Brightwell Nightline Moon filter 1.25 in, observatory grade

Specifications

Description

The Nightline Moon filter is a neutral density filter in a 1.25 in threaded cell. With an optical density of 0.9, it transmits about 13 percent of the light evenly across the visible spectrum, which reduces the glare of a bright Moon without changing its colors.

Through a telescope, the Moon near full can be bright enough to make your eye uncomfortable and to wash out low-contrast detail. Dimming it lets your pupil open slightly and lets you see subtle shades on the lunar surface, such as differences in color between the maria and the rays around bright craters.

The filter also helps when you observe the Moon in a large telescope at low magnification, where the image is especially bright. In small telescopes at high magnification, the Moon is dimmer and you may prefer to observe without the filter.

Neutral density filters are useful on bright planets too. Venus is bright enough to cause glare in most telescopes, and a lunar filter tones it down so you can see its phase more clearly.

A Moon filter is not a solar filter. It does not block enough light or the infrared and ultraviolet that can damage your eye. Never point a telescope with only a Moon filter at the Sun.

The filter screws into the barrel of any 1.25 in eyepiece. The gray absorbing glass is coated on both sides to reduce reflections. Because it reduces light rather than selecting wavelengths, it works the same in any telescope design and with any eyepiece.

The Nightline Moon filter is an inexpensive accessory that makes lunar sessions more comfortable, especially for children and visitors at outreach events.

About this configuration

All configurations of the Brightwell Nightline Moon filter 1.25 in share the same filter glass design; the options change the grade, the coating, and the case. The observatory grade comes with a measured transmission curve for the individual filter in the box. Observatory grade glass has the tightest flatness tolerance in the Nightline line, which suits imaging with long focal lengths. A standard coating suits visual use, where small reflections between filter and eyepiece rarely show.

A padded pouch is easy to open with cold fingers, which helps when you swap filters during a session. The passband in the 2024 edition matches the other years; the changes concern the housing only. The transmission figures in the specifications apply to every configuration of the Nightline line.

What's in the box

• ND 0.9 lunar filter in 1.25 in threaded cell
• Plastic storage case
• Test report with measured transmission curve
• Padded pouch

Compatibility notes

• Threads into 1.25 in eyepieces and diagonals (M28.5 x 0.6).
• Not a solar filter: never use it to observe the Sun.