This is a question about my secondary mirror and the Mel's calculator.
Starting this thread following Don's suggestion to do so, good idea.
For your diagonal, a 2.1" may still be good for 12" instead of 12.5". Others may have better supported opinions to go one way or another. If you start a thread just for that, those who know most might reply. Here is a screenshot form Mel's calculator showing both. The 9.5" distance from diagonal to focal plane might be the deciding factor.
This is a screenshot of the calculator with my settings + a GSO focuser Crayford, my first choice of focuser.
My objective is to have a general purpose telescope good on Planets and DSOs, for visual only and not really for tasks requiring a very large fully illuminated field such as photography and variable star observing.
BUT I don't want to notice any dimming at the outer edge of the field while observing.
Which secondary would you recommend between the 2.14 and 2.6 minor axis for my project?
That depends on the intercept distance (secondary to focal plane) and the size of the field in your lowest power eyepiece.
One way to use Mel's calculator is to enter the fieldstop of your lowest power eyepiece in the illuminated field size (e.g. 31mm Nagler = 1.65"), and select the diagonal size that doesn't descend below -0.4 magnitudes at the edge of the field shown.
As to what the intercept distance is, it is the sum of:
--clearance to inner tube wall
--thickness of tube
--height of base of focuser off the tube
--height of focuser racked all the way in
--1" additional to focal plane to allow things like barlows and coma correctors to come to focus and have nearsighted friends look through the scope.
So, take a 12" scope. It is hard to see an intercept distance in a properly designed scope will be less than about 114mm.
As a rule of thumb, I always add 4.5" (114mm) to the mirror radius.
A minimum of 19mm clearance between mirror and tube to keep upper opening from vignetting the incoming light and allow ventilation of the tube without significant air currents in the optical path.
A usual 25mm between the base of the focuser and the I.D. of the upper tube assembly to allow for the focuser board and ring and to keep the drawtube out of the optical path of the scope.
A low profile focuser 45mm tall. In reality, many are taller.
A focal plane clearance of 25mm over the focuser.
If any of those numbers is larger than that, then you will add length to the intercept distance. If the scope is used for photography, the secondary is larger (as it is in your example).
A purely visual 12" f/5 scope would be fine with a much smaller 66mm secondary, but that would assume a shorter intercept distance.
So, plugging in the numbers, and keeping the edge of field light loss to < 0.4 magnitudes,