Optical Glass Types
By Vic Maris
Some people ask if Stellarvue apo triplets are made with FPL53. While the answer is currently yes, there are three other glass types that provide the same level of performance based on our experience and testing. Moreover, glass type alone cannot determine the quality of a telescope. Telescope objectives that are poorly figured due to hurried production give poor performances even if they use the best glass. The reason I am writing this is to clear up some common misconceptions regarding glass type and what makes a premier telescope. So let's start at the beginning:
In 1609 Galileo first used a telescope with a simple, one-element objective to view the heavens. A simple lens like this acts like a prism, dispersing the colors that make up white light, and shows much false color.
In 1668 Isaac Newton invented the reflecting telescope, which used mirrors instead of lenses. This eliminated false color. Newton said refractors will always have false color. However, what Newton did not realize is that when combining glasses that disperse colors differently, false color can be cancelled out.
In the early 1700s, Chester Moore Hall believed it was possible to combine different glasses to reduce false color. He combined a crown glass lens with a flint glass lens, making a two element objective lens. His objective eliminated some but not all of the false color. This new type of objective lens was called an "achromat" meaning no color. While two conventional glasses can reduce color it does not get rid of it entirely as shown at the right.
It was Ernst Abbe the head of the Zeiss company who developed a truly color-free lens in 1868. It was a microscope lens that used the standard optical glasses available at that time. It required ten elements but it was truly color free.
As time went by glass technology improved, allowing for the invention of new (and expensive) types of glass, including low-dispersion glass. This glass disperses color less than standard glasses as depicted in this diagram. Since the colors are closer together, fewer elements are needed to make a color-free telescope. Remember, in a telescope color dispersion is bad so low dispersion glass is good!
Telescopes that are truly color free are called Apochromatic telescopes or apos. An apo can be made with as few as two or three elements if one of them is an extra low or super low dispersion glass.
Ernst Abbe established what an apochromatic lens must do. To be truly apochromatic the objective must focus three different colors in the same spot and certain optical defects (spherical aberration and coma) must be corrected in two different places on the visual spectrum Those are the rules. Sorry, if your apo has noticeable spherical aberration it really is not an apo.
So now that we having done a very brief summary history of glass types you can see why people want the best and lowest dispersion glass. Those who believe the best glass is FPL53 think they should have a lens made with that. But is FPL53 truly unique?
Lets look at how various glass types disperse color. We will compare the "Abbe Number" of commonly used glass types. The Abbe Number is an approximate measure of a material's dispersion. The higher the number, the lower the dispersion.
In this chart, standard glass is shown on the far left. To the right of it are two commonly used extra low dispersion (ED) glasses. The four glass types on the far right are all super low dispersion (SD) glasses. They are the lowest dispersion glasses having an Abbe number that is practically and essentially the same.
The Abbe number is only one measurement of glass performance. There are many other factors that come into play in developing a world class refractor. For example, the mating glass that is used has a large impact on an objective lens's performance.
Ohara FPL53 versus Hoya FCD100: Five years ago I was skeptical of the relatively new Hoya FCD100. My more recent experience has shown me that FCD100 and Ohara FPL53 in sizes up to 130 mm are equivalent. Here is how I determined this:
Our best doublets were once made with Russian OK4 glass which is similar to Ohara FPL52 which is now discontinued. When we mated the OK4 with a Lanthanum element our customers noted it had exceptional color correction. Many referred to this lens humorously as "the doublet that thinks it's a triplet." About five years ago I decided to take it a step further so we introduced doublets made with FPL53 and Lanthanum. We then switched from FPL53 to FCD100. We sold hundreds of these FCD doublets in three different sizes (80 mm, 102 mm and 125 mm). The telescopes made with FPL53 and FCD100 performed exactly the same. So we realized that at least in these sizes, the two glasses performed identically.
But how would it perform when imaging? We produced a number of our SVX130T refractors in 2020 with FCD100. Performance (visually and photographically) was identical to those made with FPL53. So based on this experience, I realized that it is ridiculous for people to insist on FPL53 over FCD100 for telescopes up to 130 mm. Can FCD100 be used for larger objectives? I have not tested FCD100 in sizes larger than 130 mm so I have no opinion on that yet.
Over the past two decades we have made our 130 mm apochromats with a lot of different glass types including Russian OK4, Schott FK51, Ohara FPL51 and its equivalents, Ohara FPL53 and now Hoya FCD100. The highest contrast was obtained with the last two glass types mated with a borosilicate crown element in the front and a lanthanum element in the back. We made some oil spaced FK51 and FPL53 triplets but most were air spaced. Each of these telescopes were color free when used visually. When used for astrophotography, the air spaced lenses had better color correction.
Glass type is not the most important thing: When someone chooses a relatively inexpensive, mass produced telescope because it is advertised as using FPL53 over a telescope using FCD100 that is much more accurately figured, the customer is clearly using the wrong criteria to determine quality. Let's imagine the less expensive telescope has optical defects. Choosing the less accurate optic simply because it uses a particular glass type is silly when both glasses are equivalent. The difference between two telescopes under a steady sky will be obvious. In this case the FCD100 telescope will win, not because it has better glass but because a lot more time has been spent addressing the issues to bring it to a higher standard.
Glass we will use in 2021: As a result of our experience with FCD100 over the past five years, we will be using more of it.
Currently we are using FCD100 for our doublets and SVX130T. The SVX180T uses FPL55 (see below) and the rest FPL53.
Ohara FPL53 versus FPL55. Investing in optical glass in large sizes is expensive and risky. Ohara is now making a new SD glass, FPL55. Based on the specs the only real difference between FPL53 and FPL55 is the latter can maintain its performance in larger sizes. So I will be making our new SVX180T using FPL55 since it appears to be a safer choice in this size range.
I hope that by sharing my experiences over the last 23 years I can help my customers make a more informed decision.