Filters work by blocking a specific part of the color spectrum, thus significantly enhancing the remaining wavelengths. Colored filters work by absorption/transmission, and instantly tell you which part of the spectrum they are reflecting, and therefore transmitting. The so-called light-pollution reduction and nebulae filters are very selective in the wavelengths they transmit. For these it is best to refer to the manufacturer's specifications on a given filter.
Colored filters are referred to by their Wratten numbers. The Wratten system was developed by Kodak in 1909 and has been the standard ever since. Filters used for photography, astronomy, and other applications all use this same standard. So here, then, is a summary of the filters most commonly used in astronomy.
#8 Light Yellow (83% transmission)
This is used for enhancing the detail in red and orange features in the belts of Jupiter. It is also useful in increasing the contrast in the maria on Mars, and increasing the resolution of detail on Uranus and Neptune in telescopes with 10" or more of aperture. This is a great filter for enhancing lunar detail, too, particularly on telescopes with 8" of aperture or less.
#11 Yellow-Green (78% transmission)
This filter is great for bringing out surface details on Jupiter, and to some degree, Saturn. It darkens the maria on Mars and does slightly improve visual detail on Uranus and Neptune, again in telescopes with 10" of aperture or more. This filter is what I used primarily for observations of Jupiter after it was pummeled by Comet Shoemaker-Levy in 1994. It brought out the impact areas in excruciating detail.
#12 Yellow (74% transmission)
Enhances red and orange features on Jupiter and Saturn, while blocking blue and green wavelengths. It also lightens the red and orange features on Mars, while reducing, or blocking, the transmission of blue and green areas; this increases the contrast between the two. It also enhances the blue clouds in the Martian atmosphere. This is one of my favorite Mars filters for that reason. Very nice for increasing contrast in lunar features also, in telescopes of 6" of aperture and above.
#15 Deep Yellow (67% transmission)
This filter is used to bring out Martian surface features, and the polar ice caps. It can also be used to enhance the orange and red features, bands and festoons, on Jupiter and Saturn, and for low-contrast cloud detail on Venus. Try it also on lunar surfaces; it works nicely to improve the contrast. Try this one for daylight observation of Venus and Mercury.
#21 Orange (46% transmission)
The #21 orange reduces transmission of blue and green wavelengths, thus increasing the contrast between these areas and red or yellow or orange areas. It is great on Mars because of this. It sharpens the boundaries between these areas on the planet's surface. I also use it on Jupiter to sharpen the contrast in the belts and to bring out the Great Red Spot. It will also slightly increase surface details on Saturn. This one behaves very similarly to the #15 but gives slightly more contrast.
#23A Light Red (25% transmission)
This is another great filter for use on Mars, Jupiter, and Saturn, but because of lowered light transmission, probably shouldn't be used on a scope of 6" of aperture or smaller. It performs many of the same functions as the #21 and the #15, but again, with more contrast than do either of these. It is also a great one to try for daylight observations of Mercury and Venus because it increases the contrast between these planets and the bright blue sky.
#25A Red (14% transmission)
The #25A filter strongly blocks the transmission of blue and green wavelengths, which result in very sharply defined contrast between the cloud formations and the lighter-toned surface features on Jupiter. This filter is also quite useful for definition of the Martian polar ice caps and maria. However, because of the reduced light transmission, the #25A should probably only be used on telescopes with 8" of aperture, or more.Try this one on Venus. Not only does it reduce the light glare, it really does some interesting things to the clouded Venusian atmosphere.
#38A Dark Blue (17% transmission)
The #38A is 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 the contrast. It works well on Martian surface phenomena, like dust storms, and increases the contrast in the rings of Saturn. This is a good one to use on Venus, too, because of its low light transmission; it really increases the contrast of subtle cloud markings. The #38A should only be used on telescopes of 8" of aperture or more, because of the reduced light transmission.
#47 Violet (3% transmission)
This filter strongly rejects red, yellow, and green wavelengths, making it a good one to use on the Martian polar ice caps. It is THE filter of choice for observations of Venus because of its low light transmission and its ability to enhance upper atmosphere phenomena. It is also touted as being useful for providing contrast in the ring system of Saturn, but I have not found it to be particularly useful for this purpose. Great for enhancing lunar detail, also. Try this one on Jupiter and the Galilean moons. The planet is electric purple, the moons hot pink. A psychedelic trip that's LEGAL!!
#56 Light Green (53% transmission)
This filter is excellent for the observation of Martian polar ice caps and for the yellow tinted dust storms on the planet's surface. It also increases the contrast of the red and blue regions in Jupiter's atmosphere and cloud belts. Another one that is great for lunar observing also.
#58 Green (24% transmission)
This filter strongly rejects red and blue wavelengths and increases their contrast on the lighter parts of the surface of Jupiter. It is also useful for enhancing the cloud belts and polar regions on Saturn. It does a fantastic job on increasing the contrast in Mars polar ice caps and also does a reasonable job of increasing the contrast of atmospheric features on Venus. Again, because of lower levels of light transmission, this filter probably shouldn't be used on telescopes of less than 8" of aperture.
#80A Blue (30% transmission)
Many people say that if you can only buy one filter, this should be it. This is the one of two that come out of my filter case the most often. This filter is the best and most popular for the study of detail on Jupiter and Saturn. It enhances the contrast of rills and festoons in Jupiter's cloud belts, as well as details of the Great Red Spot. It also brings out detail in Saturn's belts and polar features. This filter is also very useful for lunar observing.�Try this filter to split Antares. It works very well for this purpose, especially when the two stars are at their maximum separation.
#82A Light Blue (73% transmission)
This is the second filter that, along with the #80A, comes out of my filter case the most often. It works well on Jupiter, Mars, Saturn and the moon. Its pale blue color enhances areas of low contrast and avoids significant reduction of overall light level at the same time. I find this filter extremely useful. Try this one on bright galaxies, particularly face-on spirals. I accidentally left it in an eyepiece when I swung over to take a look at M51. The detail in the spiral arms was quite pronounced over what I was used to seeing. It took me some time to figure out why. I've tried it on a number of bright galaxies since then and it really does a great job in increasing the detail in galactic structure. Also, try this one to split Antares. Either the 80A or the 82A works quite well for this.
ND96 Neutral Density (0.9 density, 13% transmission)
The neutral density filter transmits light uniformly across the entire visible spectrum. Because of this, it is an excellent filter to use for glare reduction, particularly while observing the moon with any telescope 4" of aperture and larger. Some people also use it to split difficult double stars, particularly those in which one member of the pair is significantly brighter than the other. I have tried this myself, but so far with limited success.
BROADBAND, OR LIGHT POLLUTION REDUCTION FILTERS
SkyGlow This is one of a relatively new class of filters known as light pollution reduction, or LPR, filters. It is designed to darken the background sky by blocking mercury vapor light transmission and enhancing transmission in the hydrogen beta, doubly ionized oxygen (OIII) and hydrogen alpha regions of the spectrum. What this means to the layman is that the filter increases the contrast of deep-sky objects, emission nebulae in particular, with the background light-polluted sky. This filter doesn't work particularly well on other types of objects, but does a fine job with emission and planetary nebulae, because they emit light in the hydrogen alpha, hydrogen beta, and doubly ionized oxygen wavelengths.
Broadband This is another type of LPR filter; the spectrum of wavelengths passed by the Meade Broadband is nearly identical to that of the SkyGlow. Since I very seldom observe from light polluted skies, I am probably not getting the maximum benefit from my Broadband, but even under dark skies it is quite effective in improving the contrast of emission and planetary nebula. One thing I have found out in observing planetary nebulae, though use of these filters on M1 under even a fairly light-polluted backyard sky can help bring out such diffuse nebula as M1. Under a VERY dark site. M1 will reveal detail that is not quite as clear without the filter.