# what is the magnification of a telescope without any eyepiece

Started 7 months ago | Questions thread
Re: what is the magnification of a telescope without any eyepiece

Metastro wrote:

Hi

I'm a bit confused and trying to work out how people use barlow lens' with telescopes

if magnification = Telescope fl / eyepiece fl

then a 2000mm fl telescope with no eyepiece would be:

2000 / 0 = undefined

Would you implicitally say 1 instead of 0, as the formula requires an eyepiece? then it resolves as 2000 / 1 = 2000x magnification, 2000mm / 20mm = 100x magnification etc

My actual question if you please consider this scenario:

You have a telescope, a x2 barlow lens and a camera, with no eyepiece (as you are imaging and not observing)

Telescope FL 2000mm -> x2 barlow lens -> Camera

If I'm not mistaken this would resolve as 4000x magnification

N0, it's 4000mm "FOCAL LENGTH". Using  the generic 50mm =1X estimate,  4000/50=80X

The max useful magnification is apparently 2.5x aperature in mm.

So a 200mm aperature, 2000mm fl scope, with x2 barlow, would be approximately 4 times the maximum useful magnification.

I am adament I have made several mistakes and incorrect assumptions, but given the above, I cannot see how, where or why anyone would use a barlow lens. Eyepieces are for visual observation, hense me removing them from the scenario when referring to imaging.

Thanks

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