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80mm f/15 GOTO Apochromat. Yes Apochromat

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#26 Astrojensen


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Posted 23 September 2019 - 03:25 PM

Zeiss made long focus triplet ‘apos’ a hundred years ago without using special dispersion or low dispersion glass. Those lenses were also notoriously hard to collimate. 

Quite correct, except the glass used WAS special dispersion glass, some of it was highly abnormal for its time. And yes, they were incredibly sensitive to collimation, because their internal curves were very strong. The spherical correction of the lenses can change visibly, if the spacing is changed by as little as 1/100th of a millimeter! Corrosion of the spacers often leads to one or both air spaces becoming too thin. These objectives should NOT rattle in their cells!! If they do, the spacers have corroded and become too thin and must be replaced. A daunting task, if one does not have extensive knowledge of optics, both practical and theoretical. 


Zeiss themselves admitted, that these objectives (triplet B type) were so sensitive, they really weren't suitable for amateurs, but should be considered as highly sensitive scientific measuring devices! 


The doublet A type by comparison is much easier to work with and in practice, in smaller apertures, is free from visible false color. Its major downside is that it can't be made much faster than about f/17, while the B triplet was commonly f/15 and often f/12. 



Clear skies!
Thomas, Denmark

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#27 Terra Nova

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Posted 23 September 2019 - 10:30 PM

Thomas, can you share some information with us on the actual composition or type of melt Zeiss used in the three elements of the B objective. Was it a fluorite derivative? I had always assumed they used various borosilicates or other leaded fused silicates that were available at that time in the elements and have never found a source stating the actual composition. It was my assumption that fluorite glasses didn’t become available until the 1920s.

#28 luxo II

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Posted 24 September 2019 - 01:19 AM

It’s... complicated ... http://www.astrozeis...jectives_1.html

Then this thread suggests there may have been some variability in the quality of the colour correction https://www.cloudyni...in-1920s-zeiss/

Several other old threads are worth a read, if interested.

It all suggests to me Zeiss had some excellent designs to start with and were probably tailoring these designs to the specific melts they had to work with in production - and they knew how. Results being very good to excellent as one would expect.

Fluorite was not used, that much is evident.

Edited by luxo II, 24 September 2019 - 07:56 AM.

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