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Hi Chris,
As Malcom mentioned, 2" eyepieces offer a wider field of view in the longer focal lengths. This is first due to the size of the barrel. The narrower the barrel, the more resticted the field size is. The second reason is the achieveable field of view the EP has.
As an example in the 1.25", take the Plossl design. It's typical field of view is 50degrees. With this barrel size, the longest focal length it can have to achieve the maximum true field size is around 32mm. You can get longer focal length Plossls, but the field size you'll see through it is actually smaller when compared with the 32mm. The amount of sky you'll see is actually the exact same, just the image size is smaller.
2" EPs do offer a greater amount of sky that can be seen, but there is a limit there too. That is why there are also 3" and even 4" eyepieces!
There is also another limiting factor if you use a cassegrain optical system - the size of the bore through the primary. If it is equal to or smaller than the inside diameter of a metal eyepiece barrel, your field of view will also be limited as this bore acts as a quasi barrel in front of the actual EP barrel.
Just going back once more to the second factor of the achieveable field of view (FOV) of an eyepiece. As I mentioned, the typical Plossl has a size of FOV of 50degrees. There are EP designs that can go all the way to 120degrees. But, all these fields that are achieveable by various designs are still limited to a maximum amount of sky that the barrel will allow. That is why 68degree EPs in the 1.25" barrel have a maximum focal length of 24mm. 80degree EPs, again in 1.25", 16mm, and 100degree is 13mm. Yet all these EPs will also show the exact same amount of sky.
In the 2" barrel size, the longest focal length showing the maximum amount of sky allowable in the 2" barrel, for the 68degree is 41mm, 80degree is 31mm and 100 degree is 21mm. Approximately. There is also another factor, the 'field stop' of an EP, but for this post I won't go there for now.
One last word on EP design. Plossls were the bees-knees in EP design back in the 70's. They were at the time the best quality available. Today, this optical design is average in quality, and the cost of production is bugger all for cheap ones. As mentioned, there are limitations in usablility with the Plossl design that newer designs using newer exotic glass types are able to overcome and also provide better quality image. This is so also the case for contemporary 'cheap' eyepieces. There are limits, of cause, to what a cheaper EP can do versus a more expensive one, but that ends up coming to a judgement call in personal preference. I, for one, do not subscribe to the 'expensive only' circus. I understand the limitations of various EP designs, scope designs and human eye capabilities, and am very comfortable with my modest EPs, and am very productive with them.
If you can, get to a star party and take your scope along. There I'm sure you'd be able to borrow a few eyepieces to use in your scope. Try to chase down EPs of both the cheaper and more expensive types. You'll be able to make a more informed opinion on where you'd like to go EP wise.
Mental.
Last edited by mental4astro; 18-10-2011 at 10:14 AM.
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. Mental, you've cleared some things up in my head............muchly appreciated!!!
