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for cleaning lik to use zeiss wipes, but have also had god success with blue windex..im guessing they are dried dew marks -do they look wispy/spider web nb tis can be fungus
the adv of refractors is they are lightweight, easier to mount & you don't need to worry about collimation as much
cons can be they have a longer focal ratio e.g. f7, also they have a relatively short focal length so are not as suitable for planets/ deep galaxies- they also cost a lot more per inch of aperture
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