I agree with Jon here. A few months ago I purchased a new Astro-Tech AT130EDT. That is a 130mm f/7 ED Triplet with "regular ED" FK-61 glass, or its optical equivalent, as the center element. I sent it back for a refund after a couple days. That scope showed as much bluish fringing around bright targets as would be expected (and tolerated) with a 4" f/10 achromat, and I only do visual observation. I don't want to imagine what a camera sensor might have seen. The larger in aperture these scopes get, the more critical it is for them to have SD glass objectives, whether they are Doublets or Triplets. Here are some notes I put together regarding Doublets that help to predict how well a scope should perform regarding its level of residual CA:

Color Blur JPEG Notes.jpg

Using your values for the secondary spectrum of different glass combinations, it is easy to figure the color blur for a given generic design, assuming the color blur for ordinary crown/flint achromats will meet the Sidgwick criterion (CB = 3XAiry disc) if the focal ratio is 3 times the aperture in inches. Thus a 5" crown/flint achromat will have a CB of 3 if its focal ratio is 3X5 = 15.

Or we can use the Conrady criterion, which requires a CB = 1.8 with a focal ratio equal to 5 times the aperture in inches. Thus a 5" crown/flint combo will need to be f/25 for a CB of 1.8X the Airy disc.

For the more modern designs the CB can be determined relative to either of these two criteria. First note that the CB is inversely proportional to the focal ratio. Second, we see that for a given focal ratio, the modern glass combinations will provide a smaller CB based on the ratio of the secondary spectra. For example, the ED combination will yield a CB 1/3 that of crown/flint for a given focal ratio. 2000/6000 = 1/3.

For my AT 125 EDL I use the secondary spectrum ratio of 1/6.5 (2000/13000). However, its focal ratio is 7.8, so if it were a crown/flint design its CB would be (15/7.8)X3 = 5.77. But the FCD100/ Lanthanum glass reduces the secondary spectrum by 1/6.5. So the CB for the AT 125 EDL is 5.77/6.5 = 0.887.

Note that we get the same result if we use the Conrady criterion, (5" needs to be f/25 for a CB = 1.8). Doing the math: (25/7.8)X1.8/6.5 = 0.887.

If you want a formula (using the Sidgwick criterion): CB = (F1X3)/(F2XS), where F1 is the focal ratio of the modern design, F2 is the required focal ratio of the crown/flint design for a color blur of 3, and S is the inverse ratio of the secondary spectra. (For the 125 EDL S = 6.5).

My 102 f/11 Altair Starwave is an ED design with FPL51; thus, I use S=3, 1/ (2000/6000). Its design color blur is:

CB = (12X3)/(11X3) = 1.1.

Dom Q.