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Hi,
Firstly the term "APO" gets bandied around by scope manufacturers and has become degraded as a term.
It actually means all colours come to focus at the same or extremely closely the same point.
A Chinese ED doublet will not be able to comply with that definition. It will be sharp and a pleasant view but it will have some colour on bright objects to one degree or another.
There has been a lot of discussion about this point over the years in refractor Yahoo groups.
Generally the scale for refractors is something like this:
1. Astrophysics , TEC. Takahashi triplet (TOA or TSA series) refractors - true APOs
2. TMB Stellarvue triplets -true APOs
3. Takahashi fluorite doublets
4. Other ED doublets a fair drop down from #3.
5. Achromats.
So a Chinese ED doublet should really be called a semi-APO but of course that does not make good marketing. Not to say it wouldn't be a pleasing scope as 127mm aperture for a refractor is getting very useable.
A Mewlon though has no lens elements so there is no false colour, and the spot size is low near the centre and weakens as it goes off centre.
Below a Mewlon 250 the focus is by moving the primary mirror which is fine for visual but probably a pain for imaging (mirror shift).
I have had several Tak scopes and a mount and they were all fabulous scopes and very haveable. Whereas a Chinese scope is usually great value but a bit rough around the edges.
Greg.
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