It appears that no one can agree on what a SEMI-Apo telescope is.
If you read any advert doublets (apo ones, ???) such as the Skywatcher ED and Takahashi FS-60 as just two of many hundreds of examples are "APOchromats", and yet there are arguments that a real APO is a triplet.
From other people
One comment is "A semi-apo tends to be a doublet (achro) with "fancy" glass. FPL-51 or FPL-53 generally.
2 wavelengths are defined for the focal length usually Red and Blue but that allows the Green to drift off from the ideal. It is the amount of this "drift" that they claim makes it semi-apo. In an ED scope this difference is small, and hopefully small enough that the eye does not detect it, so it looks CA free."
Another is
"There is no formal definition of a "semi-apochromatic.""
Another comment
"The Red and Blue are at 0 whereas the Green has a difference of about 0.005 as shown
Now if that were for a normal achro then a semi-apo would have a difference of say 0.002 or 0.003, but less then an achro, however still more then an apo which should be Red=Blue=Green, so 0 difference."
"Manufacturers claiming semi-apo are fine, they at least state what you are buying, but there a a few that stated apo for what is a doublet and so cannot be apo.. "
So are manufacturers misleading us or have we become accustomed to believing a doublet with rare glass can be an APO.
Therefore do I have one proper APO and one .......... Semi APO or two APOS
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