Hello Chris, this is going to wander a bit as it is 05:30 in the morning and I am having breakfast
I will not touch on fluorite as it is a whole topic on itself.
OK-4, FPL-51,53,55, FCD-1, FCD-100, FK-61 are the most common extra low dispersion glasses used in our optics.
These of themselves do nothing to make a good refractor without a proper mating element(s).
All of these are ED glass, the SD designation is strictly marketing to separate you from your $$.
The one we hear/ read about the most is Lanthanum BUT Lanthanum can be a glass or it can be added to a base glass melt or it can just be doped onto the glass like a coating. Each method has different corrective properties.
There are other backing glasses, many of which are more expensive than the ED glass itself. A good example is Schott glass N-ZK7 which is typically 3x the price of FPL-53 (meaningless but I will give context later)
So in reality we have to put a lot of trust into the manufacturers or spend a bit of time learning some of the specifics to see if it will meet our expectations.
Grade 1-4(Perfect-Good), Scratch Dig 1-4(amount of internal bubbles-scratches)
Optical extra low dispersion glass usually comes in at least 4 different grades and in 4 different scratch/ dig ratios.
The best grade- s/d ratio is usually for military, high end scientific instruments and small amounts to boutique optic houses. Grade 3-4 is normally for consumer and mass produced consumer optics and they may have a s/d ratio between 2-4 depending on usage.
Grade 2 is common for premium/ boutique optical houses (Astro Physics, TEC, Agema, AOM, CFF, Takahashi etc.)
and s/d ratio of 1-2.
We would all like to think that we have the best glass available in our refractors but seldom is it the case.
One issue I see happening a lot due to marketing, is that many people equate the term apochromatic with the quality of the product when it only refers to the colour correction. Three elements also do not make a apochromat!
Cost of FPL glass
Glass manufacturers base their cost on their cheapest glass which for O’Hara is S-BSL7 (Schott BK7)
If S-BSL7 cost is 1, then FPL-51 is 11x and FPL-53 is 18x more. So the Schott N-ZK7 is 54x the cost!!
Now the thing to understand is this is the price per kg with Grade 4 glass. As the grade and scratch/ dig ratio improve the price increases exponentially with
aperture as well.
One example is the AOM 130/ 1200 FPL-51 doublet using grade 1 glass, the cost difference for the 160/ 1600 FPL-51 lens using grade 1 was 350%!!!
ED glass main function is to allow making a short focal length telescope. Used in longer focal lengths it eases the designers choices for mating elements and makes negating SA and other aberrations easier.
Telescopes can be made apochromatic using only standard glasses, even the ‘
apo’ refractor that got Astro-Physics on the market had no ED glass in it.
If you have any questions let me know, I understand this wanders a bit but the morning caffeine has not kicked in yet.
So the eventual question is, how do you think SVBony can offer that size telescope at that price?
