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MOON FILTER / VARIABLE POLARIZER
LEFT TO RIGHT:2 single Moon filters (6% and 13%), Antares variable polarizer,
and a Meade variable polarizer.
If you have ever tried to view the Moon through binoculars or through a larger telescope at a lower power, you know that the image can be so bright that it hurts your eyes to look at it without a filter. A Moon filter, sometimes called a neutral density filter, screws into the eyepiece. It has a dark tint that reduces the amount of light that reaches the eyepiece. They come in different tints depending on what you need for the magnification selected for your telescope. Like most filters it is available in both 1.25" and 2" sizes.
A variable polarizing filter consists of 2 individual filters mounted to each other so that you can turn them in relation to each other. If you hold the filter up to a light and slowly turn the filters, you will see the light grow darker, then brighter again as the filters rotate. This gives you the ability to adjust how much of the light you want blocked. Most filters require you to remove the eyepiece to access the filters for adjustment or changing.
I purchased a Meade variable polarizer shaped like a very short barlow lense which has a lever allowing you to adjust it while the eyepiece is still installed. Unfortunately it increases the length of the light path and only works on a few of my telescopes using a 1.25" diagonal. Even a 2" diagonal with a 1.25" adapter adds enough length to the end of the focusser to make it impossible to focus with any eyepiece.
If you don't have a moon filter or variable polarizer and want a cheap and easy to make alternative without spending a lot of money, look in my "DO IT YOURSELF" section near the bottom of my home page for a simple, inexpensive, and very easy to make alternative..