What a moon filter is
A moon filter is an accessory that reduces the amount of light reaching your eye when you look at the Moon through a telescope. You can think of it as sunglasses for your telescope. Most are neutral density filters, meaning they dim the light fairly evenly across colors, or polarizing filters, which can be adjusted. They usually screw into the bottom of an eyepiece barrel, in 1.25-inch or 2-inch sizes.
What a moon filter does and does not do
Moon filters can:
- Reduce glare and make the Moon more comfortable to look at, especially near full Moon.
- Lessen the lingering bright afterimage some people notice after looking away from the eyepiece.
- Sometimes improve how much fine surface detail you can pick out, because a dimmer image can be easier on the eye.
They do not:
- Add resolution or magnification.
- Make the Sun safe to look at. They are not solar filters.
- Make a big difference for everyone. Many observers find the effect small.
Types: fixed neutral density and variable polarizing
Fixed neutral density filters pass a set percentage of light. Common values include roughly 13%, 18% and 25%, and some sellers offer higher or lower values. A lower percentage is darker.
Variable polarizing filters are made of two linked polarizing elements. Rotating one against the other changes the transmission. Typical ranges listed by sellers run from around 1% to 40%, which lets you adjust to the Moon’s phase and your telescope.
Some observers also use a light colored filter, such as yellow, on the Moon for contrast. That changes the color of the view, so it is a matter of taste.
Do you need one?
Probably not at first. Opinion among experienced observers is mixed, and many say they rarely use one. The Moon is at its brightest around full phase, and a large aperture or low magnification makes it more intense. For a small telescope in a crescent or quarter phase, the view is often comfortable without any filter.
A moon filter is more likely to be worth it if:
- You use a larger telescope.
- You are sensitive to bright light.
- You observe a lot near full Moon.
- You run public viewing nights where many people look through the same eyepiece.
How to choose a transmission level
- Smaller telescopes generally suit lighter filters (a higher transmission percentage). Larger telescopes generally suit darker ones.
- Check how the seller labels the value. Transmission means the share of light that passes, so a 13% filter passes about 13% and blocks the rest.
- If you are undecided, a variable polarizing filter lets you adjust and avoids picking a single number.
- Try observing without one first, so you know whether you actually need it.
How to use it
- Screw the filter onto the barrel thread of your eyepiece.
- Insert the eyepiece into the focuser or diagonal as normal.
- Focus, then for a variable filter rotate the adjustable part until the brightness is comfortable.
Variable filters can be awkward in some focusers, since you need to be able to turn one part. Some observers mount one half on the diagonal and one on the eyepiece, though this depends on your equipment.
Alternatives to a filter
- Increase magnification. A higher power spreads the light and darkens the image.
- Observe at crescent or quarter phases. Detail along the terminator (the light-dark line) is often easier to see, and it is less bright.
- Let your eyes adapt. Some observers look at faint objects first and leave the Moon until the end of the session.
- Reduce the aperture with a mask, if your telescope design allows it.
Moon filters are not solar filters
Please do not confuse the two. A moon filter is not designed to make the Sun safe. Solar observing needs a dedicated full-aperture solar filter, mounted over the front of the telescope, from a reputable maker and used according to its instructions. Small filters that fit an eyepiece are not safe for the Sun because heat can build up and crack them. Avoid homemade solar filters. For more on the full range of filters, see the telescope filters guide.
FAQ
Can you use a telescope to look at the Moon? Yes. The Moon is one of the easiest and most rewarding targets, even in a small telescope or binoculars.
What can I use to look at the Moon? Your naked eye, binoculars, or a telescope. A filter is optional.
How do I make a sun filter for a telescope? Do not make one. Use a purchased full-aperture solar filter designed for the purpose and follow its instructions.
What types of solar filters exist? The common safe types for telescopes are full-aperture filters made from solar film or glass, and specialist solar telescopes or wedges. Details and suitability vary, so check with the manufacturer or an astronomy club. If you are also curious about color filters for planets, see the telescope color filter chart.