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objective design

Lonewolf4

By Lonewolf4
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michael.h.f.wilkinson

michael.h.f.wilkinson

Posted
Posted

This is a bit of a grey area, and different manufacturers have different ideas. Apochromat was initially only used for triplet designs, in which three wavelengths were perfectly corrected, whereas achromat refers to doublet designs which corrected just two. Confusingly, super-achromat refers to a lens that corrects for four wavelengths, and requires at least 4 lenses. With the advent of ED glass and fluorite lenses, doublets could provide colour corrections close to triplets, and these are sometimes referred to as semi-apochromat, although I have also heard the same term used for certain fast, triplet lenses for binoculars. I assume these may use the triplet design to achieve a compromise between full apochromatic colour correction and controll of achromatic aberrations such as spherical aberration.

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John

John

Posted
Posted

Hi and welcome to the forum :icon_biggrin:

This is a complex topic as the other posters have mentioned. It's possible to read lots of stuff and still not get a great deal of concensus !

This is quite an interesting piece on the subject:

http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/Apo_vs_Achro.html

A definition of apochromat by the late great optical designer Thomas Back:

http://www.csun.edu/~rprovin/tmb/definition.html

Sorry that these still have a degree of complexity within them.

Personally I'm not sure that the term "semi-apochromat" is helpful but thats just my opinion.

Of course newtonian and catadioptric scopes are usually apochromatic as well.

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Tim

Tim

Posted
Posted

Hello, welcome to SGL :)

It's a good question. Like John I dislike the term "Semi-Apochromat", either something is apochromatic, or it isn't....

In the most basic terms an apochromatic telescope will bring 3 wavelengths of light into close focus, so that you dont get colour halos around bright objects at the eyepiece or camera.

Most apochromatic telescopes use 3 lenses to achieve this, but it can also be accomplished effectively with two lenses, using glass such as FPL53.

Regards

Tim

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glowjet

glowjet

Posted
Posted

Hi Lonewolf and welcome to SGL, there is one other difference between APO and ED objectives and that is you need much deeper pockets for the APO, very nice if you can afford them, but many have been quite pleased with observing and imaging, using scopes fitted with the ED lens system, enjoy the forum :) 

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