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Number of lenses


dawson
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Why is it (or it may mot be) that we worry about how many surfaces the light travels through or off in the telescope, but when it comes to eye pieces the more lenses it has the better quality it generally is?

I'd have thought the same principe would have applied to the light path wherever it was.

Thanks for any help.

James

(Still very green about all this)

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I'm sure somebody will chip in with the science but, light becomes degraded as it passes from one medium to another e.g. air to glass and back again. Several lens elements constitutes many such transitions so keeping them to a minimum will maintain the quality of light.

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The lenses in a scope do the focusing of light and are coated to reduce the natural reflections within the glass. Good lenses have coatings on both sides and use high quality glass that let as much light as possible through. They also correct, filter, and focus different wavelengths (colours) of light in the same focal plane. Each coating naturally dims the light available by a very tiny percentage if well optimised.


 


Eyepiece lenses do the magnifying of the focused image produced by the scope. But you need more of them to produce a well corrected view at the required magnification and fov. A single lens isn't enough to make all the adjustments of the light needed for a good flat image at the eyepiece cos magnifying always distorts the light - especially at the edges of the fov.


 


I don't know the maths or the physics but the glass is bent one way or another to correct successive distortions through each lens used. They too must have well optimised coatings further reducing the light available. It's the expertise and very fine engineering you pay for when buying something like a TV eyepiece with seven or more lenses that produce incredible views. Especially when giving pin sharp images at 100 degree or more fov's from edge to edge.


 


Personally I don't worry about it, but hope that my laymans understanding helps explain it :)


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Agree with Kim, couldnt put it better myself. A cheap eyepiece with poor quality glass will reduce the brightness however a good quality eyepiece with top glass and coatings will be better. Yes even then there is some degredation but much less

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Guest Tweedledum

You also need to check refraction and refractive index. Each time light enters / exits a different material (or atmosphere) it is bent (refracted) by the qualities of the substance, hence you get multiple lens's to correct the lightpath and to bring the different wavelengths of light together correctly.

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