|
David, you seem to be misunderstanding the differences between prism and mirror diagonals. A mirror diagonal is a single surface mirror that reflects the maximum amount of light possible. A prism, however, is a three surface item. Light enters the prism, and some light is lost being reflected off the first surface. It then strikes the 'diagonal' surface, and goes out through a third surface, where again some more light is lost by being reflected back. While optical coatings have reduced these reflections, prism diagonals are not as efficient as mirror diagonals. Prism diagonals are now not sought after by people who want the best possible light transmission.
There a few things you need to understand about eyepieces and what the limitations are of the barrel size between 1.25", 2" and 3". Yes, there are eyepieces with a 3" barrel. There are eyepieces with a 4.5" barrel too.
Take two straws, same length, but one is wider in diameter than the other. If you look through one, and then the other, you will notice that you will have a wider field of view with the larger diameter one. This is the same with eyepieces. The maximum true field of view offered by a 1.25" eyepiece will be smaller than a 2". This maximum is seen when one looks at the longest focal length possible for a barrel size and the apparent field of view (AFOV). The longest focal length in a 1.25" barrel size that will give 68deg AFOV is just 24mm. In the 2" format, this is 42mm.
Of course, there are longer focal lengths in these formats. There are 32mm and even 40mm 1.25" eyepieces, but these won't show any more of the sky than a 24mm 68deg eyepiece. With each increase in focal length though, the AFOV gets smaller and smaller. Same happens with 2" eyepieces.
Now, things don't stop here! With a C8, the limiting factor is not a 2" barrel, but the diameter of the bore in the primary mirror. It is actually smaller than the maximum diameter possible with a 2" barrel. This means that the maximum true field of view will be a little smaller in a C8.
That 56mm eyepiece will work well in your C8. The very edge of the field of view may not be very sharp, but it is upto you if this is acceptable or not. It's not like you will be looking through a brick. It will actually throw up a very nice image. But it will only perform at its best from a dark site. It is not an eyepiece for light polluted skies. This is common to very long focal length eyepieces - they concentrate the ambient light pollution too, not just the stars.
|