What does "ND 0.9" mean for the Explore Scientific filter?
ND 0.9 refers to the filter's optical density. This is a logarithmic scale, and an ND 0.9 filter transmits approximately 12.5% of the light that enters it, blocking the other 87.5%. This is a significant but not total reduction, making it a great general-purpose "moon filter" for a wide range of telescopes.
Will the Explore Scientific ND 0.9 filter help for viewing planets like Jupiter or Saturn?
Generally, a neutral density filter is not recommended for planetary viewing. While planets can be bright, an ND filter will dim them without enhancing any specific features. For planets, you are better served by color filters (like a light blue #80A or green #58) which can increase the contrast of specific atmospheric details like Jupiter's cloud bands or Mars' polar caps.
Will the Explore Scientific ND 0.9 filter improve views of the full Moon in my 8" f/6 Dobsonian?
Yes, absolutely. An 8" telescope gathers a large amount of light, and a full Moon will be dazzlingly bright. The ND 0.9 filter is an excellent match for this aperture, cutting the glare to a comfortable level. This will relieve eye strain and allow you to resolve much finer details along the terminator and within bright ray craters like Tycho and Copernicus.
I'm trying to split the double star Antares with my 4" refractor. Will this ND 0.9 filter help?
It can. Antares (Alpha Scorpii) is a classic example of an unequal double, with a bright red primary and a much fainter secondary companion. A 4" telescope has the resolution to split them, but the glare from the primary is often the problem. The ND 0.9 filter will reduce that glare, giving you a better chance to spot the faint companion, Antares B, nestled close to the primary star.
Does the Explore Scientific ND 0.9 filter change the color of the Moon?
No, it does not. The "Neutral" in Neutral Density means it is designed to reduce the intensity of all wavelengths of light equally. This ensures that the view is simply dimmer, but the natural grey and white tones of the lunar surface are preserved without introducing a color cast.
Can I stack the Explore Scientific ND 0.9 filter with other 1.25" filters?
Yes. The filter has standard 1.25" filter threads (M28.5x0.6) on both sides, allowing you to stack it with other filters. For instance, you could stack it with a color filter to see if it helps with planetary viewing, or even stack it with another ND filter for an even greater dimming effect on a very large telescope.
