30mm vs 40mm for 1.25" barrel size

[singlespeeder]

Hi Everyone,

I'm thinking about adding in a low-magnification eyepiece for DSO viewing for my scopes and initially thought about a 40mm focal length but then I get confused when looking around.  A lot of the longer focal length EPs are for 2" barrel sizes and indeed on FLO they recommend:

"30 and 40mm:  due to the 1.25" barrel size the 30 and 40mm eyepieces show almost the same area of sky so, for visual use, the 30mm is the better choice.  The 40mm is however useful when a compact camera is positioned over the eyepiece for afocal photography."

Do many people do afocal photography?  I'm now thinking 30mm is the way to go.

Your views?

Adam

[John]

The 1.25" barrel size does indeed constrain the field of view so even longer focal length eyepieces can't show any more sky despite providing lower magnification. Thats the main advantage of the 2" eyepiece size - the field of view can be around 45% wider than a 1.25" eyepiece of the same focal length can provide.

In the 1.25" format the 32mm plossl gives an apparent field of 50 / 52 degrees which, to my eye, is still a pleasant perspective. The 40mm's in the 1.25" fitting max out at 43 degrees which seems somewhat narrower and rather like looking down a well !

I don't do photography throuhg the scope I'm afaid so I can't help there.

[kurtz9]

Personally if I couldn't use a 2" EP the longest FL I'd probably have is 24mm.... a 68 degree AFOV 24mm like the ES68-24 is about the maximum field you can get in a 1.25" EP anyway, so why lower the magnification if you won't fit any more sky in?

[ronin]

At the 30-40mm size you are talking of a plossl. Owing to the restriction by the barrel (field stop) the final field of view you get is almost exactly the same.

Owing to the percentage increase and decrease something like the 25mm BST or X-Cel will give the same field as the 30mm plossl.

So as far as makes little difference:
40mm plossl = 30mm plossl = 25mm BST or X-Cel.

I think the widest you can get is the Antares W70 at 25mm, not however a common eyepiece. I am only aware of it as I have a W70 4.3mm for use on a small scope.

As to the afocal imaging, I would suggest you seperate out viewing and afocal otherwise one or both will be compromised to an extent.