Also noticed Orion has a 2X Barlow lens> sorta' a supercharger for amplyfying the IMAGE U will observe from your basic F/4 telescope> F/4 means Focal length is 4 times C.A. or only 18" to keep scope length short b/c the image produced by the primary has to be in a position where your eyepiece can observe it> The MSS had a 4 foot (48") focal ... More @Wikipedia
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