One of the more common questions asked is, “What color filter is best when observing the planets?” With so many different colored filters available it can be confusing, but luckily there is one obvious choice that is a cut above the other, orange.
Orange is the best filter and a favorite for this author, not because of our close proximity to Syracuse and the Orangemen, but because it is so versatile. It is not so dark as to block too much of the light that is being transmitted, making it highly useful for smaller telescopes, but dark enough to cut down on annoying glare. An orange filter darkens the background sky when observing a planet in twilight and steadies the atmospheric turbulence a bit. Filters have a specific Wratten filter number corresponding to the color and orange is often labeled as W21 or #21.
➢ Mercury: Using an orange filter when Mercury is near half phase increases the chance of seeing subtle detail because most of the observing with this planet is done in twilight when it is low to the horizon and atmospheric turbulence is a factor. An orange filter will darken the sky and steady the seeing enough for better viewing.
➢ Venus: An orange filter will cut the dazzling glare of Venus’ brilliant clouds and gives a more pleasing, warm color. By not dazzling the eyes, they become more relaxed, therefore increasing the chance of seeing cloud detail. By softening the terminator, the filter allows the duskiness along the terminator to be readily seen for a 3-D effect. Dusky bands near the poles and patches near the equator may be seen, but will pose a challenge.
➢ Mars: A red filter is the best for Mars, but Mars is often small and not overly brilliant except near opposition, therefore an orange filter reigns supreme for smaller telescopes. Details on Mars are tiny and subtle, so steady viewing is mandatory. An orange filter helps to enhance the darker surface features such as Syrtis Major, which allows the ice caps to stand out.
➢ Jupiter: It is always easy to see some detail on Jupiter, but the belts and dusky polar hoods will be enhanced along with the dusky shading along the limb. This will make Jupiter stand out like a globe instead of a disk, making it seemingly float in space among the stars.
➢ Saturn: There is never much cloud detail to be seen on Saturn, but an orange filter will help darken the belts and polar hoods; the rings will stand out sharply.
➢ Uranus & Neptune: The discs of these worlds are too small and too dim to reveal detail except with telescopes of at least 16” of aperture. Little if any enhancement would be seen visually with an orange filter.
An orange filter is also good for color-correcting the Sun when using a Mylar solar filter and the Moon looks pleasing with an orange hue with most of the glare filtered out. Combine a #21 orange filter with a #58 green filter for Mars when it is at its largest and brightest as it will bring out a wealth of detail while preserving its natural color. This color combination also works great with the Moon as it will cut out the entire glare and renders it a warm hue.
Filters are not very expensive with a single 1¼” glass filter costing about $15. They simply screw onto the bottom of any standard 1¼” eyepiece or screw together with each other to create interesting and useful color combinations. Filters are a useful tool for astronomers, but if the weather is poor, they cannot magically make details appear. If the budget is limited to only purchasing one, then an orange filter is the preferred choice.