I really enjoy your posts, but this one has me scratching my head. There are two AT-130 f7 scopes. One of them doesn't specify glass. It is likely FLP-51 or equivalent, and I assume this is the one you are referencing. How then do you come up with the claim that it has a PS of 0.95? I am sure that the design may have a PS of 0.95 but this doesn't mean that the product does. The AT scope using FCD100 has a specified strehl of 0.95. This is for a single wavelength. So, if the more expensive, SD glass scope only has one wavelength at 0.95, what would make us think that the less expensive scope with ED glass and no strehl guarantee of any type has a PS of 0.95?
This isn't to argue over the AT scopes, it is to question whether or not you actually have demonstrated a flaw in the chart which was referenced and suggests PS = 0.95 as sensibly perfect. I guess it's also asking, if we assume all you wrote to be correct, does it make sense to call something "sensibly perfect" when color error can be easily detected? Best Regards.
Just to clarify, there is Strehl," and there is Polychromatic Strehl. What we are discussing in this thread is polychromatic Strhel.
The Strehl that AT is promising is that the scope will have a minimum of .95 in terms of the manufacturing tolerances, but this figure is given for only one color in the spectrum. This means that if they use green, then they guarantee that the scope will have a minimum Sthrel in green light of .95. That figure does not tell you what will happen with the other colors though, it only states the quality of the lenses from the manufacturing process.
Polychormatic Strhel is set by the design of the lenses used in the system. It includes characteristics like the number of elements, the types of glass being used, the curves on the surfaces, and the spacing between the lenses (oil or air). This figure is states the ability of the system to focus as much light as possible into the Airy Disk and is calculated. It is nothing to do with the manufactured Strehl. No matter how perfectly made, the scope will never exceed the polychromatic Strehl performance.
Let's go back to the AT scope. Suppose that the lens is perfectly made. This means the green will be near perfect (between .98 and .999 depending on the design), but the blue light will only have a Strehl of between .68 and .88, depending on the types of glasses used. Blue will have between .68 and .88 waves of spherical aberration. Red will have between .78 and .85 waves of spherical aberration. So, no matter how perfect you make the scope in manufacture, the Strehl for these colors will never be higher than the design allows. The polychromatic Strehl weights the different colors from a visual use, and for these designs, the polychromatic Strehl will be somewhere around .95.
The FPL-53 triplet using BSL7 as the mating element, assuming it is made to the same manufacturing quality, will have .999674 Sthrel in green, .912 in blue (better then the best ED glass triplet) and .906 in Red. This combination will give a polychromatic Strhel of .973739. No matter how perfect the instrument is made, it will not ever exceed this performance. It is the design that determines the best possible performance assuming that the quality of manufacture is the same. BSL7 Is the logical choice for an FPL-53 scope because it gives excellent performance and is costs less than other alternatives.
We can't really know the mating glasses used in the AT scope, but this is a possible clue. This is quote from the page I will link below:, with emphasis added by me.
With the triplet using H-LaK51A for both outside elements, contrast level at the photopic sensitivity level would be the same as with the other ED triplets having 0.95+ poly-Strehl, but with more of violet fringing noticeable.
I have zero idea of what glasses are being used in the AT scope, and I don't know if they are using H-LLaK51A for the mating elements, but due to the reports of color fringing on the AT 130 using ED glass, I have to consider that this is the possible mating elements used, but that is nothing more than a guess. As can be seen, this is a clear statement from a gifted optical engineer that says that even with a Polychromatic Strehl of .95, some violet fringing may be seen, and exactly how much would depend on the mating elements.
All of these figures are available on this page: https://www.telescop...escopes.htm#its
This is why I spent the extra $400 for the Photoline triplet. I wanted to be assured of having the best possible polychromatic Sthrel I could get at a reasonable price (The page above did not cover the TS scope when I bought it, but I used my own calculations to derive the polychromatic Strehl, and I came very close to the .973 value. I also had the reports from AT-130 owners that there was some violet fringing. Some of this could have been from optical quality because at the time, I don't think the AT scope had a mimimum Strehl gurantee. The Photoline was also binoviewer ready, so that was a big plus for me).
Edited by Eddgie, 12 May 2025 - 12:02 PM.
