A filter is an optical part, glass or plastic, whose role is to change the nature of light rays passing through it, to improve the observations. It stops some wavelengths (colors) and thus absorb some of the light. Apart sunscreens that attach to the lens, and the diameter varies depending on the model, all other types of filters are screwed onto the eyepiece (50.8mm or 31.75mm).

Light ] The human eye can not perceive a small part of the electromagnetic spectrum visible light. It is an electromagnetic wave which is characterized by its wavelength. Visible light is itself composed of seven colors of the rainbow in the sky, each with its own wavelength. Its spectrum ranges from red (wavelength = 780 nm) to violet (380 nm) through orange, yellow, green, blue and indigo.
More a star is warmer, it releases energy and its wavelength is short. This explains why the hottest stars tend to blue or white, and cooler to the red.

Bandwidth ] spectrum that a filter allows to pass. Bandwidth is thus calculated by subtracting the two wavelengths which delimits. It is expressed in angstrom or nanometer (1 angstrom = 0.1 nm = 10 -10 m).

Bandwidth = Upper limit wavelength – low limit wavelength


For example, h-alpha filters have a bandwidth of less than 1 angstrom.

Transmission rate ] Percentage of light that passes the filter.

Night Observation


A neutral density filter (or gray filter) is a filter that reduces light in all wavelengths. It is commonly used to reduce the brightness of the moon, whose light, which is safe, becomes annoying during prolonged observations. That is why it is often called “lunar filter”. But more generally, this filter is used when a drop in brightness is needed to enhance the contrast of the image: planetary or solar observation (with a full aperture solar filter or telescope). The neutral filter has the main advantage of respecting the natural colors of the object observed.

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Polarizing filters allow you to lower the brightness of the observed objects. The polarizing filter has a polarization direction. Only waves whose direction of vibration is parallel to that of the filter can pass through. Damping increases image contrast and facilitates the observation of fine detail. the system is composed of two glass plates polarized very thin, superposed each in a rotatable carrier. The rotation of one element relative to the other changes the overall transmission of light by a factor of from 1% to 40%. They are used for lunar observation, planetary, solar (with full aperture solar telescope or filter) and also for the observation of the planets in the daytime (mercury, venus, mars, jupiter and saturn). By rotating the filter, there is a position where it will darken the sky and increasing the contrast between the planet and the latter. Finally, it also has an interest in earth observation, in particular by strong light in the summer or in the presence of snow.

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This type of filter is used in lunar and planetary observation. Tinted glass slides are designed to prevent the passage of the color complementary to that which it is desired to make. for example, if one wishes to better see the red spot of jupiter, use a blue filter to make contrast with the rest of the image. All these filters can beings used as visual photography, alone or combined. numbering is standardized and corresponds to a color and a transmission rate.

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Lighting cities deliver their light in a narrow wavelength window. The interference filters can mitigate this light pollution to better see the deep sky objects. They are also called nebulae and filters can be used for visual observation and astrophotography.

There are 2 main types of interference filters :
1) Broadband filter
As the name suggests, this filter has a wide bandwidth. This means that the amount of light passing through is still important. It is versatile and can increase the contrast of nebulae, galaxies and globular clusters, despite light pollution.

2) Narrowband filters
These filters are much more selective. Having a narrow bandwidth (10 to 20 nm), they eliminate a major portion of the light pollution. The sky seems so much more black and contrasts with the observed objects, which are essentially the planetary and diffuse nebulae.

The main types of narrowband interference filters:

► OIII filter : it only passes light of ionized oxygen three times, which provides a significant contrast gain on diffuse nebulae and planetary very low light.
► H-alpha filter (light emitted by ionized hydrogen) : This allows only the wavelengths greater than 650 nm, emitted by nebulae . the eye there is not sensitive enough, this filter is only for astrophotography, and allows you to take good quality pictures even in town.
(> Note : Do not confuse these with solar H-alpha filters)
► UHC filter : It isolates the oxygen lines III (496 and 501nm) and skate Hydrogen-beta (486nm) emitted by planetary nebulae and most emission nebulae.
► H-beta filter : It is a very narrow filter that isolates the H-beta hydrogen line (486nm). It is often the only way to observe some nebulae of very low intensity (E.g. Horsehead Nebula).
► Comet filter : Filter with very narrow bandwidth (25nm) that separates the wavelengths the OIII line (501nm) and the two lines cyan (RK 511nm and 514 nm). It contrasts the ionized gaseous comet tails that are so difficult to see.

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Solar Filters


+++ To observe the sun, it is strongly advised to never use of filters is screwed onto the eyepiece (sometimes provided with instruments for beginners) but only those that are placed at the entrance of the tube directly on the lens. Indeed, the major heat damage in the optical tube could burst the filter, remove any protection from the sun and the eye, and burn your retina.
+++ Use a filter having an optical density equal to or greater than 5 (see “optical density” below) for visual observation.
+++ Remember to undo the researcher or hide the front and rear.
+++ Never leave children unattended near the instrument.
+++ Before starting observation, always check that the filter is properly installed and has no bites or scratches. Indeed, a sunscreen is very fragile and has a limited lifespan. so it is better not to touch it with your fingers.

► Optical Density
The optical density of the filter is its ability to block the light. It is synonymous with “release rate”, but is used only for the neutral density filter, which absorbs light radiation in the same way regardless of wavelength. It can be written “ND” (neutral density).

Density 3 (or ND3) = 1/1000
Density 4 (or ND4) 1/10 000
Density 5 (or ND5) = 1/100 000
Density 6 (or ND6) = 1/1 000 000

>> The minimum density for visual observation of the sun is 5. It allows only 1/100 000th of the light. sunscreens with lower density are very dangerous to the eyes, and are only used in photography.

Full aperture filter ] It is not up to the eyepiece, but at the entrance of the objective and the same diameter as the latter. It prevents almost all of the sunlight to enter and the front open farm tube telescopes (eg newton). This prevents overheating of the inside of the instrument and minimizes internal turbulence.

Filter off-axis ] It lets light by a hole whose diameter is much lower than that of the tube, and is offset from celui- thereof. by reducing the diameter of the tube, the lens cover significantly reduces the light intensity and the heat that enters the telescope. However, the image of the sun does not appear darker so far and its scope is identical to that obtained with the full opening.


The metallic filter virtually no heat because it is reflecting the sun’s rays. It can be observed stains, faculaires beaches and solar granulation in the photosphere.

► In glass
It is composed of a glass disc fixed in a frame. after the polishing, the glass receives several thin metal layers of different types on its inner face, and then looks like a two-way mirror. The advantage of this type of filter is that it restores a white light, fairly close to the true color of the sun.
► In aluminised polyester
Flexible plastic film (polymer) coated with a thin layer of aluminum, which is cut at will, and thus adapts to any type of instrument (binoculars or telescope), regardless of its diameter. Depending on the model, the sun will take a bluish white or slightly orange. The latter is preferred by astronomers because it is that which is closest to reality. Depending on the product, the optical quality obtained can be very good, even superior to glass filters. However, the material is relatively fragile. Certain filter aluminized polymer are set in a metal barrel to prevent warping of the sheet, although this does not affect the quality of the image.


It is a flexible polymer sheet black, dyed, which unlike metallic filters is by light absorption. The color of sunlight transmitted by the filter is natural enough, but its optical quality is rather average.

>> The polyester filter is sold in roll or A4. He cut as easily as a sheet of paper and can protect any optical telephoto, telescope, telescope, finder, binoculars, camera, glasses, etc. Just fix it with adequate support, or a simple elastic.


For a more detailed observation of the sun, use special filters that let through only some “spectral lines”, that is to say, certain specific wavelengths. One of it is the h-alpha, which is emitted when the hydrogen is ionized, and has a wavelength of 656.3 nanometers. It is therefore in the red portion of the visible spectrum. The area of the sun that proves this wavelength is the chromosphere, bottom of what may be called the solar atmosphere. The chromosphere overlooks the photosphere region from which most of the sunlight we receive. This filter allows to isolate the emission line of ionized hydrogen and get rid of the glare of the photosphere. Many training courses that reflect the chromosphere of the activity are visible simultaneously: tasks, rash, filaments, spicules and prominences.

This solar interference filter passes only calcium k line of singly ionized (Ca-K) at 393.4 nm. This wavelength is used to display faculae, tasks and structure “orange peel” deeper levels of the chromosphere. The K line vision is more suited to photography.

For a safe observation with instruments of larger diameter, it is advisable to add an eye on ir cut filter and anti-UV. The baader planetarium establishments produce an interference filter (UV/IR-cut), which is perfect for this type of observation.

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