Every amateur astronomer should have a few good color filters in their eyepiece case. Color filters can be especially useful when observing planets. They help to reduce glare from bright planets like Jupiter and can bring out subtle contrast not as readily visible in natural color. While the color exaggeration may be distracting at first, it can really help you to distinguish those tiny details on Mars and help to see cloud formations on Jupiter. The following 4 filters (specified by their "Wratten" number) are my favorite color filters for both planetary and deep sky use.
#23A Red Filter
The #23A filter is a red filter and is very useful for observing Mars. This filter will help to increase the contrast between the Martian surface and the dark Maria. It can also be used on Jupiter and Saturn. This filter's relatively low light transmission (25%) makes it well tailored for larger scopes. Therefore, those with smaller scopes (130mm and smaller) may want to try the #21 orange filter instead.
#80A Blue Filter
The #80A Blue filter is an excellent filter for use on Jupiter and Saturn. This filter greatly increases contrast in Jupiter's festoons and other cloud formations. It will also help with contrast in the Great Red Spot. On Saturn, it helps to increase contrast in the cloud belts and may help you see some polar features. This filter also has a relatively low light transmission (30%), so users of smaller scopes may want to try the next filter instead.
#82A Light Blue Filter
The #82A light blue filter is similar to the #80A, except it has a higher light transmission of 73%. This filter is excellent for use on Jupiter and Saturn, especially in smaller scopes. I prefer this filter over the #80A even in my 10" dob, though, since the color difference is noticeable, yet very subtle. Surprisingly, this filter can have interesting effects on some bright deep sky objects, too. I have used it on some brighter galaxies, such as M31, with positive results. This trick, however, will only work in large scopes (10" and larger). I'm curious to see what this filter would do for M45.
ND96 Neutral Density Filter
This filter is not really a "color" filter, but rather a "neutral density" filter which transmits the same across the spectrum. Its low light transmission (13%), however, has earned it the nickname of "moon filter". This filter is great for reducing glare from bright objects like the moon and Jupiter. Using only a ND96 filter on Jupiter will have astonishing results in the form of greatly increased contrast. This is really an effect of the eye, however. Since the image isn't so blindingly bright, it is easier to see minute details. On some larger scopes you may want to try stacking ND96 filters while viewing the moon for more comfortable viewing.
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