NOTE: This blog has been updated since it was first written in 2012 - mainly to include the use of Hoya FCD-100 glass which appeared on the scene in 2016 - Nick
I recently had an interesting phone conversation with a customer. He asked me about the differences between the various types of ED glass we use in our refractors and how we grade the optical quality of our various telescopes. We had a long chat, and because our new website has a blog facility (which I'm trying out), I thought I'd write a little about glass types first, and hopefully get to the second part in another blog entry. Here goes – it’s the same basic conversation we had, but a bit more structured.
Our customer first asked why there are other refractors on the market which look similar and whether they have the same optics inside. These days most of the mechanical components are the same – in fact they are often standard parts available "off the shelf" so to speak from various machining companies around the world. This includes tubes, tube rings and focusers, of which there are many diameters and varieties. However, just because a telescope looks the same on the outside, that doesn’t mean it has the same optics. I’ve spoken with several customers who have bought “unbranded” telescopes which look the same on the outside, but which definitely have inferior optical quality.
We use several optical contractors from around the world, and sometimes different companies provide components for a single refractor, for example focuser, optical tube & mounting rings, and the lens cell itself. Optical contractors are capable of making lenses with different types of glass, different geometries and varying grades of surface accuracy and optical correction - depending on what we specify in the design brief, and of course what we’re prepared to pay. We tend to be quite demanding when it comes to the overall quality of the optics, and our contractors know that we test their optics extensively. An “Interferogram” (a snapshot of the optics showing their overall correction) can be provided to the customer if desired and just recently has become standard on Wave Series scopes. We’re so demanding that we’re considered to be one of the most difficult clients to work with, often raising issues that no other client raises, and always buying the best production service a contractor is prepared to offer. We often bring in expertise to solve problems, and provide design files to tweak the production specs.
So while the standard mechanics such as tube rings and focusers are relatively easy to source in quantity, and are used by several companies to make a refractor – this is by no means representative of the optical quality of the refractor in question. We have even been asked to modify refractors bought from various warehouse optical companies, using various industrial size online consumer market places who charge a fee for the transaction. These supersize online marketplaces charge a fee which must be absorbed by the manufacturer and the prices are low.
Optics are offered in four relative “grades” in our experience. “Budget high volume production” we refer to as grade 1 and if made in the Far East, this can be intended for the local domestic market. We then encounter the next level which is slightly higher and intended for export, but high volume export of hundreds of units. We call that “grade 2”. Then we have “proper export quality” where the optics go through final checks, however the manufacturers aren’t prepared to offer a set guarantee of quality. That's "grade 3". Then finally we have “grade 4” optics which are special small production runs of the highest possible quality.
At this point in time, our contractors tell us they have never been asked to attain such levels of quality by any other company. We call the 4th grade the “Altair Wave Series”. The optics have a guaranteed optical quality level on leaving the factory, and the manufacturer knows that they will be returned if they aren’t up to standard. This level of quality is only possible in small production runs because it involves manual finishing and testing by an optician in charge of a particular production run – that’s why each batch is small – to keep it manageable. Each batch of glass is unique in it’s properties, bought in advance from Ohara, or whatever glass company is being used, and the optics are hand figured to give a near-perfect star test and interferometer test. A final test interferogram image is provided with each telescope. Each telescope has a serial number engraved on the lens cell. The cost for these optics is significantly higher than the other levels of quality we’ve been offered before, and is increasing all the time, as skilled craftsmen tend to charge more for their services. Now really is the best time to buy premium refractor optics. Let’s cover ED glass types next.
What is ED glass and why do you use the stuff in refractors? A sensible question asked by customers all the time! Firstly I need to explain what "ED" refers to. It stands for "Extra-low Dispersion". Dispersion is a property of any glass, and it's the extent whereby it separates and spreads out the colours of the rainbow in a normal beam of white light. With high Dispersion you get something called “colour fringing” around bright objects. This isn’t necessarily a bad thing by the way – and some of the most experienced visual observers of planets and the moon don’t mind it at all – however it does detract a little from the image and in the interests of quality, we tend to try and reduce it as much as possible to give a cleaner image with more “natural” colours. The degree to which the colour separates out can be controlled, and the whole purpose of a refractor is to reduce dispersion and colour separation as much as possible in the in-focus image. That gives a purer image with less colour aberrations visible. (The type of colour aberration you can see is also affected by your age, the “type” of eye you have, the eyepiece, and so-on, but that’s another subject I’ll tackle at another time). The important thing is that Dispersion can be controlled by using multiple lens elements, and/or by adding various rare earth elements to glasses to make "Extra Dispersion" ED glass. ED glass is rated in many ways which I won’t go into here, but one important factor is it’s “Abbe number”. This is an index which indicates how well a glass type controls dispersion when it bends light. The higher the Abbe number, the better. Unfortunately the Universe (once again!) has conspired to make materials with higher Abbe numbers harder to make and work with. That’s why, as a general rule, the higher the Abbe number is, the more expensive the lens element is.
Here are the relative Abbe numbers of commonly used refractor ED glass types, ranked by Abbe number, give or take a few points:
CaF2 “Fluorite” (Various manufacturers): 94.99 (incredibly expensive and hard to work with, often has inhomogeneity, internal stresses, can fracture during polishing, not very stable in a chemical or thermal sense).
S-FPL-53 (Ohara Japan): 94.94 (more even internal structure, freedom from internal stresses can be attained by baking and annealing, more chemically stable, less expensive and easier to work with than Fluorite, but harder to work with than S-FPL51).
S-FPL-51 (Ohara Japan): 81.54 (easier to work with, and much less expensive than S-FPL53, very stable by comparison).
FCD-100 (Hoya): 94.66 (less expensive than S-PL53, thanks Hoya!)
By combining multiple lens elements and using ED glasses in the same refractor lens cell, we can get very good or essentially perfect “colour correction” in a refractor, such that there are no false colours visible in the in-focus image. This is particularly evident in our FPL-53 based triplet refractors which have perfect colour correction to the eye both inside and outside of focus, and amongst the best photographic colour correction on the market today, especially in the red and blue ends of the spectrum which CCD cameras are very sensitive to. This is borne out in the profusion of images taken with our telescopes on the internet. It’s important to remember, however, that glass type and the Abbe number of that glass type isn’t the only factor in controlling colour correction or optical quality overall. The geometry of the surface being polished, the relative spacing between the lens elements, coatings, surface smoothness, and deviation from the ideal design, plus the final figuring process are all very important. In fact, we are able to get near perfect colour correction in focus in our FPL-51 based triplets, and better blue colour correction than many FCD1 and H-FK61 based lenses we’ve tested. This is achieved by using a better match of mating elements and geometry, and taking time to adjust the lens arrangement and surfaces during production. Once again, this is borne out by the images taken with our larger FPL-51 based refractors which show better blue channel colour correction than some competing refractors with FCD1 and H-FK61 lenses. The point is that, while a lower dispersion element helps, it's not the only factor. But for now, let’s concentrate on the main ED glass types we use:
Glass types used in Altair refractors. You've probably noticed that we list the various glass types used in our refractors, and that we generally use glass by “Ohara”, a Japanese glass-making company, and Schott, a German company. We use Ohara FPL-53 "Super ED" glass in our 80mm F6 Wave Series ED Triplet (AA80-480EDT) and our 102mm F7 Wave Series Triplet (AA102-715EDT) refractors. We use Ohara FPL-51 ED glass in our 115mm Wave Series ED Triplet (AA115-805EDT), and our 130mm Wave Series ED Triplet (AA130-915EDT). Here’s some comments on these materials based on our experience:
S-FPL-53 (or FPL-53 for short) is also referred to as "ED" "Super ED" or "Synthetic Fluorite" or "SF" glass. This is the best stuff in terms of dispersion characteristics with a high Abbe number. It’s “easier” to design a lens on paper using S-FPL53 with almost-perfect colour correction than most other glasses in the glass catalogues. However it’s actually harder to figure a lens using FPL-53 than it is using S-FPL51 and it’s harder to get it to maintain its shape as temperature changes. To go off on a slight tangent, the customer mentioned he’d seen telescopes advertised as using “Fluorite” lenses, and asked me why we don't use Fluorite in our refractors (this is a material with legendary low-dispersion properties). Having spoken with, and used the same contractors who produce refractors for other companies, we know that some manufacturers claim that they use fluorite in their lenses, when it usually isn't. Controversial stuff, but there's a lot of hype around this subject, and we get questions about this a lot. Fluorite costs a lot more than FPL-53 (which is jolly expensive stuff in its own right) and is less chemically, thermally and physically stable than FPL53 too. These properties make it a lot harder to work into a good smooth, well figured lens. It’s also incredibly difficult to source, if not impossible these days, thanks to FPL-53, which has largely replaced it. Things have moved on, and the optics industry wanted a better, more stable material with the same qualities, so they made Synthetic Fluorite! Due to the fact that the Abbe numbers are almost identical for both materials, plus the better batch-to-batch consistency offered by FPL-53, lower toxicity during manufacture, freedom from crystal ‘pull-out’ and inhomogeneity problems which can render a finished lens useless, we consider overall that FPL-53 is a better and more responsible material to use for lens construction. Not only is the figuring and finishing process easier with FPL-53 compared to Fluorite, but the amount of final spherical figuring required is lessened, because the batch to batch consistency in dispersion and other characteristics is closer to the "ideal" required by the optical design specs.
To sum it up, FPL-53 offers almost indistinguishable performance compared to Fluorite/CaF2, in an astronomical refractor, it’s more stable, has better physical characteristics, cost far less, and it's environmentally better to work with. We like S-FPL-53, though we wish it cost less because it's a lot more expensive than the rest.
FCD-100. The relatively new Extra Low Dispersion Hoya FCD-100 glass has an Abbe number of 94.66 compared to 94.99 of S-FPL53, and in for example, a triplet refractor lens, a similar level of colour correction can be achieved by means of the different curvatures of the mating elements, therefore colour correction is very similar in an FCD-100 based triplet lens compared to an S-FPL53 based triplet lens. The price is however cheaper, thank you Hoya! We are using this in the Starwave 80mm Triplet refractor with very good results for imaging and visual use. Colour correction performance is better than multi-element photographic lenses costing a LOT more, in a recent test, but the telescope is considerably cheaper and more contrasty.
FPL-51 has a slightly lower Abbe Number of 81.54. Oh no! I hear you say. Surely this means inferior performance? Well, not quite. There’s more to it than that. It's harder, more stable stuff, and expands and contracts less so maintains its shape better (geometry is an important factor in colour correction). Larger chunks of glass often require more stability to perform well because they lose and gain heat, flex and move within their mountings. A large piece of glass varies in size more when the temperature changes than a small one. Because FPL-51 is more thermally and physically stable than FPL-53, it doesn’t change shape as much when the temperature and of course that makes it easier to “work” than FPL-53. These attributes, and because it's a lot cheaper than SFL-53, make it ideal for use in larger refractors like our 115mm and 130mm. Because these refractors are adjusted and figured the comparative colour correction is close but not quite as good as our smaller FPL-53 (or FCD100) based designs. Most folks cannot tell the difference, and many bloggers and reviewers state these scopes have perfect colour correction with only some slight colour visible in the out-of-focus image. Seeing the telescope is going to be used in-focus (we hope!) that’s not a problem at all. FPL-51 is a good material to use in larger refractors of longer focal length because, as the focal length increases so it becomes easier to get better colour correction. In fact, some very high end triplets of over 130mm aperture and a longer focal length of F7 or more, use FPL-51 based triplet lenses, and with good reason. Therefore, should you want more aperture, and therefore light gathering power and resolution, then our larger Wave Series models can be a very good way to go, provided your mount can cope with the additional weight.
The point is, that while a higher Abbe number and glass type also help with achieving colour correction (and make it easier to do so), the glass type isn't the only factor to consider. Cost per inch of aperture, focal ratio, lens design and spacing, thermal stability, and many other factors should also be considered when choosing a telescope. The skill of the optician making the optical set, the experience of the optical designer, and the experience of the manufacturer / contract holder are all critical.
In the Altair Wave Series refractors, the optics and how they function as a unit make a substantial difference to performance, and that’s where we spend our effort and money. This results in a higher production cost per unit, and also a smaller production batch. We often order in batches of 20-40x, instead of 250x plus like other companies do. We demand evidence that optics fit the test-plates before delivery. We pay for the attention of an independent optician who will adjust each batch of lenses for final assembly, whose job it is to ensure the quality is the best they can deliver. Sometimes there are gaps between batch availability when larger manufacturers place large orders, and the price per units is almost always increasing for each production run as the cost of materials worldwide increases, as does the cost of skilled optical labour. The proof, we think, is self-evident.
In our Starwave refractor lines, the adjustments are done at our optical supplier factories, with the exception of the 4-element "quads" (EDQ-R for example), which are still done by independent contractors because they are particularly involved to collimate. Still, we guarantee the telescope will arrive collimated and performing out of the box.
As always, the best advice is to talk to us about what you want to use your telescope for, your portability requirements, camera type or visual observing targets, and what scope you’re upgrading from. We’ll then help you choose the right model for your requirement and budget.
Nick, Altair Astro