You'll also see claims that scopes using "FPL-53" have better color correction that scopes using "FPL-51" and the like. This, too, is baloney.
Oh Jim, oh Jim..
It's time for the other half of the story, the facts, the reality..
FPL-53 is more expensive and has superior optical properties that allow for better color correction within a particular set of design constraints. An 80mm F/7 FPL-53 doublet can have and most likely will have significantly better color correction than an 80mm F/7 doublet based on FCD1, HFK-61 or FPL-51. You could make an 80mm F/10 using FPL-51, no one does, and achieve similar color correction to an 80mm F/7 FPL-53 doublet but it would be F/10 and not F/7.
And too, the manufacturer can be most important. At the top of the heap you will find a company like Astro-Physics where each batch of glass is measured for homogeneity as well as it's optical properties. If the glass is not of the highest quality, it is discarded. The design of the lens is altered based on the measured properties of the individual melt of glass. The optics are all done in house from the design to the final hand figuring of the objective which is done by the owner himself on an interferometer.
There is a lot to know about refractors, not only optical but also mechanical design.. in general, you get what you pay for.. The key is knowing what you want and what you are willing to pay. Most are quite good, some are better values for essentially the same product, the balance between mechanical and optical quality varies.. Companies like Astro-Physics, Tec, Takahashi and TeleVue are manufacturers, they do the design, they may have an optical house oversees fabricate the optical elements to the specs and then the assemble the scope. Most other brands are, as Jim says, really vendors, they buy from a manufacturer who has a product line, they might spec out a scope slightly differently than the next guy...
Actually Roland says that he cannot afford to use the highest grade glasses from Ohara.
He also agrees that focusing on the ED element alone is pointless.
"There is no other glass readily available and in general production that would be superior to these two. They are both very close in terms of optical characteristics with an ultra-slight edge going to the FPL53. The only other material that is superior in a very small way is Fluorite. All three can produce triplets of essentially zero color error. They can also produce zero color error doublets, but the focal ratio must be considerably longer because no suitable mates exist to provide fast focal ratios and zero color both at the same time in a doublet.
It is senseless to argue about these glasses because the overall correction is due to the mating element, not the ED. Why not lets argue about suitable mates, yes?
More on the importance of the non-ED glasses used, from Roland:
">>Would you post a sample recipe for objectives in this design class --- for
ATMOS? I am playing around, but not really hitting anything that has this
performance. It would be fun to take such a design and see how decentering
or changing melt characteristics changes it in comparison to changes in a
Sure, can do. In fact, I take it you will be starting a new Atmos Yahoo group,
in which case we can exchange some intersting ideas.
In the case of this particular fluorite design, the color correction depends
entirely on the mating element. Traditionally, one would choose a mate that had
the same partial dispersion. Until recently, the only viable mates were ZKN7
While ZKN7 is readily available from Schott and other sources, KZFSN2 (or the
new N-KZFS2) have some undesireable weathering problems. The surface is highly
affected by moisture, and even if coated, the moisture will penetrate through
the pinholes that all coatings have and eventually cause opaque spots on the
glass. One can place the fluorite out front in the hopes of shielding the KZ
glass in back, but anyone who has ever seen a refractor lens the morning after
a moist night will know that the back of the glass will have condensate on it.
Two other approaches are possible. One could use a Lanthanum glass, which
results in quite low sphero-chromatism, as well as good color correction, and
the other would be to use a glass not quite a perfect match, but at least twice
as good as the one that Tak uses. This would be Shott K10. The result is quite
a nice level of color correction with almost perfect spherical using an
airspace of a few millimeters.
There are other glasses such as Scott K4, which are out of production, but may
be poured by the Chinese for some special purpose. I am in the process of
inquiring what materials are available from the Chinese glass makers for some
future products. It seems that both Schott and Ohara, for environmental
reasons, have shied away from making those glasses that could produce really
good results with fluorite or FPL53, and perhaps the Chinese are now stepping
into the void.
It really is all about the mating glass. Truly.
There are great triplets being made with FPL-51, too. The TEC 160EDs and 200EDs, for instance. I'd much rather have one of these than a Chinese FPL-53 triplet such as the Sky Watcher 150mm. Sure, the TECs are slower. And for visual that's a GREAT thing.
Jared Wilson along these lines:
"Speaking generally, refractors made with this material (it's an Ohara catalog number by the way--other manufactures may have very similar materials) have a very good reputation for quality. This is for two reasons:
1) It has the potential, when coupled with the right mating element, to produce extremely good color correction
2) Because it is intrinsically expensive, most manufacturers only use it when they are producing fairly high-end (or very high-end) refractors. So while it doesn't make a telescope perform well by itself, it is often taken as an indication of overall quality. If you weren't setting out to make a top notch scope, why would you bother to spec in such an expensive glass (or so the argument goes)?
The problem with the emphasis on FPL-53 or any other uber glass is that the heavy marketing emphasis can cause the material to take on a mystique of its own. Just as fluorite did in the past. Then people start to believe that you can't make a top quality refractor unless it includes FPL-53, and that simply isn't the case.
Why should you care about FPL-53? Because scopes that use it are usually of very good quality and have the potential to show very little chromatic aberration. Just don't fool yourself into thinking this is the only way to achieve good performance in a refractor. There are other materials that work very well also."
Slower scope, too, have significant advantages; most importantly greatly reduced spherochromatism. Fast doublets and triplets have reduced maximum potential polychromatic Strehls. Fast scopes are beneficial for imaging. For visual, they are actually something of a liability. Sure, portability is improved, but the reality is a scope up to 4" is not much less portable at f/10 than it is at f/7.
So, again, whenever a manufacturer or OEM (i.e., re-brander) advertises only the type and brand of ED glasses used, they are telling you nothing important or of real value with respect to the quality of the instrument they are hawking. It's meaningless marketing drivel intended to mislead the uninformed. Buy quality, not glass type (or is that hype?).