Lunar and Planetary Filters
Posted by Brian Ventrudo
To bring out contrast in the clouds of Jupiter and Saturn, or see fine detail on the surface of Mars, you should consider a set of inexpensive colored-glass planetary filters that thread into your eyepieces. Here’s a quick guide to choosing a few filters that will help you get the most out of your telescope.
• Planetary filters are simply optical-quality glass, possibly with an anti-reflection coating, mounted in a ring that threads into the back of your eyepiece. Make sure you get the right size. If you have eyepieces with 1.25″ diameter, get filters made for that diameter.
• The glass in planetary filters passes certain colors of visible light while blocking others. These color filters are labeled in the same way as photographic filters. For example, a light yellow filter is labeled as No. 8, the same as a corresponding Kodak Wratten No. 8 photographic filter. The numbering system is not complicated, nor is it particularly descriptive; it’s just a standard way, rooted in history, of naming colored filters.
• Do you need a full set of 10 or 20 color filters? Not at all. Just three or four filters are all that you need to tease out detail on the brighter planets.
• The No. 23A or 25 light red filter is great for bringing out contrast between the light and dark areas on Mars. It also darkens the blue sky background making it easier to see Venus in the daytime.
• The No. 56 or 58 light-green filter brings out the great red Spot and cloud bands on Jupiter.
• The No. 80A blue filter may let you see high clouds near the limb of Mars, and can bring out detail in the belts and oval clouds of Jupiter. If you have a big telescope, this filter may also let you see some detail in the clouds of Venus at high magnification.
• You might consider another kind of filter, called a neutral density filter, for looking at the Moon. Why would you need a filter to see the moon? Because in all but the smallest telescopes, the moon is uncomfortably bright, especially between first and last quarter phases. A deep yellow color filter, either the No. 12 or No. 15, is a good choice for the Moon. But here’s a better choice: a neutral density filter, which blocks a substantial portion of broadband light from the Moon.
• A variable polarizing filter provides another alternative for lunar observing. This device contains crossed polarizers that rotate to alter the amount of light reaching your eye. Very useful for dimming the light from the moon to a comfortable level no matter what its phase.
• Lunar and colored planetary filters are only $15-$20, roughly: much less expensive than deep sky filters.
Good To Know
If you have a achromatic refractor which tends to show blue or violet fringes when you look at bright objects like the moon, Venus, and Jupiter, you might consider getting a No. 8 or No. 11 light-yellow filter to keep the unwanted color to a minimum.
When I first used color filters, truth be told, I couldn’t see much difference. But with a little practice, especially at higher magnification, I learned to see far more detail on Mars and Jupiter at high magnification. I found a small set of color filters are well worth the investment. If you like looking at the Moon and planets, you should give these filters a try.