I would not know the brand but it sounds like the generic 80mm scopes out there by the 1000's made by Synta in China , great little wide field scopes , not to hot on high power lunar/planetary views , ok if you dont crank the magnification past 70x max .
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Oh yea at 80mm dia and 500mm focal length so its a f/6.25 .
500mm divided by 80mm = 6.25 , so its a f/6.25 not f/5 .
hope this helps , any chance of a photo ?
Brian.