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Optical Filter Guide - AstroShop

Eyepiece Filters and how they improve viewing - Go to Quick Guide
The following guide will help you in determining which basic filter set may be best suited to your needs.

It should be noted that for small aperture instruments, filters with higher light transmission characteristics will be better suited so as not to darken the view too greatly. A sometimes unavoidable side affect is that many filters can alter trueness of colours and this should be taken into account when using them. In other words, it is the intended resulting detail a filter reveals when passing or blocking specific wavelengths that is important.    

Click Here to see our overall selection of astronomical filters.

Objects Examples Best Filter for Viewing
Stars & Star Clusters M13, M11 None
Diffuse Nebulae Lagoon, Swan Baader or ProStar OIII (light polluted sky) CLS or UHC (dark sky)
Planetary Nebulae Dumbbell, Ring Baader or ProStar OIII (light polluted sky) CLS or UHC (dark sky)
Faint Planetary Nebulae NGC 7293, Abell 33, Jones 1 Baader or ProStar OIII
Reflection Nebulae Pleiades, Trifid, M78 None
Spiral Galaxies M33, M101 None
Faint Nebulae Veil, Rosette, N. American OIII (light polluted sky) UHC (dark sky) CLS
Extremely Faint Nebulae California, Horsehead H-Beta

The Deep-Sky Hydrogen-Beta , Oxygen III , and Ultra High Contrast Filters are the result of 20 years of steady design improvements, and continue to deliver the highest performance of all anti-light pollution filters obtainable today. The following information recommends which filter to use on which celestial objects, and explains how filter transmissions differ.


#8 Light Yellow (83% transmission) Used to enhance detail in red and orange features in the belts of Jupiter. Is useful for increasing mare contrast on Mars and improving disc resolution of Uranus and Neptune with larger telescopes. Can be used to enhance lunar detail also.

#11 Yellow-Green (78% transmission) Excellent for bringing out surface details on Jupiter.
Darkens the mare regions on Mars.

#12 Yellow (74% transmission) Enhances red and orange features of Jupiter and Saturn, while blocking blue and green wavelengths. It also lightens red and orange features on Mars, while reducing, the transmission of blue and green areas. This filter increases the contrast between the two. Also enhances the blue clouds in the Martian atmosphere. Can also be used to increase contrast in lunar features with telescopes of 150mm aperture and larger.

#21 Orange (46% transmission) Reduces transmission of blue and green wavelengths, thus increasing the contrast between these areas and red or yellow or orange areas. Works very well with Mars. Sharpens the boundaries between these areas on the planet's surface and can sharpen the contrast in the belts of Jupiter also enhancing the Great Red Spot. Has slightly more contrast than the Wratten #15.

#23A Light Red (25% transmission) A popular filter for use when observing Mars, Jupiter, and Saturn. Due to the lower light transmission of this filter, it is best used with telescopes larger than 130mm aperture. A red almost dark orange filter, it performs many of the same functions as the #21 and the #15 but with somewhat more contrast. Can be a useful filter for daylight observations of Mercury and Venus. improving contrast between these planets and the pale blue sky.

#25 Red (14% transmission) The #25 and #25A filter strongly blocks the transmission of blue and green wavelengths which results in very sharply defined contrast between the cloud formations and the lighter hue features of Jupiter. Also quite useful for improving definition of the Martian polar ice caps and darker mare surface regions.

#38A Dark Blue (17% transmission) Very good for use on Jupiter because it strongly rejects red and orange wavelengths in the belts and in the Great Red Spot, thus increasing contrast. It works well on Martian surface phenomena, like dust storms, and increases contrast in the rings of Saturn. Most suitable for Venus given its low light transmission and readily increases contrast of subtle cloud markings.

#47 Violet (3% transmission) Strongly rejects red, yellow, and green wavelengths, making it suitable to use on the Martian polar ice caps. It is the best filter for observations of Venus due to its low light transmission and ability to enhance upper atmosphere phenomena. Also can be used for enhancing lunar detail. Presents interesting views of Jupiter and its satellites.

#56 Light Green (53% transmission) Very good for observing Martian polar ice caps and yellow tinted dust storms on the planet's surface. Also increases the contrast of the red and blue regions in Jupiter's atmosphere and cloud belts.

#58 Green (24% transmission) Strongly rejects red and blue wavelengths increasing their contrast on the lighter regions of Jupiter's cloud tops. Useful for enhancing the cloud belts and polar regions of Saturn. Great for increasing contrast in polar ice caps of Mars and does a reasonable job at improving contrast of atmospheric features on Venus.

#80A Blue (30% transmission) A magnificent all-round filter. Perhaps the best economical filter for studying planetary detail of Jupiter and Saturn. Enhances contrast of festoons and other disturbances in Jupiter's cloud belts, along with detail of the Great Red Spot. Works great with Saturn revealing terrific detail in its belts and polar features. Can be useful for lunar observations.

#82A Light Blue (73% transmission) Another outstanding performer like the #80A and works well with Jupiter, Mars, Saturn and the Moon. Its pale blue colour enhances low contrast areas and avoids significant reduction of overall light level at the same time.

ND96 Neutral Density (0.9 density, 13% transmission) The neutral density filter transmits light uniformly across the entire visible spectrum making it an excellent tool for glare reduction and studying bright localised lunar surface regions around times of full Moon. Suitable also to improve contrast when splitting difficult double stars.


Deep Sky UHC & CLS filters

Intended for viewing nebulae from light-polluted skies.
Blocks all mercury vapour and high & low pressure sodium vapour lamp light, neon lights and airglow, while transmitting the rest of the visible spectrum. The best all-around visual light pollution filter for use in urban skies.
This filter also provides high-contrast views of the Martian polar caps

Ultra High Contrast Filters

Baader and ProStar narrow band pass filter isolates the two doubly ionized oxygen lines (496 and 501nm) and the hydrogen-beta line (486nm) emitted by planetary and most emission nebulae. Provides superb views of the Orion, Lagoon, Swan and other extended nebulae. The best all-around dark-sky nebular filter available.

Oxygen III Filters

Narrow band pass filter (11nm) isolates just the two doubly ionized oxygen lines (496nm and 501nm) emitted by planetary and extremely faint nebulae.Produces near-photographic views of the Ring Neb, Dumbbell Neb and Orion Neb etc.

Hydrogen-Beta Filters

Extremely narrow bandpass filter isolating the hydrogen-beta line alone (486nm).Excellent for viewing the Horsehead, Cocoon and California Nebulae. Often the only way to view certain nebulae and are best used under clear skies with large aperture telescopes.

See specifications for ProStar Narrow band filters here


Object Features Recommended Filter
Mercury Planet/Sky Contrast #23A Light Red
Features #25 Red

Venus Clouds #38A Deep Blue
#47 Violet
#58 Green
Planet/Sky Contrast #25 Red
Terminator #25 Red

Moon Detail #56 Light Green
Feature Contrast #8 Light Yellow
#12 Yellow
#15 Deep Yellow
#80A Blue
Low Contrast Features #82A Light Blue
Glare Reduction ND13 Neutral Density

Mars Clouds #15 Deep Yellow
Maria #8 Light Yellow
#15 Deep Yellow
#11 Yellow-Green
#21 Orange
#23A Light Red
#25 Red
Blue-Green Areas #12 Yellow
#23A Light Red
Dust Storms #38A Deep Blue
#56 Light Green
Polar Caps #15 Deep Yellow
#25 Red
#47 Violet
#56 Light Green
#58 Green
Deep Sky Filter
Low Contrast Features #82A Light Blue

Jupiter Clouds #11 Yellow-Green
Belts #8 Light Yellow
#15 Deep Yellow
#21 Orange
#23A Light Red
#25 Red
#38A Deep Blue
#56 Light Green
#80A Blue
Rilles #80A Blue
Festoons #80A Blue
Atmosphere #56 Light Green
Red-Orange Features #12 Yellow
Orange-Red Zonal #8 Light Yellow
Red/Blue Contrast #11 Yellow-Green
Blue/Light Contrast #25 Red
Great Red Spot #38A Deep Blue
#80A Blue
Galilean Moon Transits #25 Red
Red/Blue/Light Contrast #56 Light Green
#58 Green
Polar Regions #21 Orange
#23A Light Red
Disc #38A Deep Blue
Low Contrast Features #82A Light Blue

Saturn Clouds #11 Yellow-Green
#12 Yellow
#25 Red
Belts #15 Deep Yellow
#21 Orange
#23A Light Red
#38A Deep Blue
#58 Green
#80A Blue
Polar regions #21 Orange
#23A Light Red
#58 Green
#80A Blue
Rings #47 Violet
Cassini Division #11 Yellow-Green
Red/Blue Contrast #11 Yellow-Green
Red/Orange Features #12 Yellow
Low Contrast Features #82A Light Blue

Uranus Dusky detail #8 Light Yellow
#15 Deep Yellow

Neptune Dusky detail #8 Light Yellow
#15 Deep Yellow

Filter Suggestions

CLS filter for reducing light pollution and highly improved visual contrast.
UHC-S filter

Ultra-High Contrast for light polluted skies and improved contrast.


Semi APO filter for reduction of visible purple / blue halo in achromat telescopes. Improves visual contrast


Variable Polarizing
Adjustable light transmission for viewing the Moon or double stars and more


Contrast Booster
Outstanding visual contrast booster for viewing the Moon and planets. Also improves achromat refractor performance.


Moon Sky-Glow
Classic Moon & Sky Glow filter with IR cut coatings for subtle reduction of scattered sky light offering improved visual contrast.


Selection of dye glass 1.25 inch filters for a multitude of uses. See information at left.


LRGB set

Luminence, Red, Green & Blue filters for colour imaging with a monochrome camera.

CLS filter

Broadband High Contrast filter for light polluted skies and improved visual contrast.

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