Tim LeVane said:
Are scopes with ED glass apos or just really good achromats?
The scope market seems to be flooded with terms from semi APO to semi ED.
Newcomers such as myself are over whelmed when trying to make an informed choice on what to buy.
It is confusing, isn't it? Ideally, the company should provide you with a graph showing the variation of focus with wavelength (chromatic focal shift). A typical achromat has a focal shift between green and the common blue and red focus of about 1 part in 2000 or so. If a maker or seller is claiming "ED" or "semi-APO" (a rather loosely used term that would be better buried) the chromatic focal shift should be considerably less.
Sometimes they will say they are using a fluor-crown or true fluorite. If so, you are probably getting better color correction, especially if they specify the fluor-crown is FPL53. Of course, manufacturers can buy these substrates at different quality levels, and the certified ones with high homogenity cost more.
A set of narrow band interference filters could be used to check the focal shift. I have not seen any reviews that do this. This check would easily show the lenses that are simply achromats, but would be difficult to do with the small focal shifts of true APOs.
I'd stick with someone who is willing to provide some measure of the chromatic focal shift, whether it is a graph or simply a number measuring the shift over some specified part of the spectrum. If they give you a song and dance and no data, walk away.
Unfortunately, consumers are often swayed by the promise of a low price and something better, and don't insist on the information they need to make an informed decision.
Clear skies, Alan
