Opteka's 500mm f/8 pre-set long-focus lens is the only product I would consider from them; this is a lens type and specification that has been offered for sale since the 1970's (along with a 400mm f/6.3 lens with pre-set diaphragm). I think Opteka is just the trade name, not a true brand-name. I have one, branded as Phoenix. Loooong, slender barrel, probably four elements, 22 ounce weight, not bad for a $99 T-mount 500mm f/8, all-glass type lens (all-glass meaning NOT a mirror or catadioptric type of lens). Of course it is f/8, it only focuses down to 33 feet as the minimum focusing distance or MFD, a bugger to get into focus, and fairly prone to shake issues....but, again--$99 with T-mount and 2x converter.
The 500mm f/8 lenses have middling image quality, but are fun to play with a bit. The ultra-long zoom lenses Opteka offers are slow aperture-wise, almost to the point that they're pretty difficult to deploy except under super-bright lighting conditions; focusing an f/11~f/16 lens is extremely challenging on today's d-slr cameras. BUT--there are some times when one can use this kinda lens....moon shots...long-range shots when the atmosphere is clear, and so on.
ATMOSPHERIC factors can make a 500mm f/8 lens look about as good as a $10,000 500mm f/4 lens when the shooting distance is a mile or more....there's so muck gunk in the air (pollen,dust,particulate,haze,fog,smoke), or the heat waves are so bad that Top Grade and $10k looks damned close to $99 and 1960's-tech, as in these 500mm f/8 lenses.