactured by Kodak in 1909.
Professional filters are also colored, but their bandpass centers are placed around other midpoints (such as in the UBVRI and Cousins systems).
Some of common color filters and their uses are:
Neutral density filters, also known in astronomy as Moon filters, are another approach for contrast enhancement and glare reduction. They work simply by blocking some of the object's light to enhance the contrast. Neutral density filters are mainly used in traditional photography, but are used in astronomy to enhance lunar and planetary observations.
Polarizing filters adjust the brightness of images to a better level for observing, but much less so than solar filters. With these types of filter, the range of transmission varies from 3% to 40%. They are usually used for the observation of the Moon, but may also be used for planetary observation. They consist of two polarizing layers in a rotating aluminum cell, which changes the amount of transmission of the filter by rotating them. This reduction in brightness and improvement in contrast can reveal the lunar surface features and details, especially when it is near full. Polarizing filters should not be used in place of solar filters designed specially for observing the sun.
Narrowband filters are astronomical filters which transmit only a narrow band of spectral lines from the spectrum (usually 22 nm or less). It is mainly used for nebulae observation. Emission nebulae mainly radiate the doubly ionized oxygen in the visible spectrum, which emits near 500 nm wavelength. These nebulae also radiate weakly at 486 nm, the Hydrogen-beta line. There are three main types of Narrowband filters: Ultra-high contrast (UHC), Oxygen-III & Hydrogen-beta, and Hydrogen-alpha, the narrowest of the three filters with 8 nm range. The UHC filters range from 484 to 506 nm. It transmits both the O-III and H-beta spectral lines, blocks a large fraction of light pollution, and brings the details of planetary nebulae and most of emission nebulae under a dark sky.
The broadband, or light pollution reduction (LPR), filters are nebular filters that block the light pollution in the sky and transmit the H-alpha, H-beta, and O III spectral lines, which makes observing nebulae from the city and light polluted skies possible. These filters block the Sodium and Mercury vapor light, and also block the natural skyglow such as the auroral light. The broadband filters differ from the narrowband with the range of wavelengths transmission. LED lighting is more broadband so this is not blocked although white LEDs have from themselves a considerably lower output around 480 nm which is close to O III and H-beta wavelength. The broadband filters have a wider range because the narrower transmission range causes a fainter image of sky objects, and since the work of these filters is revealing the details of nebulae from light polluted skies, it has a wider transmission for more brightness. These filters are particularly designed for nebulae observing, are not useful with other deep sky objects. However, it can improve the contrast between the DSOs and the background sky, which may clarify the image.