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MAKE A VARIABLE POLARIZER
A home-made "variable polarizer", and dew cap with center cap removed.
I once saw a posting by someone on an astronomy forum seeking some advice for a purchase. They owned an Orion XT8 and had 4 eyepieces for it: 25mm, 20mm, 10mm and 9mm. They could either afford a 2X barlow lense or a variable polarizing filter for the moon, and were wondering which to get. A variable polarizer is great for viewing the Moon, but I can't
think of any other application for one. A barlow lense on the other hand would be like adding 4 new eyepieces to his equipment: a 12.5mm, another 10mm, a 5mm and a 4.5mm. These additional magnifications would come in handy for viewing the Moon, planets, and even deep space objects.
Since a variable polarizer just controls the amount of light entering the eyepiece, I thought that you could do the same thing by just controlling the amount of light that enters the tube. Some larger telescopes come with a removable cap in the center of the main dew cap. It can be used at low powers to view the moon without hurting your eyes, but it is too small for any high power viewing. It occurred to me that by using a simple piece of cardboard or a paper plate and partially covering the end of the telescope tube, you could vary the amount of light entering it without compromising image quality. The cardboard would not be visible in the eyepiece image since it is out of focus, much like the small spider frame that holds the secondary mirror in a Newtonian is not visible when the observing target is in focus.
I suggested to the person who made the posting that they try this by taping a paper plate to the end of the scope and moving it to adjust the brightness of the image. I'm glad to say that it worked, and they can now also buy the barlow lense. It may not be a high tech solution, but it really does work quite well, especially when you consider that the only cost involved is a piece of thin cardboard or bristal board and a piece of masking tape.
Technically, this is not a variable polarizer (I won't get into a long winded, technical and probably boring explanation of how a variable polarizer works), but the result is the same. Both can adjust the amount of light reaching the eyepiece.