The moon is one of the first things people point at when they get their telescope up and running. It’s large, it’s easy to get a look at and its bright. It can be bright, maybe too bright.
Telescopes naturally gather and enhance light, which is why they should never be used to look at the sun. At night when the moon is full and at its brightest, the moon can be too bright for certain telescopes. At worse, it gives the user a headache, and at best, it can make the moon look washed out and fuzzy.
The remedy for this is a moon filter.
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What Exactly Is It?
Most new telescope users do not realize that the majority of eyepieces are machined to fit filters to the end of them. Most are suitable to fit one of the two most common filter sizes, one and a quarter inches (1.25″) and two inches (2″). Moon filters are one of these attachments that can fit the eyepiece.
Moon filters are eyepiece accessories that are specifically designed to enhance the view of the moon via a telescope. There are many kinds of filters for telescopes that each enhance a different thing that is being viewed in the sky.
The moon filters work by cutting down on the harsh white light that the moon gives off during bright periods. When the moon is full or nearly full, the light it gives off is extremely bright and can illuminate the night very well by itself. The brightest time is also the best viewing time as it is the largest and gives the best viewing field.
Moon filters are designed to cut down on that bright light, which in turn will increase contrast bringing out the finer details of the moon during a viewing. This works like any kind of glasses that people wear in their day to day lives. Sunglasses filter sunlight, 3D glasses work with a specific movie or show to create 3D images for the wearer. Similarly, certain telescope filters shade or enhance different kinds of lights that are found in the sky.
The moon filter is primarily a shade and is most similar to a light pollution filter. They are designed to cut through that hazy white light that usually fills the dark skies.
Are There Different Kinds?
Four main different variables go into telescope filters. There is fixed transmission versus variable transmission as well as neutral versus colored filters.
Fixed transmission filters allow a specific fixed amount of light through the filter at any given time. This is usually denoted as a percentage of light that it is designed to let through. In comparison, variable filters can be adjusted to allow for different amounts of light to shine through depending on the state of the moon during viewing.
Most moon filters are neutral density filters meaning they do not affect the color during viewing. This is because moonlight is mainly white. There are colored filters for telescopes, but they are mostly used to view more colored objects in space, such as different stars and certain planets.
How do you use them?
Using most filters on telescopes is fairly simple. First, decide which filter is the best for viewing. Secondly, attach the filter. Third, enjoy the view.
If that sounds overly simple, that is because it is. Moon filters are overwhelmingly circular, neutral, and fixed transmission lenses. Just pick which size fits the eyepiece and screw it on to begin viewing. Again, most are 1.25″ or 2″ diameter and should be threaded for easy attachment.
If a non-circular or variable filter is being considered, just double check that it will fit the specific eyepiece being used, but most of them are close to universal.
Once it is attached properly, refocus the scope and get ready to view the moon.
Here are the best simple moon filters for immediate use.
Celestron 94105 Neutral Density Moon Filter 1.25”
This is Celestron’s take on the basic universal moon filter. It is a standard circular 1.25” neutral density filter that will be able to fit most eyepieces.
This filter allows 13% light transmission, which is a darker filter than some. This is the most common filter level for moon viewing and is the ideal for the light of a full moon.
Celestron is a very reputable company that makes high-quality attachments for their telescopes. Even this simple moon filter shows Celestron’s expertise and design aesthetic in its simplicity and form.
It comes in at a low price and is easily available from multiple online retailers.Check Latest Price
Celestron 94107 Variable Polarizing Filter 1.25”
In addition to the neutral fixed transmission filter that Celestron offers, there is also its variable transmission filter. The variable transmission filter is a more versatile option than the fixed neutral density filter and can allow for more diverse viewings than its simple cousin.
This filter has a variable transmission range of 1% to 40%. This is a very large range that can shut out almost all light for very bright viewings or let in about half-light for dimmer viewings that still require a filter. This would be a great moon filter for viewing all of the different phases of the moon as the brightness fluctuates, so can the filter.
This filter is extremely easy to use as it screws on tight and then allows for the transmission level to be adjusted by simply twisting the ring. There is no fussing or changing different lens caps or anything like that. It is all done by turning the filter.
In addition to being a great filter for the moon, for some extra money, this filter can be used for a lot of different applications when viewing the sky. This is the only filter on this list not to advertise itself specifically as a moon filter. It is not a colored filter, so it will only work when different light levels are required, but the range and ease of use make this a very versatile addition to any astronomer’s collection.Check Latest Price
Orion 05662 1.25” 13 Percent Transmission Moon Filter
Here we have another strong brand in Orion giving their take on the simple circular moon filter. Like the Celestron version, this one is a neutral density circular filter designed with the moon in mind. It also has a 13% transmission rate.
The big difference between the two is that Celestron specifically says their version will fit all Celestron telescopes while the Orion says it should fit all 1.25” telescopes. I am not sure if the Celestron one is exclusively made for their telescopes alone, but I know that the Orion one is advertised as universal. If you do not have a Celestron telescope and do not want to risk getting a filter that might not fit, then this model from Orion is the equivalent.
Other than that, these two models are almost identical. They offer the same threaded construction, same black casing, and the same transmission level. The Orion filter is a hair more expensive than the Celestron, but if the Celestron one is not universal, then the price disparity makes sense.
This is a solid, basic moon filter that can be used quickly and easily by anyone.Check Latest Price
Orion 05598 1.25” 25 Percent Transmission Moon Filter
The other Orion model on this list is a similar one but is a higher transmission level version. Instead of allowing just 13% of light through, this filter is a 25% transmission filter allowing more light in.
For telescopes with smaller apertures, a higher transmission rate can be better. Larger aperture telescopes allow more light in so they need more light filtered out when looking at bright objects, but if the telescope has a much smaller aperture and doesn’t allow as much light in, then, it doesn’t need as much filtered out. Some of the 13% filters would make the views too dim with the size of their light gathering capability.
That is where this model comes in to bridge that gap. On smaller telescopes, this 25% transmission filter will give a similar view to the 13% filter on a larger telescope. Or get both and create your variable lens with two different levels of transmission.Check Latest Price
Many astronomers swear by filters and their ability to enhance a night viewing session. All of these moon filters are standard, easy to use, easy to obtain, and cheap. They are perfect for any beginner astronomer who wants to get a better look at the moon and practice their craft.
They are also an easy way to jump into other kinds of filters. Moon filters are commonly included in larger filter sets that come with colored filters, sun filters, and other types of eyepiece accessories, and this is a great way to get familiar with how and why filters work on different kinds of objects.
At the end of the day, the moon is a rich, ripe target for astronomy, and a filter can only add to the viewing experience. Whether it is to get different kinds of views of our closest neighbor or to filter out the glare of a bright full moon, moon filters can be a useful tool for any backyard astronomer.