I was reading about the Lunt Herschel wedges, and at least one dealer claims they should only be used with air-spaced refractors of 150mm or less. I find this peculiar. I note that AP makes no such claim about their ultradeluxe Baader wedge. It doesn't seem likely to me that an oiled objective would get any hotter than an air-spaced one. What issues are involved here?
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