Astronomical filter

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 it’s not unknown for them to 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."[2]

Solar filters are used to safely observe and photograph the Sun, which despite being white, may appear 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,[3] 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[1] invisible through common filters. These H-alpha filters are much narrower than those use for night H-alpha observing (see Nebular filters below), passing only 0.05 nm (0.5 angstrom) for one common model,[4] compared with 3 nm-12 nm or more for night filters. Due to the narrow bandpass and temperature shifts often telescopes like that are tunable within about a ±0.05 nm.

NASA included the following filters on the Solar Dynamics Observatory, of which only one is visible to human eyes (450.0 nm):[5] 450.0 nm, 170.0 nm, 160.0 nm, 33.5 nm, 30.4 nm, 19.3 nm, 21.1 nm, 17.1 nm, 13.1 nm, and 9.4 nm. These were chosen for temperature, instead of particular emission lines, as are many narrowband filters such as the H-alpha line mentioned above.

The three main spectral lines that narrow-band filters transmit

Narrow-band filters are astronomical filters which transmit only a narrow band of spectral lines from the spectrum (usually 22 nm bandwidth, or less). They are 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 two main types of Narrowband filters: Ultra-high contrast (UHC), and specific emission line(s) filters.

Specific emission line (or lines) filters are used to isolate line or lines of specific elements or molecules to allow for being able to see the distribution within Nebula. This is a common method to produce false color images. Common filters are often use for the Hubble Space Telescope, forming the so-called HST-pallet, with colors assigned as such: Red = S-II; Green = H-alpha; Blue = O-III. These filters will commonly be specified with a second figure in nm, which refers to how wide a band is passed, which may cause it to exclude or include other lines. For example, H-alpha at 656 nm, may pick up N-II (at 658–654 nm), some filters will block most of the N-II if they are 3 nm wide.[9]

Commonly used lines / filters are:

• H-Alpha Hα / Ha (656 nm) from the Balmer series is emitted by HII Regions and is one of the stronger sources.
• H-Beta Hβ / Hb (486 nm) from the Balmer series is visible from stronger sources.
• O-III (496 nm and 501 nm) filters allow for both of the Oxygen-III lines to pass through. This is strong in many Emission nebulae.
• S-II (672 nm) filters show the Sulfur-II line.

Less common lines/filters:

• He-II (468 nm)[10]
• He-I: (587 nm) [10]
• O-I: (630 nm) [10]
• Ar-III: (713 nm) [10]
• CA-II Ca-K/Ca-H: (393 and 396 nm)[11] For solar observing, shows the sun with the K and H Fraunhofer lines
• N-II (658 nm and 654 nm) Often included in wider H-alpha filters[9]
• Methane (889 nm)[12] allowing clouds to be seen on the gas giants, Venus and (with filter) the Sun.

Known commonly as UHC filters, these filters consist of things which allow multiple strong common emission lines to pass through, which also has the effect of the similar Light Pollution Reduction filters (see below) of blocking most light sources.

The UHC filters range from 484 to 506 nm.[6] It transmits both the O-III and H-beta spectral lines, blocks a large fraction of light pollution, and brings the details of planetary nebula and most of emission nebulae under a dark sky.[13]

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 allows observing nebulae from the city and light polluted skies.[1] These filters block the Sodium and Mercury vapor light, and also block natural skyglow such as the auroral light.[14] Broadband filters differ from narrowband with the range of wavelengths transmission. LED lighting is more broadband so it is not blocked, although white LEDs have a considerably lower output around 480 nm, which is close to O III and H-beta wavelength. Broadband filters have a wider range because a narrow 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.[6] These filters are particularly designed for nebulae observing, and not useful with other deep sky objects. However, they can improve the contrast between the DSOs and the background sky, which may clarify the image.