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I have always used a 2" UV/IR filter on the front of my 2" diagonal with my ED80. The cone of light entering through 2" filter at this point is very large, so no harm comes to the filter by way of heat. 80mm is on the threshold of filter/no filter, so if you only do casual a filter may not be necessary, but if you spend a significant amount of time viewing through the eyepiece, then a filter would be wise. I'd still suggest using a filter either way. The image won't degrade and you have your eyes to keep safe.
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