I wouldn't mind spending a little time with PS to swallow those donuts. But the IQ just doesn't seems to get me aroused (correct me if I am wrong). My Tamron SP500 Mirror been sleeping the last 6 months. About time I find him a new home.
Camera specific mirror lenses and astronomical Cassegrain style mirror telescopes aren't in the same league optically. While I agree that mirror lenses tend to leave much to be desired with regards to resolution, almost every astronomical Maksutov Cassegrain built in the last 30 years is 1/3 wave lambda or better, which means far higher surface polish quality than even the best professional grade camera lenses.
Additionally, and I'm actually not sure of the exact measurements, but looking at an image of a lens such as the Tammy SP-500, it appears that the central obstruction is close to 40-45% of the objective diameter. That much obstruction is contrast
and resolution robbing. Mak Cass scopes seldom go above 35%, and some as low as 25%. With smaller sizes like this resolution is less impacted. While contrast may remain lackluster, we thankfully have plenty of software options to help correct this.
Touching on the comment by Tom (tjd) regarding glare and internal reflection, there in some validity. However a simple dew shield will remove most of the oblique angle daylight, thus improving contrast significantly.
I think the image is good, and that Rick's idea of passing this along for interested low budget birders is a valid one. Paul is correct regarding the donut shaped bokeh, obviously a product of the secondary mirror in the light path. It does make for some occasionally unpleasant bg's, but doesn't take away from use as a life list lens, or simply for learning long lens technique. Best of all is that you can just throw a diagonal and eyepiece in the back for use as a spotting scope.
Obviously no lens does everything perfectly, and every astronomical scope used for birding has it's own unique limitations. But for size, weight, resolution, and ease of use, I'd say small (90-130mm) Mak Cass scopes are a great option for starting birders.
Good post Rick. Thanks.