FOV

[Astro Imp]

I have read the sticky on eyepieces and think I followed the gist.

One thing I didn't undrstand, it is often mentioned that eyepicese have a certain FOV, doesn't the focal length of the scope have a bearing on the FOV.

Sorry if this is a silly question but I'm a complete newbie here and there is so much to learn.

Thanks in advance.

Alan.

[Moonshane]

Hi Alan

You are actually correct but only understanding part of the 'picture'.

There's FOV and A(parent)FOV

AFOV is what you see when you hold an eyepiece to your eyeball and up to the ceiling or a wall. This is usually 50 degrees for plossls, 60 degrees, 68 degrees 82 degrees up to 100 degrees for the Ethos etc. This is fixed and is what you refer to as the eyepiece FOV.

FOV 'proper' is a combination of the focal length of the telescope and the AFOV and is effectively the amount of sky you fit into the view when looking through your scope. A scope with a longer focal length will result in less FOV than one with a shorter tube when using the same eyepiece. An eyepiece with a larger AFOV will give more field of view than an eyepiece with a smaller AFOV in the same scope.

Some examples:

Focal Length of scope = 500mm

Two eyepieces - 25mm with AFOV of 50 degrees = FOV of 2.5 degrees

and 25mm with AFOV of 82 degrees = FOV of 4.1 degrees - magnification in both cases = 20x

Focal Length of scope = 1000mm

Two eyepieces - 25mm with AFOV of 50 degrees = FOV of 1.25 and 25mm with AFOV of 82 degrees = FOV of 2.05 degrees - magnification in both cases = 40x

As you see focal length of the scope also affects magnification.

FOV = (AFOV x focal length of eyepiece) divided by focal length of telescope (this is technically not quite the calculation and a little out but close enough)

Magnification = focal length of telescope divided by focal length of eyepiece

Focal ratio of telescope = focal length of telescope divided by aperture of telescope

Exit pupil = focal length of eyepiece divided by focal ratio of telescope

Hope this helps.

Cheers

Shane

[Astro Imp]

Great answer.

Thanks for clearing this up.

Alan.