Quote:
Originally Posted by ThunderStorm
Ah...most of 2" eyepiece are 20+mm eyepiece?
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Many, but not all
take a look at the diagram on
this page from Televue
Basically, the field stop is a limiting factor in the cylindrical "beam" of light that flows from the telescope objective to your eye, which limits the maximum field of view of the optical system (note: there can be other factors too), and is determined by considering the focal length of the eyepiece and its AFOV.
For a 1.25" setup (diagonal + eyepiece), the maximum beam size is 27mm across since the physical barrel eats up some space...this is sufficient for most of the shorter focal length eyepieces, and popular designs up to 24mm@68*AFOV (Panoptic and others) and 32mm@50*AFOV (plossls). To cram in more sky than that, it needs a larger barrel...
So we have a 2" setup (diagonal + eyepiece), with the maximum beam size of about 46mm across...this increase in the maximum beam size of about 70% is more significant than it sounds, as it increases the field
area, the circle of sky you see in the eyepiece, by almost 3x (actually, 2.9x). There are a lot of eyepieces out there that take advantage of the extra sky "real estate" available. From the 20mm 82 degree designs upwards, all the way through to the 40/41mm 68-70 degree AFOV (Panoptic/Paragon/Pentax/etc) and the 55mm plossls.
To confuse the issue further, some eyepieces have a 2" barrel for physical reasons, generally when they're big and heavy and the maker feels they'd be precarious in a 1.25" focusser/diagonal ... ES 100 degree range is an example here where the whole range have a 2" barrel, even down to the 5.5mm model!