In reference to the 127 f7.5 Air-Spaced Triplet ED Apochromatic Refractor. How does the HOYA ED glass compare with the FPL-53 glass?
Ohara FPL53 is a very low dispersion glass (with a refractive index value of 95 which is close to fluorite). The Hoya FCD1 ED Glass (used by Explore Scientific) is also a premium low dispersion glass (with a value that is more like the equivalent to Ohara FPL51, or Schott N-PK52 with a refractive index of about 81). So if you just compare the refractive index numbers between the two, you may at first glance choose FPL53, but there’s more. If someone is comparing these glass types with ED refractors, they also need to compare how many elements are in the set. If a designer is using Ohara FPL53 in an ED doublet, even though it has a much higher refractive index number, it may not perform any better than Hoya FCD1 in an air-spaced triplet design such as ours. So the person trying to weigh the differences needs to look at price, and hopefully get a chance to look through them to really see the visual performance. We always encourage people to look through our optics and directly compare th