My RedCat 51 arrived today. Much drama, but that's another story. This is just a few first impressions -- as a lens, not as a telescope -- it's cloudy here tonight so no stars.
This thing is sharp! I'm not trying to do any official testing -- not my bag and others more capable have promised something much more analytical -- but I did compare it side-by-side using the same Nikon D850 body attached to my 200mm f/2G AF-S VR Nikkor lens set at f/4.5 and to my two 300mm f/4 Nikkor lenses (f/4D AF-S and f/4E PF AF-S VR), both at f/4, wide open. It easily is sharper than either of the two 300mm lenses (I don't have a 300f/2.8 to run an informal comparison) and competition against the 200f/2 @ f/4.5, f/4 and f/2.8 is too close to call. Comparing the 200f/2 at f/2 against the RC51 250f/4.9 is bit unfair; however, the RC51 was the winner. I was really amazed as the 200f/2 is considered one of Nikon's very sharpest lenses. I'm not bothering to post images -- they're boring -- a rusty hubcap leaning against a wooden fence about 200 yards from my camera with the lens firmly mounted using its base's A/S side to an FLM CB58 ball head on my PMG TR424L tripod with two cinder-blocks as ballast beneath the apex.
Now some suggestions: (1) A minor manufacturing improvement might be etching three marks on the RC tripod mount corresponding with vertical and horizontal at either side and four corresponding alignment marks on the lens, itself. I realize there is that whole rotating mount thing with all 360 degrees incorporated into the lens design; but sometimes, especially with the camera mounted and the len's tripod mount on a tripod, it's much easier to loosen the mount retaining screw and rotate the lens a quarter turn. Having those marks engraved/etched/painted would add a penny to production cost (maybe) and a great deal of convenience to the land-based end user.
And, (2) there are a lot of Allen wrenches needed for this thing. Not that I don't have quite a collection at hand, but having them at hand and having to guesstimate as I approach each screw is mildly annoying. Couldn't WO have specified the sizes used somewhere in the meager documentation?
Overall, I like it. Fit and finish are exemplary. It's very easy to use. And packaging is stellar (forgive the pun). I'm impressed. And I'm a jaded photographer with a history of 60 years of photographic equipment under my belt, including high-end gear from Contax, Hasselblad, Leica, Linhof, Rollei and Schneider as well as all the used and abused Japanese brands. I don't impress easily.
Just my two cents....