Astronomical filter

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. They are usually made from a durable glass or a plastic film which transmits 1/100,000th of the light. Film filters are used over the aperture of the telescope and do not heat up significantly, whereas glass filters are used near the objective end causing them to heat up greatly and it is not unknown for them to shatter from thermal shock. Glass solar filters are therefore not recommended by most experts and some stockists refuse to sell them or remove them from telescope packages. "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."

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Wikipedia