Let me see if I can explain this simple terms.
When you are looking at the target line with your mark 1 eyeball, you really don't see shimmer. You can see it in extreme conditions, such as the tailpipe of a jet aircraft at full power. You can also see a mirage with your naked eye, looking down a long stretch of paved road and you think you see a lake or water up ahead. That is a mirage. There are also cases of inversion, where you see an object above ground down the road, another mirage.
For us shooters, what we call mirage is the distortion of the image in our optics caused by the shimmer out there and made worse by the use of optics. When we increase the magnification on an optic, we bend the light more and this causes increased color dispersion in the optic, which further exacerbates the effect of shimmer and messes with the IQ of the optics.
(Please note that "bending the light" is done by the objective lens group. As we increase the magnification, we are examining a smaller portion of the FFP and the defects in the IQ are made more apparent. When we decrease the magnification, the observed section grows, and the details shrink along with the distortion.)
Low end optics do not control the CA very well and as you further examine a smaller portion, the IQ worsens dramatically. I had a long-running thread on this site about my experiences and findings using different types of glass and mirage. ED glass does a decent job of controlling CA across the entire spectrum, and I found that I could stay at 40X all the time, regardless of "mirage" conditions. The IQ deteriorated, of course, but not as quickly as non-ED glass. When people had to wind down their scopes, the IQ was more usable. I could stay at 40X.
Next came Super ED glass which actually contains a great deal of fluorite in a glass matrix. The CA is extremely well controlled in riflescopes with Super ED glass (the March High Master and Majesta scopes are the only ones with Super ED glass currently). In my case, I found myself using 50X all the time with my March 10-60X56 HM. In the Majesta, there is more pixie dust in the formula, and a lot of people shoot it at or near 80X in "mirage" conditions, where lesser scopes are wound down to the 20-some X or less.
When it comes to spotting scopes, as I discussed in that long-running thread, I selected Kowa as my latest spotting scope because they are the only ones with pure fluorite crystal glass, which has the highest Abbe number and thus the lowest color dispersion. It's a tick above Super ED glass, but it's pricey, fragile, and subject to temperature variations. That's not a problem for spotting scopes, can't say the same for a riflescope which is all about shock. The Kowa 883 is excellent in "mirage" conditions.
The neat thing about ED, Super ED and CaF2 glass is that since the CA is better controlled, you are able to actually discern the slightest twinge of "mirage" which will show up as a wind river moving in one direction or another. The definition is so much better.
If you want the best spotting scope to discern mirage and still be able to see the movement, Kowa with their CaF2 glass is the way to go. I will also add that other top tier optics that do not use ED/Super ED/CaF2 glass, can control CA very well but only for 2 or 3 specific wavelengths.
I hope that explains it. I tried to keep it simple and didn't even talk about the circle of confusion.
