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One thing you need to understand about eyepieces is how barrel diameter affects things.
Take two straws, same length but one larger diameter than the other. Looking through each, you'll notice that you see a wider field through the larger diameter than the smaller one. With eyepieces it is the same. There comes a point in eyepiece focal length that you just cannot go any wider in the True Field of View. You can go longer in focal length, but the AFOV just gets smaller, and the true field of view stays the same.
There is another thing (a few more actually, but one at a time), and that is as you increase foacal length you also reduce contrast in the resulting image. This means you start to battle with the background sky glow. Increasing magnification also increases contrast that can help us see fainter things.
Now, barrel size vs AFOV. As we now know that we can only see a limited amount through a straw, it also means that the widest AFOV is also limited. You cannot get a 40mm 1.25" EP with an AFOV of 68°. The longest focal length that gives an AFOV of 68° in 1.25"barrel is 24mm.
Plossls typically have an AFOV of 52°. But there is also a limit to this. The longest focal length that will yield an AFOV of 52° in 1.25" barrel is 32mm. Longer focal lengths only give smaller AFOV.
While at first it may seem like a good deal, just be aware of the limitations. You need to decide if it really suits your requirements. You may be better off with a 32mm plossl.
2" eyepieces have the same limitations, allbeit the TFOV is wider. The longest focal length giving a 68° AFOV is 42mm here.
Note also, that as you increase AFOV you reduce the maximum focal length shortens:
82° AFOV in 1.25" is approx. 16mm
82° in 2" is approx. 31mm
100° in 1.25" is approx. 13mm
100° in 2" is approx. 21mm
Having said all this, there still is a place for longer focal lengths. Certain EP designs in a given scope design can yield an image with fewer aberrations. You also need to be careful here to as there can be other consequences to this too.
Alex.
Last edited by mental4astro; 04-01-2015 at 04:22 PM.
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