You can also try using 2" eyepieces with a proper adapter for a greater TFOV even though most Maks use only 1.25" eyepieces. I have a Meade ETX-125 Mak and can attach 2" eyepieces to it using the 27mm. rear photo port. Even with this reduced 27mm opening my 2" 32mm Erfle eyepiece shows a 1.14º (68.4 arc-minute) TFOV ... with no noticeable vignetting at night ... versus a .9º (54 arc-minute) TFOV using a 1.25" 40mm eyepiece. That represents a real increase in the actual viewing area of 62%. Also, the stars remain sharp and pinpoint across the entire FOV without any coma visible! I'm sure with the "right" 2" eyepiece, an even greater TFOV is obtainable.
Check out Siebert Optics 's eyepiece adapter page for more information on using 2" eyepieces on Maksutov scopes:
It isn't visable vignetting that one should be concerned about. It is the loss of aperture that occurs when the mirror has to be moved so far forward that the light cone is cut off by the baffles.
The baffles are so far away that you don't see this as vignetting, but the result is that the rays comeing from the edge of the mirror can no longer even reach the very center of the mirror.
Again, this does not look like classic vignetting, but the apeture can easily be reduced by an inch or more.
How do you know if you are in this state? The easiest way is to defocus a star at the center of the field. Now, start to move the star away from the center of the field and watch the outside edge.
If you immediatly see that the outside of the Fresnel pattern starts to experience a smootly curved "Slice" that gets worse as you move away from the center, then your scope is very likely in a reduced apeture mode. To know how much, you would need to measure it with a laser, but again, depending on the scope, the reduction could be small, or it could be as much as an inch or even more, depending on the scopes exact configuration.
Most small MCTs simply do not tolerate a lot of back focus.
And by the way, some of the imported MCTs are already working at reduced apeture (118mm vs the 127mm that they are sold as) so for these scopes, putting a 2" diagonal may not show additional damage because the systems were already improperly designed even for a 1.25" eyepecie use.
My point though is that seeing vignetting is not the main issue. Having the apeture reduce is the main danger.
If you are going to try to turn a scope that was not designed into a wide field scope at the expense of reducing its apeture, why not just buy a smaller wide field scope? In the end, it will not only give wider fields, but will almost always have a better illuminated field.
Another test once can do is to start with a very dim cluster at the center of the field. Find the faintest stars in the cluster.
Now, drift the cluster to the edge of the field and see how many of those stars disappear before they get to the field stop.
Some might say that they only care about what is at the center of the field, but if this is the case, they why bother with the wider field to begin with? My personal belief is that the best views are when the field has all of the stars around the target shown as well as the target item. This gives "Contest" to the target. The target appears in "Space" rather than as just something in the middle of nowhwere.
That is of course a highly personal preference, but the more I started understanding howbadly poor off axis illumination was affecting my enjoyment, I stopped using configurations that damaged this characterist.
The most serious performance hit though when pushing a scope with baffles beyond the design limit is the aperture and contrast loss that starts to occur.