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I am not a big fan of filters for planetary, mostly because I feel they cause a slight loss in resolution when placed in the train. For Mars though there are two filters that I use at times: Baader Contrast Booster and the Vernonscope Magenta Filter. Of these, the Magenta is probably the favorite as it defines things up without too much light loss. But I find that I use a filter, only when the seeing is not as steady as I like. When the atmosphere is very steady and transparent, I feel no filter is best. When it's a little off, then the filter can help...especially when you use the highest magnifications. So recently, when at about 250x in my 4" APO on Mars, the filter was a distinct benefit. Back off to 150x, and preferred the no filter view.
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