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Eyepiece question that puzzles me..


mdstuart
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Some of my eyepieces have quite small lenses on the underside that goes in to the focus tube.

How can you be sure that when you are in focus the whole of the light cone is falling on glass. Do some eyepiece designs result in light hitting the surround rather than the lens and hence light loss?:D

Mark

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My understanding is that you move the focuser till the light is entering the glass of the magnifying lens at the focal point. If some of the cone at that point is outside the magnifying lens diameter (eg looking at the moon) then you only see part of the object (eg close up of a moon crater rather then the whole moon).

I could be wrong though - maybe someone can explain it better :D

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The focused image from the objective is tiny, the EP is used to examine this image, much like a magnifying glass is used. The higher the magnification (shorter the focal length of the EP) the less of the focused objective image is shown in the FOV.

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The small field lens on high power eyepieces limits the field of view.

You can appreciate the need to have the optics well aligned and collimated when you look at the size of the lens in say a 4mm eyepiece; not much of a target area to get precisely into focus on the optical axis.

Ken

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Great question, and one I've always been afraid to ask!!

The small field lens on high power eyepieces limits the field of view.

Which also results in light loss? Is this why higher mag views are dimmer?

Andrew

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Which also results in light loss? Is this why higher mag views are dimmer?

No. Higher mag views are dimmer because you are spreading the light out over more area (receptors in your eye). The eyepiece should let the full light cone from the telescope though irrespective of focal length -- whether it actually does or not comes down to the design of course :D

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In general terms, the light cone diameter of a F5 telescope for instance, will be 1" if 5" inside focus. Again generally, an eyepiece will focus on the focal plane of the objective at a distance similar to that of the eyepiece focl length. In the case of a 25mm eyepiece this will be around 1" inside the focal plane where the cone diameter is only 1/5" so no vignetting of the objective. This obviously gets tighter with very short focus eyepieces but most modern designs have reasonable sized field lenses. Longer than F5, the potential problem is much less.

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If the field lens is right up close to the focal plane this is not even necessary. Suppose I have a 5mm EP with 60 deg AFOV, and an F/5 scope. The field stop then needs to be 5mm. If the field lens is 2 mm from the field stop it needs to have a free aperture of 5 mm + 2mm/5 = 5.4mm. If it is 5mm from the field stop, it needs to be 5mm + 5mm/5 = 6mm. For a 2.5mm EP which is just a scaled version of the above, you would arrive at 2.7 and 3 mm respectively

Edited by michael.h.f.wilkinson
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