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Different FOV with two 4.8 Naglers

Started by kmorgan, 09/06/2002 08:25AM
Posted 09/06/2002 08:25AM Opening Post
Anyone know of a reason that two different 4.8mm Naglers should have different fields of view? Last night I was out observing with my friend. We were comparing the eyepieces and noticed that my friend's older e.p. had a wider field than mine.

Posted 09/08/2002 11:55AM #1
Hi, Kevin.

Various factors could be at work, here. I'll make one critical assumption:
-the eyepieces' fields were compared in the *same* scope, using the *same* diagonal (if any). Is this correct?

Even "identical" scopes from the same manufacturer can have slightly different focal lengths. Different focal lengths will yield different magnifications with a given eyepiece, and (thusly) different fields of view will result.

Likewise, if the eyepieces themselves have slightly different focal lengths, they will yield different magnifications in a given scope.

Another possibility is that the field stops differ slightly in diameter &/or position. (I.e., their apparent fields of view differ.) You can't directly measure the Naglers' field stops (they're located between elements, within the eyepiece body), but you can look through both eyepieces (like binoculars) and compare the visible circular fields defined by their stops. As the circles begin to merge, you should be able to tell if one of the circles is larger than the other. The eyepiece with the larger circle (field stop) will yield the larger f.o.v. If both circles are the same size, then their apparent fields are also the same.

If the scope used for comparison was a moving-mirror SCT or Maksutov, then yet another wrinkle arises. If the eyepieces aren't exactly parfocal (i.e., if the scope needs to be re-focused when switching from one eyepiece to another), then the resulting effective focal length of the scope is changing --slightly-- for each eyepiece. Hence, different magnifications, different fields of view.

Assuming the same scope/set-up was used for comparison, then the different fields of view you noted were likely caused by differences in the eyepieces themselves (field stops &/or focal lengths). The fact that they're not the same age supports that possibility.

Hope this helps a little. Best wishes.
-Dan