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Astigmatism Adjustments

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The following procedure was written by Curt Palme. 

One of the most confusing setup aspects of CRTs is trying to explain what astigmatism is, and how to set it up properly. I’ll do my best to explain my version here, as shown on an NEC PG projector.

As with your vision, you want a round dot to appear round on the face of a CRT. You don’t want it to flare in any direction , appear oval or triangular. Depending on your projector make and model, the astig magnets on the necks of each CRT, the electronic focus and astigmatism control via the remote control will affect the focusing and astigmatism quality that you get on the face of the tube.

Looking at the image below, you have a 2 pole and 4 pole astigmatism magnet on the neck of the tube. The screwdriver points to the astig magnet assembly. Some projectors (Marquee and NEC XG) have an additional 2 magnets called 6 pole within that magnet assembly.

You’ll see that there are two concentric tabs that slide around the tube, and with and against each other. From the factory, there is a silicone blob that holds these tabs into place. Now usually these astig magnets do NOT need adjusting, so if they are still factory sealed, leave them alone. If you are changing out a CRT though, then break the seal without breaking the plastic tabs off and free up the tabs so that they can be moved. Note however that it’s quite possible that the astig on a replaced tube will be very close in setting to the original CRT, so you might consider only breaking the silicone blob after you confirm that the astig magnets actually need adjusting.

The pair of tabs/magnets closest to the CRT socket are called the 2 pole magnets and have the most affect on astigmatism. The second set closest to the lens are the 4 pole ones, and are more or less a fine tuning of the 2 pole ones. The 6 pole magnets on some projectors are an even finer tuning of astig.

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