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justin138

Eyepiece question

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I have a 40mm 1.25" eyepiece and what I've noticed is that you have to have your eye dead centre in the FOV otherwise everything goes black when you move your head slightly! I compared it to a 25mm which didn't exhibit the same effect.

Is there a name for this? What should I look for in my next eyepiece purchase to avoid this as it's a bit annoying having to keep ones head so still!

Thanks for your patience folks with what is no doubt a silly question!

Justin

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It's called vignetting. the largest field stop in a 1.25" barrel is.... 1.25"! 40mm is larger than this so you get vignetting. It gets worse the wider your field of view. The largest 1.25" 82 degree nagler is 16mm, the largest 68 degree panoptic is 24mm. After these sizes you have to move to 2" eyepeices to get the larger fov

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Thanks Gordon, should have figured that out really. I guess trying to fit 40mm into 31.75mm is pushing it a bit!

(Note to self: post after 10am once brain is engaged and coffee has taken effect!)

Justin

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To be honest i'm not sure why people sell eyepieces larger than 32mm (at 50 degrees) in a 1.25" barrel. You do get reduced magnification and a wider field of view but at the cost of vignetting.

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I thought this phenomena was called 'kidney beaning'.

Isn't vignetting when, no matter where you put your eye, you can't see the whole FOV.?

I have a 40mm EP and wether I use it with a 1 1/4 or 2 ins diagonal I get the full FOV. If I move my eye slightly I get a temporary partial blackout - shaped like a kidney bean. I'm probably wrong but I thought it was more down to the EP's design?

MD

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actually after re-reading the original question you could be right MD, i just picked up that it's a 40mm eyepiece which often show vignetting. If you can see the whole FOV when your eye is dead centre then i'm wrong and MD is right, it's kidney beaning. You can tell if only half of the FOV goes black. Hard to tell without looking through the eyepiece myself though.

I had terrible kidney beaning with the celestron x-cel range of eyepieces and sold the ones i had.

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I have a 40mm EP and wether I use it with a 1 1/4 or 2 ins diagonal I get the full FOV. If I move my eye slightly I get a temporary partial blackout

MD

Exactly what I am experiencing with my 40mm, such a nuisance. Will get a 25mm next time....

Justin

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Sounds like the kidney bean effect.

If your eyepiece suffers from this, eye positioning and distance from eyepiece is critical. Some eyepieces are more prone to kidney beaning than others, some are so well corrected that the effect is almost entirely removed.

I find it occurs most with eyepieces that try to achieve too much for their price - cheap wide-fields seem particularly prone.

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