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Filters
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Wratten numbers are a labelling system for optical filters, usually for photographic use.
They are named for the man who founded the first company, Frederick Wratten, a British inventor. Wratten and partner C. E. K. Mees sold their company to Eastman Kodak in 1912, and Kodak continued to produce Wratten filters for decades. Even now, as of 2006, Wratten filters are still produced by Kodak, and sold under license through the Tiffen corporation.
Wratten filters are very much an active part of observational astronomy.

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ColourRecommended minimum apertureRecommended for use on.
#8 Light Yellow(83% transmission):50mm ApertureUseful in observing red and orange-coloured phenomena in the belts of Jupiter and in enhancing the level of observable detail of small orange-red zonal features within the belts of the planet. Also results in improved resolution of detail on Uranus and Neptune in instruments of 10" aperture and larger. A popular filter for the enhancement of features of the Moon, particularly in telescopes of 8" aperture and smaller.
#11 Yellow-Green(78% transmission):60mmApertureContrasts well with the red and blue characteristics of surface features on Jupiter and Saturn. Darkens the maria visible on Mars and improves visible detail on Uranus and Neptune in larger telescopes.
#12 Yellow(74% transmission):80mm ApertureContrast strongly with blue-coloured features on Jupiter and Saturn, while enhancing red and orange features. Lightens red-orange features of Mars, while reducing or blocking the transmission and thereby increasing the contrast of blue-green areas. Useful in increasing the contrast of lunar features in telescopes 6" aperture or larger. Yellow filters can also help tame some of the colour excess seen in short focal ratio achromats.
#15A Dark Yellow120mm ApertureSimilar to yellow, suitable for bigger aperture telescopes.Removes false colourin Fraunhofer refractors.
#21 Orange(46% transmission):80mm ApertureReduces or blocks transmission of blue-green wavelengths. Use on Jupiter or Saturn to enhance detail in the belts and polar regions. Sharpens boundaries between yellow-orange areas and blue-green regions on Mars, resulting in a darkening of edge-detail in the maria.
#23A Light Red(25% transmission):60mm ApertureOn telescopes of 6" aperture and larger the #23A filter does approximately the same functions as the #21 filter, but with stronger contrast and enhancement of marginally defined blue-green surface detail. Useful primarily on Jupiter, Saturn and Mars. Increases contrast between Mercury and bright blue sky during daylight observations or during twilight.
#25A Red(14% transmission): (3 stops)80mm ApertureThe #25A filter strongly blocks the transmission of blue and blue-green wavelengths, resulting in a very sharply defined contrast between blue-tinted cloud formations on Jupiter and the lighter-toned features on the disc. Also useful for delineation of the Martian polar ice caps and maria. Because of its relatively low total light transmission, the #25A should be employed on telescopes of 8" aperture or larger.
Dark Red #29120mm ApertureSimilar to light red, suitable for telescopes of 150mm aperture and larger. This filter will make for an extreme contrast.
#82A Light Blue(73% transmission): (1/3 stops)50mm ApertureUseful on the Moon, Mars, Jupiter and Saturn, this subtle pale blue filter enhances areas of low contrast while avoiding significant reduction of overall image brightness. It is also reportedly used with success on deep-sky objects such as spiral galaxies to enhance details in spiral structure.
#80A Blue(30% transmission): (2-stops)70mm Aperture

The most popular filter for the study of Jupiter and Saturn. 3200 K to 5500 K. Enhances contrast of rills and festoons in Jupiter's cloud belts as well as details of the Red Spot. Brings out detail in Saturn's belts and polar phenomena. Very useful as a contrast-enhancing lunar filter.

#80B Blue: (1-2/3 stops)70mm Aperture

Similar to 80A; 3400 K to 5500 K.

#80C Blue: (1-stop)70mm Aperture

Similar to 80A; 3800 K to 5500 K.

#38A Dark Blue(17% transmission):100mm ApertureA popular filter for study of Jupiter's disc, owing to the filter's strong rejection of red and orange wavelengths. Increases contrast between belt structures and enhances detail of the Red Spot. Also useful for study of isolated phenomena, such as dust storms on Mars as well as the belt structure of Saturn. Increases contrast of subtle cloud markings on Venus.
#47 Violet(3% transmission):120mm Aperture

Strongly rejects red, yellow and green wavelengths, useful for the study of Martian polar cap regions and for the observation of occasional phenomena in the upper atmosphere of Venus. Enhances contrast between the rings of Saturn. Use only on telescopes of 8" aperture or larger.

#56 Light Green(53% transmission):60mm ApertureExcellent for the observation of Martian polar ice caps as well as yellow-tinted dust storms on the Martian surface. Increase contrast of red and blue regions in Jupiter's atmosphere. Also useful for enhancing lunar detail.
#58 Green(24% transmission):120mm ApertureUse on telescopes of 8" aperture or larger to reject blue- and red-toned structures on the surface of Jupiter and thereby increase their contrast relative to lighter parts of the disc. Also useful for the enhancement of Saturn's cloud belts and polar regions. Strongly increases contrast of Mars's polar ice caps and increases contrast of atmospheric phenomena on Venus. Can also be used to enhance details of Sun when observed in white (not H-Alpha) light (Note, this filter does not replace a solar filter or Herschel-prism).
#ND96:Neutral density (13% transmission):50mm ApertureVery useful on Moon to reduce glare and also for splitting close binary stars. Reduces all wavelengths to an acceptable level, recommended for telescopes with larger apertures. Stack two to get an even lower transmission.




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Wratten #8: Light Yellow Colour Filter

5.50

Large 26mm in clear aperture
Manufactured from the purest crown optical glass
Dyed-in-the-mass (not "colour coated")
Each filter is mounted in a machined aluminium cell with the Wratten No. of the filter marked on the side of the cell.
Filters may be piggybacked; or stacked; to achieve selective filtration of the visual colour spectrum.


For visual applications Astronomica Colour filters thread into the barrels of virtually all other 1.25" eyepieces. Each filter is packed in a foam-fitted plastic case for secure long-term storage.
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Achromatic Filters
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A Yellow, or Light-Yellow, filter will reduce an achromatic refractor's false colour (violet fringes mostly noticeable on bright or high contrast objects) and give crisper views.
The yellow filter will only allow yellow light to pass through it and is ideal for planetary and double star work. The filter will enhance contrast on the moon, and reveal detail on Mars, Jupiter (e.g. cloud structure and the Great Red Spot) and Saturn.

Light Yellow (Wratten #8) Filter has a 83% transmission rate
Yellow (Wratten #12, K2, Y48) Filter has a 73% transmission rate
Yellow/Green (Wratten #11) Filter


Moon through telescope - Lumicon No. 12 Deep Yellow Filter



TS-Optics 1.25" colour filter for telescopes - colour: Yellow #12 - ideal for planetary observing

12.99

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Also, a strong UV filter, such as a Haze-2A or UV17, cuts off some visible light in the violet part of the spectrum, and so has a pale yellow colour; these strong filters are more effective at cutting haze, and can reduce purple fringing in digital cameras.

Source

A tip is to fit the filter onto the front of the Star Diagonal (if it is threaded) so you can use multiple eyepieces in one session with just one filter.

An alternative solution for fast refractors with noticeable colour aberration is to effectively increase the f-ratio by stopping it down with a mask in front of the objective lens (f8 or greater will eliminate the false colour to unnoticeable amounts). It should be noted that this has the negative affect of reducing the resolution of the telescope.



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Filters
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Using Colour Filters With Telescopes
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Light Pollution Filters
On the beginning of summer 2008 I figured out that the situation from the point of light pollution from my backyard is so bad that I need the solution. On the web I found some filters that suppress the light pollution but no one good review of them.
In the end I decided for IDAS LPS filter of Hutec.

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Lunar and Planetary Filters
To bring out contrast in the clouds of Jupiter and Saturn, or see fine detail on the surface of Mars, you should consider a set of inexpensive coloured-glass planetary filters that thread into your eyepieces. Here's a quick guide to choosing a few filters that will help you get the most out of your telescope.

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Lunar and Planetary Filters
Planetary filters work by blocking a specific part of the colour spectrum, enhancing detail in other colours. Use of a deep yellow or light red filter on the planet Mars will enhance visible details in other colours. The milder neutral density filters are sometimes used with large telescopes to reduce glare. Neutral density filters are used to reduce the brightness on objects such as the moon. We recommend a lunar or neutral density filter be used for lunar observing with any 4" or larger telescope. Choose the 25% transmission filters for telescopes up to 5" in aperture, or the 13% transmission filters for telescopes of 6" or larger aperture.

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Lunar and Planetary Filters
The Lumicon Colour and Neutral Density Filters are made from renowned Schott and Hoya optical glass and allow for maximum contrast on viewing planetary and lunar detail. Individually precision ground, highly polished with maximum light transmission coatings on both sides, these filters are 100% guaranteed for life.

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