Two types of filters are often used to enhance planetary detail.  The most popular method is to use a neutral density or polarizing filter to cut down the glare from bright objects like the moon and Venus.  There are also color filters which are used to enhance specific details of certain planets.  Both types of filters are designed to thread into the bottom of an eyepiece barrel.  This makes them compatible with almost any telescope (meaning you can keep your filter collection if you upgrade to a new scope).  Most planetary observing is done at high power, which usually means using a 1.25" eyepiece, so this is the most common size for planetary filters, but they are available in 2" diameter as well.

Neutral Density & Polarizing Filters

Neutral density filters simply block a certain percentage of light.  This attenuates the brightness of objects, which can allow much more detail to be observed.  The moon, Venus, Jupiter, and Mars can all benefit from the use of such a filter.  Neutral density filters are available with various light transmissions, usually ranging from 50% to 13%.  They are rated by ND numbers.  The higher the ND number, the darker the filter.  See the table below for conversion from ND number to percent light transmission.

A more popular method is to use polarizing filters.  Polarizers are a combination of two filters which, when rotated relative to one another, change the amount of light that passes through the filters.  In this way, a range of light transmissions can be achieved.  The typical transmission range for a set of polarizers is from 1% to 40%.  This means they can block more light than a typical 0.9ND (13%) neutral density filter.  However, at very low light transmissions polarizers will cause a slight blue tint because only the smallest wavelengths (blue light) can pass through.  However, they are neutral in color over most of their range and work very well for lunar and planetary observing.

Above:  A set of polarizing filters

Color Filters

Most observers find that a polarizing filter works best for enhancing planetary detail because it cuts out the excessive glare and allows more detail to be seen.  For this reason, these are the most popular filters.  However, certain planetary detail can be enhanced by the use of color filters.  These filters block certain colors of light while transmitting others, increasing the contrast of certain features.  They work best on planets with distinctive colors such as Mars and Jupiter.

How Color Filters Work

A color filter allows certain wavelengths of light to pass, thereby highlighting certain features of the object being viewed.  For example, a blue filter transmits blue light while blocking red and green light.  This would allow blue planetary details to pass through while red and green details would become darker, causing the blue details to stand out.  Similarly, a red filter allows red light to pass and blocks green and blue light.  This would increase the contrast of any red planetary features.

Above:  Mars as seen through blue and red color filters.  Note the bluish clouds near the south pole and right edge of the planet are enhanced by the blue filter.  The red filter enhances contrast between the light (red-colored) and dark surface features.

Types of Color Filters

The most popular color filters are red and blue, since these enhance the prominent details on Mars and Jupiter, such as the blue clouds of Mars and the Great Red Spot on Jupiter.  Other colors tend to have a more subtle effect or a less common application, but there are many to choose from depending on the type of observing to be done.  Color filters are given Wratten numbers, which come from the use of these filters for photographic applications.  The chart below lists the most popular color filters and their common uses.

Subtle colors such as the #8 Light Yellow and #80A Medium Blue work well at bringing out some detail while retaining a somewhat normal appearance to the planet's natural colors.  These can be combined with a polarizer or neutral density to cut down glare and improve contrast.

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