FAQ - APO and ED Definition

[funoooo]

Refer to God Mirror comment:
________________________________________
Dear Mr. BingSze,

No, by modern layman definition, your telescope is an APO telescope.

Originally APO means " Apochromatic objective lens" - it means a lens capable of bringing more than 2 colors into a common focus. Only triplet lens can finish such a job.

Strictly speaking ED doublet only brings 2 colors into a common focus. We should describe ED doublet as "Achromatic" instead of "Apochromatic". However modern SMALL ( smaller than 6 inch aperture ) ED doublet exhibits such a small residue color that people starts to use the term "apochromatic" even in ED doublet. The term "Achromatic" is only left to describe classical crown-flint doublet.

Hence in layman terms, your telescope is an apochromatic telescope. And, your telescope, I belief, is exactly the one which appears in the German test report.

Once again, congratulation.

Best regards
Chan Yuk Lun