I ended up purchasing the tiny Vixen refractor instead of a Canon EF300 for a number of reasons, some of which could even sound reasonable.
Since had troubles at finding informations about it (there are some Japanese reviews, but I can not read the language and google translator is not very reliable...), thought that could be useful to add my little contribution for those who can understand my "pidgin" English.
So, done with the foreword, can start listing what it is included in the box, and what instead I needed.
The fl55 box contain the refractor and a dovetail bar, Vixen-type, so for the purpose of astrophotography it is almost mandatory to purchase one of the "kits" (I chose reducer+flattener); I also puchased an adapter ring for Canon DSLRs, which acts also as angle adjuster: a brilliant solution for small telescopes.
The kit contains the optical elements and an extension tube replacing the one provided with the refractor; the tube ensures that the distances between flattener, reducer and sensor are the right ones; the optical groups are screwed at its ends, and the tube is the screwed to the refractor's drawtube.
Since the flattener protrudes in the drawtube, care is required to avoid scratching the dark matte paint.
Overall think that it is not a bad design (especially if compared to what has been done for my FC100DF), but there are still a few rough spots.
The dovetail bar can be unscrewed, freeing a couple of 1/8" threaded holes and a single 3/8" one; however take note that, with the photographic kit and the eos1100 body, the center of weight is located a couple of centimeters behind the focuser's knobs, so to balance the rig a rather long plate with eccentric screws is required (with my setup and just the flattener the center of weight falls ca 9cm behind the rearmost 1/8" screw); this is a most annoying aspect, since I had to unscrew a knob to be able to balance the refractor on the Astrotrac mount.
Since it is almost full Moon I focused on star testing the refractor, leaving to more favorable times the evaluation of its widefield astrophotographic performances.
To star test the fl55 had to unscrew the eyepiece holder and screw the stock extension tube: an inferior option if compared to Takahashi's quick release system.
I chose Polaris as a target due to its negligible apparent motion and peformed the test though a 4mm Abbe eyepiece and a Baader Solar Continuum filter (green filter peaked at 540nm); later added a 2x and 3x Barlow lens and last removed the extension tube and replied the test with a Baader prism and a TAL mirror stardiagonal to identify the most fitting one.
At prime focus the refractor provided a very good diffraction pattern; then applied a 1.5mm defocus (the value suggested by Suiter for f/6 telescopes, but also the size of my smallest Allen wrench, so I chose this for better reproducibilty) and observed slightly asymmetrical patterns.
I had estimate a value better than 1/6th wavelenght and maybe close to 1/8th wavelelnght for low-order spherical aberration, with patterns consistent with overcorrection.
The FL55SS showed an excellent correction of chromatic aberration, at the same level of the FC100/740 refractor.
At last aimed the little telescope to the Moon, observing our companion at 100x (3x lens, 9mm eyepiece) and 150x (3x lens, 6mm eyepiece).
At 55mm the resolving power is very low, but the views were enjoyable nonetheless.
As it could be guessed, the small refractor performed slightly better with the mirror stardiagonal; therefore, even if do not plan to use it often for stargazing, in such cases will use the 1.25" mirror.