After reading Jeff Beish's paper on Newtonian Design Calculations (supplied by Alpal), I have run the Contrast Factor calculations for each of my Newts:
With a theoretical perfect score of 5.25 (assuming zero obstruction - admittedly not going to happen with a newt anyway):
MN190, with 0.26 obstruction ratio, gives a Contrast Factor of 2.5168.
10" Imaging Newt, with 0.25 obstruction ratio, gives a Contrast Factor of 2.6412
16" Dob, with a 0.216 obstruction ratio, gives a Contrast Factor of 3.0773.
What can be shown from these numbers:
Firstly, the two imaging scopes have larger secondary obstructions due to the fact that they are imaging scopes and thus run larger secondary mirrors than what is required for a purely visual setup as in the 16" dob.
Secondly, using the Central Spot Energy table it seems that over 70% of the energy is concentrated in the centre spot for all these scopes. Theoretical maximum Central Spot Energy is 84% but again that would be with no obstruction, which no newt can deliver.
Thirdly, the MN190 and 10" Imaging Newt are very close in terms of contrast factor but very different in terms of diffraction - something not addressed in Beish's paper. The 10" newt has a traditional spider holding the secondary where as the MN190 has no spider and thus no diffraction spikes (beause the secondary is mounted on the rear of the corrector plate).
Finally, I am not sure how Spherical and Parabolic primaries affect the image and contrast, if at all. The MN190 has a spherical primary while the others have traditional parabolic primaries. Spherical abberation is corrected of course by the Corrector plate, and is the typical design used in SCTs and Maks. Parabolics are figured to correct spherical abberation.
Conclusions, well from a visual point of view big dobs are clearly the kings of contrast factor as long as you keep the secondary size under control as you go up in size. However, there would be a question as to how this translates to image quality as conventional imaging newt design recommends larger secondaries to provide the correct spot size for imaging sensors.
It is important to remember that these calculation are related to
relative obstruction area in the pupil (see the reference paper below), and thus may have limited validity in imaging scope comparisons (especially in comparing imaging against visual scopes).
For those seeking an even more indepth look at central obstruction math here is a link for you that is beyond my aging brain:
http://www.telescope-optics.net/obstruction.htm