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The easiest way to determine the optimal longest focal length eyepiece for a reflector is using the following formula:
longest f.l. eyepiece = f/ratio of scope X 6mm
Therefore with your 6" dob, its focal ratio, say is f/6, then the longest focal length is 36mm. At a stretch a 40mm could be used as this focal length is more common than 36mm.
The reason for not going any longer is that the shadow of the secondary mirror starts to become too obvious and seriously degrades the image.
Quality wise, the first determining factor is the size focuser you have. There is a physical limit to how wide a field of view you can get with a 1.25" EP (eyepiece) in the longer focal lengths. This being determined by the size of the barrel of the eyepiece fitting into the focuser. Hence you will find really wide angle eyepiece (greater than 65 degrees) only in the 2" format in anything over 30mm.
The second determining factor is human eye physiology. Being blessed with human eyes, we have a limiting field of view of 68* until we need to start to move our head to see more. An 80* FOV is enormous, and 100*, well, that is in the relm of birds of prey and antelops.
The third factor is the focal ratio of the scope. 'Fast' scopes, those with a focal ratio typically smaller than f/6, like f/5 and f/4, begin to show an artifact called "coma". This manifests as the stars along the edge of the field of view showing little comet like tails. It is not a defect in either the scope's mirror or the eyepiece being used. It is a result of the eyepiece not being able to deal well with the 'stumpy' cone of light the primary mirror forms, resulting in the eyepiece being incapable of focusing all the light at the edge of its FOV all at the one point. This can be over come to an extent with the use of 'coma correctors', but this adds to the weigth and complexity of using a scope, not to mention balance issues.
The forth factor is $$$. If this is not a problem, then ignore the first three factors.
If $ are a factor, then keep in mind that there are many exceedingly good eyepieces out there that can perform just as well, and fulfill the complexities of the first three factors listed.
To this end I would suggest you look into the TMB Paragon 40mm (my first preference, $300 new in OZ, I've used its 30mm sibling and I want one), and the GSO Superview 42mm (second pref., $60 at Andrews Communications, I have the 30mm version). Both EPs have a 68* FOV, though the later EP is not as good for coma correcton and does suffer from astigmatism.
I must admit that an 82* FOV is amazing. However, it is not for ME. I've used one in my f/4.5 scope, and found it uncomfortable to use, and without a coma corrector, this really wide FOV is not done justice.
Try an 82* and a 68* FOV eyepieces. See what feels most comfortable to use for YOU. A Nagler EP is at least twice the price of your 6" dob.
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