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Old 07-07-2019, 07:21 AM
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astro744 is offline
Join Date: Apr 2007
Posts: 846
Originally Posted by Atmos View Post

Moving to 2" should give better off-axis views because you're using less of the usable FOV.
Could you please explain this statement. Seems contradictory and I'm not really sure what you mean by it.

With regards to 2" or 1.25" if the field stop diameter and focal length is the same then the size of the barrel makes no difference. Dual skirts exist on some eyepieces for convenience. The eyepiece performs the same in either mode (except for position of the focuser as you either have to rack in or rack out depending on which mode you are using. e.g. 12mm Nagler.

The main advantage of 2" is that the field stop diameter can be up to 46mm and is not resctricted to 27mm as for a 1.25" barrel. This means larger true fields, (more actual sky covered).

Tele Vue have Field Stop (mm) column the following table for their eyepieces.


Any eyepiece with the same field stop diameter gives the same true field for a given telescope and this is not dependent on other parameters.

True field of view = eyepiece field stop diameter telescope focal length x 57.3

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