Anastronomical filter is atelescope accessory consisting of anoptical filter used byamateur astronomers to improve the details and contrast ofcelestial objects, either for viewing or for photography.Research astronomers, on the other hand, usevarious band-passfilters forphotometry on telescopes, in order to obtain measurements which reveal objects'astrophysical properties, such asstellar classification and placement of acelestial body on itsWien curve.
Most astronomical filters work by blocking a specific part of the color spectrum above and below abandpass, significantly increasing thesignal-to-noise ratio 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 theplanets and theMoon, the polarizing filters work by adjusting the brightness, and are usually used for the Moon. The broad-band and narrow-band filters transmit the wavelengths that are emitted by thenebulae (by thehydrogen andoxygen atoms), and are frequently used for reducing the effects oflight pollution.[1]
Filters have been used in astronomy at least since thesolar eclipse of May 12, 1706.[2]
Solar filters block most of thesunlight to avoid any damage to the eyes. Proper filters are usually made from a durable glass orpolymer film that transmits only 0.00001% of the light. For safety, solar filters must be securely fitted over theobjective of arefracting telescope oraperture of areflecting telescope so that the body does not heat up significantly.
Small solar filters threaded behindeyepieces do not block the radiation entering the scope body, causing thetelescope to heat up greatly, and it is not unknown for them to shatter fromthermal shock. Therefore, most experts do not recommend such solar filters for eyepieces, and some stockists refuse to sell them and remove them from telescope packages. According toNASA: "Solar filters designed to thread into eyepieces that are often provided with inexpensivetelescopes are also unsafe. These glass filters can crack unexpectedly from overheating when the telescope is pointed at the Sun, andretinal damage can occur faster than the observer can move the eye from the eyepiece."[3]
Solar filters are used to safely observe and photograph theSun, which despite being white, may appear as a yellow-orange disk. Atelescope with these filters attached can directly and properly view details of solar features, especiallysunspots andgranulation on thesurface,[4] as well assolar eclipses andtransits of theinferior planetsMercury andVenus across the solar disk.
TheHerschel wedge is a prism-based device combined with aneutral-density filter that directs most of the heat andultraviolet rays out of the telescope, generally giving better results than most filter types. TheH-alpha filter transmits the H-alphaspectral line for viewingsolar flares andprominences[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,[5] 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 theSolar Dynamics Observatory, of which only one is visible to human eyes (450.0 nm):[6] 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.
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 showMars's maria andJupiter's belts.[7]The Wratten system is the standard number system used to refer to the color filter types. It was first manufactured byKodak in 1909.[1]
Professional filters are also colored, but their bandpass centers are placed around other midpoints (such as in theUBVRI and Cousins systems).
Some of common color filters and their uses are:[8]
Neutral density filters, also known in astronomy as Moon filters, are another approach for contrast enhancement andglare 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,[1] but may also be used for planetary observation. They consist of two polarizing layers in a rotatingaluminum cell,[9] 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.
Narrow-band filters are astronomical filters which transmit only a narrow band ofspectral lines from the spectrum (usually 22 nm bandwidth, or less). They are mainly used fornebulae observation.Emission nebulae mainly radiate the doublyionized oxygen in thevisible spectrum, which emits near 500 nm wavelength. These nebulae also radiate weakly at 486 nm, theHydrogen-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 lines of specific elements or molecules to see their distribution within Nebulae. By combining the images from different filters they may also be used to producefalse color images. Common filters are often used with theHubble Space Telescope, forming the so-called HST-palette, with colors assigned as such: Red = S-II; Green = H-alpha; Blue = O-III. These filters are commonly specified with a second figure innm, 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.[10]
Commonly used lines / filters are:
Less common lines/filters:
Known commonly asUHC filters, these filters consist of things which allow multiple strong common emission lines to pass through, which also has the effect of the similarLight Pollution Reduction filters (see below) of blocking most light sources.
The UHC filters range from 484 to 506 nm.[7] It transmits both the O-III and H-beta spectral lines, blocks a large fraction of light pollution, and brings the details ofplanetary nebula and most of emission nebulae under a dark sky.[14]
The broadband, or light pollution reduction (LPR), filters are designed to block thesodium andmercury vapor light, and also block naturalskyglow such as theauroral light.[15] This allows observing nebulae from the city and light polluted skies.[1] 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.[7] These filters are particularly designed for galaxy observation and photography, and not useful with otherdeep sky objects such as emission nebulae. However, they can still improve the contrast between the DSOs and the background sky, which may clarify the image.