Astronomy, visual and CCD, the planets of the solar system but also stellar objects are often very low contrast and collect their surface details or structures is not easy, you have to resort to "tricks". The filters are part of these devices, very useful and often used. Visible light is defined as a solid component which is the white light. This white light is however made up of different wavelengths which is the spectrum highlighted by Isaac Newton in the 18th century with the use of a prism or the beautiful nature phenomenon that is the rainbow. Depending on the application area, we will discover the power of each color or some neutral density filters commonly used.

Some filters to explore beyond the visible into the infrared, ultraviolet not stand using the eye as a detector but the CCD video camera whose detection range is much wider than human perception or isolate specific narrow frequency very visible as the h-alpha, the oxygen III, ... etc. By analyzing many amateurs images, the detection spectrum including visible light and the invisible area for an amateur seems to extend to 365 nm (near UV) to 1000 nm (IR). Current working an amateur spectrum but is not so large and is often limited strictly to visible light between 400 nm and 700 nm. These are the frequencies to which our eye is sensitive.

For planetary imaging, we will work with basic color filters to broadband for the visible or Interference filters centered on frequencies in the IR or UV. For the stellar nebulae and above, we can focus on three very narrow frequency in the visible light : The ionized hydrogen, H-alpha, the most common element in the universe radiating in the visible around 656 nm. The H-Beta radiating 486.1 nm in a 12 nm window.
The Oxygen-III, triply ionized Oxygen, beaming on the strip of 496 nm - 501 nm.

If one wishes to be even more specific, there is also the NII line frequency at 658.4 nm in only 3 nm and the IIC line at about 670 nm in 13 nm. The comet transmission frequency is the CII, two lines in emitting visible at 511 and 514 nm. These are the C2 molecules contained in the heads comets.

Color filters or neutral for visual observation

Special photographic filters for CCD

Filter

COMMENTS

Yellow (WK 12)
Valuation lunar shapes. Valuation orange and red forms bands and zones (Jupiter). Reduction of light from the blue and green areas which darken the seas, the Oasis brand and canals. Brightens the orange desert regions (Mars). Overall valuation forms (Saturn).

Orange (WK 21)
Significant increase in the contrast on the Moon (ideal color). Improved perception garlands and polar regions (Jupiter). Same effect on Mars as yellow, and more intensive. It improves the structure rings and polar regions (Saturn).

Blue (WK 38A)
Useful for the Great Red Spot (Jupiter). Increasing the contrast surface detail (Mars). Saturn improvement on the contrast of details between rings and areas. Valuation dark spots on Venus. Observation bright gas tails (comets).

Red (WK 25)
Ideal for studying blue clouds (Jupiter). Also ideal for the study of inferior planets, Mercury and Venus, which are almost never observed in a dark sky. Ideal on Mars for the diligent observation surface detail. Ideal filter for imaging CCD B&W.

Green (WK 58)
Ideal on the moon. Increased visibility of the Great Red Spot (Jupiter).

Polarizing filter
C1158
Reduction radiation and radiance. Ideal on planets and double stars.

All filters for CCD are dichroic. This means that these filters are able to isolate very precisely the frequency in each color. This is not the case with an ordinary filter as WRATTEN KODAK employees for simple visual exploration that do not block the frequencies at the extremities in the visible. These filters are natively opaque to infrared so they do not pollute the pictures (the CCD sensor is very sensitive to IR). Overall, these filters intending to photography, their production level is also very often much higher. Filters listed below are only for use coins CCD for frequencies on the immediate extremities in the visible occupied by UV (365-400 nm) and IR (700 to 1000 nm) or for cutting off the very effectively to isolate the visible :

Filter

COMMENTS

Infrared
Insulation filters the frequencies existing in IR 630 nm+, 700 nm+, 780 nm+, 800 nm+ and 1000 nm.

Ultraviolet
Insulation filter in existing UV frequency (365 nm).

UV/IR cut
Filter cutting all frequencies in infrared and ultraviolet leaving only the visible light reaching the CCD camera sensor. It can be considered as an L filter (Luminance).

Filters by visual observation and imaging

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OBSERVATION AND IMAGERY

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Arnaud FIOCRET
 

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