Telescope Filters Buying Guide

You've been studying star charts for weeks. You’ve read “Tips for Buying a Telescope,” picked out, and purchased your first telescope. You can hardly contain your enthusiasm while setting up your new scope. Finally, the moment arrives—you take your first magnified look at the night sky, prepared to be blown out of this world. It is a beautiful sight. Yet after the initial awe wears off, you can’t help feeling a bit underwhelmed. The sky is hazy, the moon is glaring white, and the nebulae you have been dreaming about all week are nowhere to be seen. Feeling tricked by all the beautiful pictures jam-packed into astronomy books and magazines, you consider leaving your new purchase out with the rest of the recyclables and picking a more reliable hobby. Before trading your telescope for a set of woodcarving knives, read this article.

One of the simplest ways to boost the quality of views offered by your telescope is the addition of filters to your setup. Whether scrutinizing the ice caps of Mars or surfing the clouds of distant nebulae, looking through a telescope filtered for its target can radically improve the viewing experience. However, choosing the proper filter for the job can be a daunting task. Never fear! After reading this article, you will be prepared to outfit your gear no matter what your favorite celestial target is.

Night Skies and Our Eyes

The quality of views offered by your telescope depends primarily upon three factors: resolving power, contrast, and sharpness. While resolving power depends largely upon the aperture of your scope, sharpness and contrast can be boosted by the application of filters. Both attributes are affected by the imperfections of human vision and what astronomer’s call seeing, the effects of light scattering in the atmosphere. Proper filtration can do wonders for both issues.

If you have ever forgotten your sunglasses on a trip to the beach, you know the benefits of filtering light before it reaches your eyes. Your retinas are loaded with light-sensitive cells and when they become exposed to large quantities of light it can be difficult, if not painful, to see. Viewing the sky at night poses the opposite problem—our eyes are under-stimulated by the light in their surroundings. It might seem counterintuitive at first that the solution to this problem involves the use of filters which, by nature, reduce the amount of light reaching your eyes. Without getting lost in the complexities of human vision, it is worth knowing a little about how our eyes work to understand why filters are an astronomer’s best friend.

You might remember from biology class that your eyes contain two kinds of photoreceptors: rods and cones. Three types of cone cells, each responding to a different wavelength of light (red, green, and blue) are responsible for our perception of color. They are fewer in number and require much brighter conditions than our rods to relay sensory information to our brains. Our cones take the reins during photopic (daylight) conditions. Scotopic (night) vision relies entirely upon our rods. Although you have about twenty times as many rods as cones, there is only one type of receptor at work, and it responds ideally to a wavelength of light situated between the blue and green spectra. If you have ever wondered why the world seems a little bluer at night, it is because your rods are working solo. Scientists call this the Purkinje Effect, named after the Czech neuroscientist, Jan Evangelista Purkinje. So, what does all of this mean for astronomers? Because scotopic vision is monochromatic, the determining factor for perception in the dark is contrast. This is where filters come into play.

Telescope filters screw into the barrel of your eyepiece and are sized accordingly. All filters operate by reflecting some light and transmitting the rest. Their value to astronomers resides in their ability to let you pick and choose which wavelengths of light reach your eye. By controlling the type and quantity of light that your eyes perceive, you will be better able to distinguish differences in low-light conditions. By reflecting certain wavelengths, transmitted light appears much brighter, increasing contrast and improving your view. Since different celestial bodies reflect or emit different wavelengths of light, filters allow you to fine-tune your instruments, depending upon where you are looking in the sky.

Turn off the Bright Lights

Unless you are planning to use your telescope thousands of miles from civilization, your overall viewing experience will be improved by the application of a light pollution reduction (LPR) filter. If you are looking for one filter to enhance your views across the board, add one of these to your bag. LPR filters reflect the wavelengths of light associated with sodium-vapor and mercury-vapor lamps, the most common types used for street illumination. If you live near or in a city, an LPR is as necessary as a tripod. While the elimination of all artificial light is impossible, LPRs will give you a noticeably darker sky to work with.

While most of this article is aimed at providing solutions for boosting our perception of light associated with distant subjects, one beloved night-time target is so bright that it requires dimming: the moon. The best options for teasing detail out of the moon are the use of polarizing and/or neutral density filters. If you have ever experimented with filters on your camera, you are probably familiar with these two photography staples. Neutral density filters (often marketed as “moon filters” for the astronomy crowd) reduce glare while leaving the colors transmitted to your eyes unaltered. Because neutral density filters uniformly reduce light across the spectrum, they will not increase contrast, but they will cut back the intensity of light reaching your eyes, allowing you to see otherwise invisible details. Polarizing filters cut back on reflection and have the added benefit of allowing you to manually adjust the filter strength. With the turn of a thumbscrew, you can choose the precise amount of filtration for optimal viewing.

Neighborhood Watch

As we move further away from Earth, the variables that affect our view of the sky begin piling up. The atmospheric conditions and physical composition of each planet present unique challenges, depending upon where you are looking and what you are looking for. Consequently, there are countless ways to enhance your views for each planet. This is where color telescope filters come in handy. Several manufacturers offer “planetary sets” that consist of varying grades of red, yellow, and blue filters. As a bonus, most sets include a neutral density filter in their lineup. Color telescope filters are described using Wratten numbers, the same industry standard used to categorize camera lens filters. A comprehensive list of filter-planet combinations would triple the length of this article. However, a few general tips should get you started. Red filters help with daytime viewing of Mercury and Venus. Yellow filters boost contrast in Neptune and Uranus while teasing out detail in the belts of Jupiter and the surface of Mars. Blue filters are the most versatile of the group, revealing dust storms on Mars, the belts of Jupiter, and the rings of Saturn. Finally, if you are interested in the stormy skies of Venus, try a violet filter. It is never a bad idea to try a couple of different filters for each subject—you may be surprised by what you see!

In a Galaxy Far, Far Away

It comes as no surprise that the most difficult (and often beautiful) celestial phenomena to observe are the ones furthest away. Narrowband and line filters add clarity to this otherwise cloudy subject. Narrowband filters block out all light except for small ranges of wavelengths associated with specific phenomena. Line filters are even more precise, blocking out all but one or two wavelengths of light. The most popular of these groups is the Oxygen III (OIII) filters, which reflect all but 496 and 501nm lines, associated with planetary and emission nebulae. Such extreme filtration provides a clean, black background for observation.

No matter what your favorite celestial subject may be, there is a filter out there that will help you get to know it better. Experimenting with the many available options is the best way to determine what works best for you. So, grab some filters and have a look!

*Note: The photographs in this article serve as simulations. Cameras register greater detail and more colors than the human eye is capable of perceiving when looking through a telescope.