She belongs to the ED family from before the turn of the century. Due to the scarcity of FLuorite crystal, a different, slightly less corrective ED glass was used, the focal ratio necessarily became
The ED variants were also called "semi-apochromats" with some derision, whatever that means.
Many Extra Low Dispersion telescopes are run as doublets, mainly for visual purposes. The use of FPL-53 glass today has more or less displaced fluorite as an ED element. The optical refraction properties of FPL-53 hardly differ from Fluorite. FPL-53 is not a crystal and therefore less subject to weathering. (Ever had crystal glasses in the dishwasher? Then you know what I mean)
Not only lenses introduce chromatic abberation. Prisms do this as well. Although many times smaller than a lens, far behind the point, but measurable. Visually visible in focus? Questionable.
Manufacturers of refractors in the last century tuned their lenses and prisms to reduce color aberration. The top brands certainly did this. When buying an older refractor, it is recommended that you check this out under the stars.
When I bought my Vixen third-hand, it came with a prism. Knowing the story above, I assumed it was fine that way. I did see a residual error, but every refractor has that. And so I didn't pay attention to it. Also, a better diagonal that I bought for the telescope, a Baader Zeiss , was also a prism for this reason.
Since I had purchased a Baader BBHS for my 120mm
Until yesterday, when I put my Vixen on the observation pedestal again, just for fun and inserted the BBHS mirror diagonal.
Aiming at Arcturus (first visible) and going inside to let things cool down. Then first to Izar, the beautiful double in Bootes. High magnification. Nice view. Back to Arcturus for a moment to recalibrate. And then I noticed that the diffraction ring was perhaps a little less colored than I was used to.
A quick star test in and out confirmed my idea. The pink glow I normally saw in the center.....was gone. ??? Until I realized that for the first time in years I had a highest end quality mirror diagonal in it instead of a highest end prism. I did burst into laughter for a moment. For years I had been observing with a prism, thinking it gave the optimal image, as intended by the manufacturer..... Now it turned out that I had been thinking for years that I was be able to see the best I could. For a check I turned to Epsilon Lyrae. The colors were more distinguishable than usual. Amazing!
Even old observers are never too old to learn!