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2" or 1.25" wide angle

Started by Paultricounty, 02/14/2003 06:02PM
Posted 02/14/2003 06:02PM Opening Post


This may be a stupid question, but I will go for it anyway because a local camera/telescope store had answered this question I believe in error. Example in one scope one uses a 1.25” diagonal and a 1.25” eyepiece with lets say 82 degree AFOV, and another scope {identical scopes} with 2” diagonal and 2” eyepiece with 82 degree AFOV, {both eyepieces with same FL} I say the true field of view is the same. The dealer says the port hole effect will be greater with the 2” diag combo. If true field is true field then it should not make a difference. If I am right then why use 2” eyepieces in certain focal lengths that are available in 1.25”?

Paul
Posted 02/14/2003 08:36PM #1
Paul,

You are correct, the true field will be the same for an eyepiece of the same specifications (FL, design, field stop, etc.) regardless of whether it has a 1.25" or 2" barrel. The 2 inch barrel is more convienient to use if you have a bunch of other 2 inch eyepieces and don't want to fiddle with adapters for 1.25" eyepieces in a 2" diagonal (you could also just leave an adapter on each 1.25" eyepiece). The only time a 2" eyepiece is really required is when you need a large AFOV and long focal length (ex: naglers 17mm and up).

Joplin
Posted 02/15/2003 09:48PM #2
I believe that you are referring to Nagler eyepieces with a hybrid 1.25"/2" barrel. This is a feature designed for safety and convenience and has nothing to do with optics. Using a focal length of less than 17mm in a 2" format yields no optical advantage.

Dave Mitsky

Chance favors the prepared mind.

De gustibus non est disputandum.
Posted 02/16/2003 05:09AM #3
What EPs are you comparing?

The key question is what is the size of the field stop in each EP, data that not all manufacturers supply, but you can find it for Televue EPs on their website.

For instance, you can check out the new 7mm T6 Nagler and see that it's field stop is a bit larger than the older version. Both are the same focal length, both have apparent fields of 82 degrees (which is probably just a nominal value) but the old version will probably show a bit less sky than the newer one.

One point is that apparent field is not that good a datum, since it depends on assumptions about eye-relief and where the observer's eye is located. Apparent fields get wider as your eye gets closer, and this gives manufacturers room to fudge their claims about apparent field a little.

I think it's certain that there could be differences in true field between the two EPs you mention, but unlikely that they would be huge differences.

Check out the field stop sizes. If you can't get the info, maybe you could compare the EPs for true field by looking out the window. That's the one advantage of buying at a store, you can get your hands on the stuff you are thinking about buying.

Steve