Nebulas term: APO

What criteria has to be met to qualify a scope to fall under the category of "APO"? Fluorite, ED Glass? My TeleVue Genesis 500mm. f/5 is in a gray area... Achro?? Does a scope have to resolve a certain wave length to qualify? Maybe less false color on bright objects?? Thank you, John M

>>>What criteria has to be met to qualify a scope to fall under the category of "APO"? Fluorite, ED Glass? My TeleVue Genesis 500mm. f/5 is in a gray area... Achro?? Does a scope have to resolve a certain wave length to qualify? Maybe less false color on bright objects?? Thank you, John M
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John:

APO is not a "nebulus" term and has a fairly strict definition. There are many who post here who are far more qualified to comment on this including Roland Christen of Astro-Physics and Roger Ceragioli. Both these guys (as well as many others) know this stuff cold...


Anyway, Apochromatic means that the objective is capable of bringing three different (widely spaced) colors to focus, ie "three color crossings." It also must be corrected for "spherical aberration" at 2 places though I believe most only achieve one...

An Achromat brings two colors to focus.

This definition does not discuss how this is achieved, the glass used, the number of elements etc. Modern Apochromatic scopes use ED or Fluorite glass in combination with other elements to achieve the three crossings but simply because a scope used ED glass or has three elements does not make it an Apochromatic scope.

Jon

Thanks Bill & Jon... for your thoughts.. JM