And this... Being the refractor forum, if you study the spots for most refractors, you very quicaly realize that very few refractors are capable of putting all of the energy into the diameter of the Airy Disk.
As I mentioned, the problem with Ray Traces is that they under-represent the peak intensity, but for charomatic abberation defocus, they tend to be much more relevant.
The problem though is understanding how much total energy is being lost and how the specific wavelengths contribute to image intensity.
For this reason, spot diagrams for refractors are not particularly useful becuase it is hard to integrate the different color plots in a way that tells us about peak intensity.
For this, a far more useful way to express design performance would be to use Polychrromatic Strhel. This method uses mathamatics to combine the energy and express the performance in a way that allows comparing apertures that spots would not easily allow.
If you go to this page, you can easily see what I mean. Figure 149 shows the plots for different colors for a variety of instruments, but by itself, that detail is not all that valuable.
If you look at Table 12 on the same page though, the author has "condensed" the data by offering the poly-chromatic Strehl figures for each design.
Now we are getting close to being able to compare apertures, but because the apertures are not similar, we need another way to express how the aperture and choromatic errors would affect the performance when the scopes were compared and this is the role of the MTF plot. Here is just such an example and no surprise, the author has used some of the same instruments and has even provided a table that correlates that performance to obstruction size.
This is the problem with spots. They are superb for comparing off axis performance because here, aberration levels are rarely diffraction limited, and now the plots will tell a great deal about how two different designs will perform.
Spots bundle size differences within the Airy Disk are almost meaningless by comparison.
But more than that, for refractive systems, the spots simply can't allow the viewer to easily integrate the system performance the way Poly-chromatic Strehl can, and MTF plots can now be used to compare how different apertures with different polychormatic Strehl ratios would differ in contrast performance.
Last but not least, manufacturers are reluctant to publish ray trace diagrams, and I can't blame them. Reflect on my post regarding stereo specsmanship above. They don't want to get into that kind of battle that often overstates the important of this or that spec (color plot).
It is sad that they do not publish poly-chromatic Strehl though, and if one were to really want the most important "Spec" for a refractor, this would be it. Don't hold your breath on that one though. I believe Yuri from TEC has given figures int the past and he should be applauded for that.
Edited by Eddgie, 29 March 2016 - 01:31 PM.