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The longer the scope with respect to any given field stop diameter (regardless of focal ratio) the smaller the true field of view (tfov). So to get larger fields of view in long scopes you need wider field stops and thus 2" or bigger. There is a related issue of exit pupil size but that is another matter. So an f8 4.5" reflector with 1.25" (field stop say 27-28mm) has a maximum TFOV of about 1.7 odd degrees. In an f5 12" reflector the same 1.25" barrel gives only about 1 degree. To get more a 2" (field stop about 48mm) allows about 1.8 degrees. This is regradless of the Aparent field of view. So putting that very large ultra wide angle in a long scope with only a 1.25" barrel means the field of view is simply cut off by the field stop.
cheers
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