Meade Oxygen-III (O-III) nebular filters provide an extraordinary level of nebular filtration on diffuse and planetary nebulae. Observations of these nebulae in a city environment with the O-III filter often yield more image contrast than observations of the same objects in a dark-sky setting without the filter. Threads onto the rear cells of Meade LX-series mirror-lens telescopes and onto Meade ED Apochromatic Refractors (using the Universal Thread Adapter), as well as to the rear cells of other Schmidt-Cassegrain brands. In this way the telescopes are converted into a basic "nebular mode," for visual observing or for photography with the T-Adapter or Off-Axis Guider. Clear aperture is 36mm.

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http://www.meade.com/catalog/meade_4000/meade_series_4000_nebular_filters.htm