The moon is a surprisingly bright object, particularly when using a telescope. You need a moon filter to darken the image to make it less painful to view. There are a lot of moon filters out there so figuring out which is the best fit for you and your budget can be hard. The following reviews hope to help you decide which moon filter to get.
About a Moon Filter
Moon filters are supposed to darken the image. They can do this in two general ways; neutral density or polarization.
Neutral density and the traditional type of filter and simply means that the filter will darken the object a specific amount in all color wavelengths. These filters contain gray glass which act just like sunglasses. Better ones darken the image without changing the color of the image, it remains neutral. These generally come in 13% and 25% versions where the lower the number, the less light it allows through.
Larger telescopes such as medium to large Newtonians and Dobsonians typically have brighter images so a lower number filter such as the 13% is the preferred filter. Smaller telescopes like a refractor or small Newtonian might be better off with a 25% model.
A polarization filter is generally variable moon filter allowing you to change the amount of light removed to suit your tastes. These filters work by removing light coming in at angles and only allowing light coming straight through the filter. Since the moon tends to scatter the light it reflects from the sun, these work surprisingly well but are more expensive.
Adjustment of a polarizing filter is done by rotating the two pieces of the filter (top and bottom) which in turn rotates the two pieces of glass inside the filter. When rotating you will see the image appear to darken and then lighten again. This allows you to get just the right amount of light.
Polarizing filters have a range of light adjustment which can go from around 1% to 40%. This range is much darker on one end and much brighter on the other end than traditional filters so it makes them perfect for any type of telescope with any aperture.
Another advantage of the polarizing filters is that since the light is no longer scattered, the image appears much sharper and clearer.
Recommended Moon Filters 1.25″
If you have a 1.25″ diagonal or only 1.25″ eyepieces then these are the filters you need.
The Gosky 1.25″ 13% Transmission Filter is an excellent choice for your first filter. With an aluminum frame and actual optical glass is is much higher quality than some others in its price range. Less expensive models typically have plastic threads and/or plastic optics. Gosky also gives you a full year warranty so if you have any problems you can get it taken care of.
If you want something that doesn’t block quite as much light such as for smaller telescopes this Orion Moon Filter 05598 1.25-Inch 25 Percent Transmission is an excellent choice. Just like the Gosky it has a metal filter cell and optical glass which cuts the light while not affecting the color. I have used one of these for years and it works very well. It is hard to beat an Orion moon filter.
For the ultimate in control this Orion 5560 1.25-Inch Variable Polarizing Moon Filter allows you to control the amount of light passing through from 1% to 40%. It manages this while reducing light scatter which increases contrast substantially.
Recommended Moon Filters 2″
Have a 2″ diagonal or 2″ eyepieces? Then you need a 2 inch moon filter.
If you have a telescope using 2″ diagonal or 2″ eyepieces then it is probably not a small telescope so you need a 13% filter at a minimum like this Orion 05594 2-Inch 13 Percent Transmission Filter. This filter is built very well and is optically excellent featuring a metal filter cell and good optical glass.
Just like their 1.25″ version, this Orion 5562 2-Inch Variable Polarizing Eyepiece Filter allows you to vary the light transmission from 1% to 40% making sure you can get the exact amount of light you want for your viewing conditions. It also substantially cuts out light scatter which greatly enhances contrast. If you do a lot of lunar viewing and have a 2″ diagonal or 2″ eyepieces, this is the filter you really want.
Believe it or not these filters are also useful with beginner astrophotography with cameras that do not have manual exposure settings. These cameras can tend to overexpose the image because there is a lot of black surrounding the moon. Using a moon filter in this situation reduces the contrast between the moon and background allowing the camera to get a better image.
I hope this article will help you choose the right moon filter for you!