Using a 2inch eyepiece in a 1.25 via adapter

[Chiefwiggum]

Hi guys new to all this would it be possible to use a 2inch eyepiece in a 1.25 focuser via an adapter? The scope is a 10inch dob 1250 focal length, the only reason I ask is because I sometimes see a great eyepiece used for a great price but they are 2inch, let's say a wide fov eyepiece , would the fov be affected and would the image still fill the whole eyepiece , I know the field stop in a 1.25 is 27mm max apparently but could someone show me the formular to work out the total fov I would see, and has anyone used a 2inch via adapter in a 1.25 or is it a waste of time , thanks guys

[janoskiss (Steve H)]

There is little point. with an adapter the FOV of a 2" EP in a 1.25" focusser will be even more limited than that of a 1.25" EP (because the 1.25" barrel of the adapter extends beyond the end of the 2" barrel of the EP). My advice would be to invest in a 2" focusser. GSO Crayford with 10:1 fine focus is good bang for buck.

[Don Pensack]

It is a waste of time because the adapter would heavily vignette the field of the 2" eyepiece.
There are some 1.25" eyepieces with field stops a bit wider than 27mm--the Baader Hyperion 24 (28.5mm), the Baader Eudiascopic 35mm (29mm), and a few others (most discontinued), but the difference is not large.
The formula you seek is:
TF=(FS/TFL)x57.296
TF=True Field in degrees
FS=Field stop in the eyepiece
TFL=Telescope Focal Length