Best ep for planetary viewing.

[cuivenion]

What's the difference between apparent and true field of view?

[John]

What's the difference between apparent and true field of view?

The Apparent Field of View (AFoV) is the figure quoted in the specs of the eyepiece, ie: 50, 60, 70, 80 degrees and so on. The True Field of View (TFoV) is the width of sky, in degrees, that a particular eyepiece will show you when used with a particular scope.

A rough calculation of the true field of view can be obtained by dividing the AFoV of an eyepiece by the magnification that it gives in the scope eg: a 32mm eyepiece with a 52 degree AFoV gives a magnification of 31.25 when used in a scope with a focal length of 1000mm. Divide 52 degrees by 31.25 and you get 1.6 which is the TFoV in degrees that the eyepiece / scope combination gives. Thats about 3x the diameter of the full moon.

[cuivenion]

Thanks .

[bingevader]

Guys thank you very much for all your replies, certainly has given me something to think about.

My other option would be to get a TeleVue 2x PowerMate, that and my 13mm Ethos would give me a 6.5mm ep reducing the AFOV down to 50 degrees (not a problem when using for planetary viewing) and should give me a magnification of 185x, which I will probably use more than those rarer clear nights when 200x mag plus will be possible while still keeping the eye relief.

Hmmmm, going to go for a think over this one.

Thanks once again.

That would definitely be another very good option!   Hopefully, you'd be getting twice the use out of the Ethos too.

This year has been very peculiar with regards to the magic 200x for me. When seeing has been bad and I haven't been able to use the 6mm EP, no focal length seems to have worked on the planets. On good nights, it's been straight back to the 6mm! Very strange.

[Guest]

I have a 6mm Williams Optics SPL eyepiece.Cannot fault it.Eye relief is very good.Nice sharp clear views.