Eyepiece filters are an invaluable aid in lunar and planetary observing. They reduce glare and light scattering, increase contrast through selective filtration, increase definition and resolution, reduce irradiation and lessen eye fatigue. Celestron's filters are made of high quality, solid plane parallel glass with excellent homogeneity. They're anti-reflection coated to prevent glaring and ghosting. All eyepiece filters are threaded to fit Celestron's, and most other manufacturer's, 1¼" eyepieces, and offer a full 26mm clear aperture.
Celestron color filters are mounted in black anodized aluminum cells with the Kodak Wratten Series Number individually engraved, and come available in 4 assorted kits packed in plastic cases. The cells of each filter are double-threaded, so they can be stacked (piggybacked) in various combinations. This allows you to create different color combinations and transmission characteristics, or to have the same color characteristic, but with a lower transmission. When stacking color filters, the effective transmission of the combination you create is equal to the product of the spectral transmission of each of the filters used.
The effectiveness of the filters depends on several factors, including: the aperture and focal length of the telescope, the magnification being used, and seeing conditions.
#12 Yellow (74% transmission): Contrasts strongly with blue-colored features on Jupiter and Saturn, while enhancing red and orange features. Lightens red-orange features on Mars, while reducing or blocking the transmission, and thereby increasing the contrast, of blue-green areas. Useful in increasing the contrast of lunar features in telescopes 6" aperture and larger.
#56 Light Green (53% transmission): Excellent for the observation of Martian polar ice caps as well as yellow-tinted dust storms on the Martian surface. Increases contrast of the red and blue regions in Jupiter’s atmosphere as well as in the cloud belts. Also useful for enhancing lunar detail.
#25A Red (14% transmission): Strongly blocks the transmission of blue and blue-green wavelengths, resulting in very sharply defined contrast between, for example, blue-tinted cloud formations on Jupiter and the lighter-toned features of the disc. Also useful for the delineation of the Martian polar ice caps and maria. Because of its relatively low total light transmission the #25A filter should be employed on telescopes of 8" aperture and larger.
Neutral Density (ND6) Filter (25% transmission): Great for the moon and observing bright planets like Venus. Since neutral density filters do not change the color of the object observed, it is excellent for use in planetary viewing. Also reduces glare, brightness, and irradiation on the Moon and planets due to the fact that it only lets 25% of the light coming through your telescope reach your eye. The filter also helps split binary stars where one of the members is significantly brighter than the other. This filter is recommended for telescopes of medium aperture (3.5" to 6") or with larger telescopes when stacked with other filters.
