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I Need The Maths Please


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Hi All,

I've just been reading the other topic about 'wider field eyepieces' and this has sparked my interest.

But what I would like to know is how you work all this out?

I know all about my scope, but how do I work out what field of view an EP will give me?

It's all confusing - an EP can be 57 degree or 70 degree etc, as well as it's diameter and size.

Is there a formula (or more than one) where I can work all this out from the bits of info I have?

eg. What EPs (and AFoV) will give me 1 degree TFoV with an f10 8" 2000mm?

Yours Roy

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OK so now I`LL be picky :wink::D

it`s not really much use working back from true FOV to eyepiece (you would need lot`s of eyepieces)

better to know the true FOV of the eyepieces you`ve already got.

Still if you REALLY need a certain FOV maybe a zoom eyepiece would be best? or work through the listed eyepices until you get your req FOV and then buy that one

Mike

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Hi Roy

You say you're interested in the maths - here's how it works! :shock:

If we use:

T = focal length of telescope

E = focal length of eyepiece

A = apparent field of view

C = calculated field of view

M = magnification for given eyepiece

The magnification for a given eyepiece, M = T/E

The true field of view of a given eyepiece is then calculated by C = A/M or C = (A x E)/T

Thus, for a given calculated field of view, E = (C x T) / A

Now, obviously there are an infinite number of solutions to this, since an eyepiece is defined by both its focal length (E) and apparent FOV (A)

But, if you know the A value for a set of eyepieces, you can easily calculate the appropriate E.

e.g. What EPs (and AFoV) will give me 1 degree TFoV with an f10 8" 2000mm?

T = 2000

C = 1

E = (C x T) / A = (1 x 2000) / A

so, if we assume A = 57, E = 2000 / 57 = 35mm (approx)

HTH

Trev

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AFOV is the apparent field of view of the eyepeice, it lets you compare the FOV eyepieces of the same/ similar focal length will give.

If you don't know the AFOV of an eyepiece, most Plossls are around 54 deg up to the 30mm models which decrease to 34 sih as the 1.25" format becomes the bottleneck.

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