Hi:
First...
Since this is a beginners forum, I am going to discuss this at a level that seems appropriate for someone who is a newbie.
There are other coma correctors out there such as the Lumicon but it is my understanding that they are less flexible/adaptable than the Paracorr and require significantly more focuser travel. Visually the Paracorr is you best bet.
One of my favorite personal scopes is a 12.5 inch F4.06 Discovery DOB, I have been dealing with many of these issues for several years...
Some thoughts/experiences:
1. In a Newtonian one usually thinks of the illuminated field of view rather than Vignetting. Vignetting is a darkening of the field caused by some restriction in the optical path and can be fairly sudden where as the illuminated field drops off slowly.
The level of the illumination at any radius from the center is a function of several factors, tube diameter, secondary diameter, focuser height.
My Discovery with a 3.1 inch secondary has 100% illuminated field of view of about 3/4 of a degree, the 75% level is at 2.2 degrees, greater than is possible with a 2 inch eyepiece. For deep sky visual work, these are good numbers.
These numbers have been derived using Dale Kellner's wonderful Newt for Windows, freeware program.
http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm
My point here is that even though one may see the entire primary from a given focuser height, that does not mean the field of view is fully illuminated, normally moving things a 1/2in or so changes the illumination levels somewhat but not drastically. If you haven't done it already, I would download and run Newt for Windows.
2. A Paracorr is a nice addition to a fast Newtonian but there are two things to be aware of:
It increases the focal length by about 15% which reduces the field of view for any given eyepiece by about 15%.
A fast Newtonian, and F4 is very fast, requires rather expensive eyepieces to achieve good edge sharpness. It is my experience that much of what is often identified as "coma" is actually eyepiece astigmatism and other eyepiece aberrations that result from the very steep light cone of a fast scope. To get the clean views edge to edge in an F4 Newtonian, one needs not only a Paracorr but some expensive eyepieces as well.
For the best views this means Nagler which means lotta money but Panoptics and some others work reasonably well.
3. An F4 scope with a Paracorr is operating at F/4.6 This means any eyepiece with a focal length longer than 4.6 x 7mm (exit pupil)=32.2 will produce an exit pupil larger than 7mm and the full aperture of the scope will not be utilized.
4. Paracorr's have some range of adjustability, for an F4 scope, it will probably live full time in the focuser so moving the primary mirror to achieve focus is a good solution. One also has to be concerned with the position of the focal plane of the eyepiece, some eyepieces like UO orthos require lots of "in" focus, many TV eyepieces seem to require significant back focus.
5. Last but not least... Getting the most out of an F4 scope takes some doing. Collimation is critical, cooldown is important, and good eyepieces are important.
But... an 12.5 inch F4 Newtonian with good optics that are cooled down and collimated, used with a Paracorr and appropriate eyepieces, will provide some truly amazing views. This is not only at low magnifications where its widefield performance can show big fields of view, but also at high magnification (200x, 300x and greater) where it can provide nice bright views that are sharp from edge to edge.
Best wishes
jon isaacs
