Filters are an accessory used in astronomy with telescopes and/or instruments such as a CCD camera, to improve the observation of celestial objects and/or to facilitate the measurement of the number of photons of selected wavelengths of light. Filters of varying wavelength width or bandpass can be used.

A range of filter choices are available depending on the type of observation: for solar, lunar, planetary, stellar and nebula observations. Filter properties and types will change depending on the specific wavelengths or regions of the electromagnetic spectrum observed. Filters are usually made of glass, acrylic or other similar transparent material and then coated as required. Filters can be placed in different locations of the optical path but typically just before the eyepiece lens or detector.

Filters are usually designated narrowband, intermediate or broadband depending on their width. The width can be designated by Full Width Half Maximum (FWHM) which is the wavelength/filter width at the transmission point that is 50% of the peak transmission.

If the range of permitted wavelengths is narrow the filter is known as narrowband (FWHM approx. 5 nm). Narrowband filters are typically designed to detect a single spectral line. e.g. [OIII] at 500 nm, H-alpha at 656 nm. If the filter transmits a broader spectrum of radiation then it is called a broadband filter (FWHM approx. 100-150 nm), e.g. the photometric Johnson-Cousins UBVRI system.

Near-IR broadband filters include the JHKLM system, with FWHM approx. 1 micron.

Other filter systems include optical: Stromgren (uvby), Sloan/SDSS (ugriz); near-IR (various variations on JHKLM typically due to atmospheric behaviour at specific observatories).

Some filters are able to reduce the intensity of the light across a broad range of wavelengths of light. These are sometimes called neutral density filters.

There are also specific filters for solar observing in which a filter may block around 99.999% of the light that falls upon it. Extreme care must be taken when using solar filters, or when viewing eclipses. Solar filters are used to protect telescopes and instrumentation from intense sunlight and importantly ensure that you don’t view the Sun through an unfiltered instrument. Solar filters must be securely attached to the front of telescopes, binoculars, or camera lens. There are three types of solar filters: metal on glass, aluminised polyester film (also known as aluminised Mylar), and black polymer. Some render the Sun white, while others impart a yellow, orange, or bluish tint. Solar filters should be purchased from reputable suppliers.

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