Color filters can be very useful for enhancing your views of the Moon and the planets. Depending on the atmospheric conditions, the telescope being used, the observer's experience, and the planet being observed or photographed, the advantages of color filters can be anywhere from subtle to dramatic. The right filter can make all the difference between seeing several small craters in the floor of Clavius on the Moon or not; seeing five or six swirls in Jupiter's belts or not; or seeing the inner Crepe Ring of Saturn or not.
Agena's 2" color/planetary filters are manufactured from the purest optical glass and are dyed-in-the-mass (not simply "color coated") for premium performance. They thread into the barrels of virtually any 2" telescope eyepiece. Each filter cell has a male and female 2" filter thread (M48x0.75) on opposite sides, so multiple filters can be stacked to achieve selective filtration of the visual color spectrum. The clear aperture is 43mm and each filter comes in a protective plastic storage case.
The #23A Light Red Filter is another great filter for use on Mars, Jupiter, and Saturn, but because of lowered light transmission, it may not perform well for 6" and smaller telescopes. It carries out many of the same functions as the #21 and the #15, but with more contrast, bringing out marginally defined blue-green surface detail.
The filter is ideal for viewing the polar ice caps and surface features of Mars. It brings out yellow dust clouds and darkens Maria, oases and canal markings. On Jupiter and Saturn, this filter is useful for studying blue clouds.
The filter also increases contrast between Mercury and the bright blue sky during daylight observations or during twilight, when the planet is near the horizon. On Venus, it occasionally reveals deformations of the terminator. For comets the filter improves definition of the dust tail.