You’re new here. You’ve read a bit. You’ve started to shop for a telescope, and now you’re seeing certain words and phrases being used by dealers and manufacturers as if they have importance, but you haven’t the foggiest as to what those words and phrases actually mean. What do you do?
You could (a) acknowledge your own lack of experience and knowledge regarding astronomical equipments and trust the greater experience and knowledge of the manufacturer or dealer, or (b) recognize that your interests and those of the manufacturer or dealer are not totally aligned (they want to sell you the product they produce or purchase the cheapest for the most money they can get you to pay, and you want to get the absolute best quality product possible for the least amount of money), and ask “what does it all mean, really?”
I’m not a scientist or even particularly technical. I’m just a consumer like you who has been buying astronomy gear for a bit longer than you have, and have made the mistake of following course (a) and more recently become interested in course (b). So what you get from me is an average consumer’s eye view of the significance or lack thereof of commonly used marketing terms.
I. “Diffraction Limited”
You’ve seen references to “diffraction limited” and gathered that it is supposed to tell you something about the optical quality of telescopes from the source employing the term, but you aren’t sure what it means. That’s okay though. I’m not sure either. In fact, no one is. Not even the experts.
I won’t go into the technicalities of what a wave is and what terms like “1/4 wave” and “1/8 wave” actually mean in a nuts-and-bolts sense. Mainly because I can’t do it justice. I will, however, say that when applied in the proper context, a smaller wave ratio indicates an optical surface closer to theoretical perfection than a larger ratio. I’ll touch on what I mean by “proper context” in a moment. For now, note that 1/8 wave > 1/4 wave.
By definition “diffraction limited” in the context of telescope optics means that flaws in the figure of the optics are small enough that errors induced by other factors (atmospherics and the like) external to the telescope mask or hide such optical flaws. The problem is, no one is really sure at what point an optic has enough flaws that those flaws are visible in images formed by the optic no matter what externally induced errors are also present.
An old “rule of thumb” derived from the work of Lord Rayleigh, but perhaps not actually posited by Rayleigh, is that a “1/4 wave” optic is “diffraction limited”. You’ll see this figure in old telescope making sources, repeated in marketing materials on some dealer and manufacturer websites, referenced in internet astronomy forum threads and the like.
Problem is, it simply ain’t true! Before we consider why it ain’t true, let’s turn back to the meaning of “1/x wave”. When I say “1/x wave” I mean 1/x wave PV or peak-to-valley. Now, please don’t ask me what that means or I’ll start saying things like “imagine a mirror were the size of Lake Superior; the surface that looked so smooth will actually appear rippled...”
Now why is it that 1/4 wave PV isn’t really diffraction limited? Well, based on the definition, a diffraction limited optic should show no image defect caused by a flaw in the optics. In other words, if 1/4 wave PV is truly diffraction limited, side-by-side, two otherwise identical scopes, one with a 1/4 wave PV optics and the other 1/8 wave PV optics, should show exactly the same thing imagewise. You shouldn’t be able to differentiate them, since neither, by definition, shows any flaws other than those caused by the atmosphere and both are subjected to the same atmospheric conditions.
In 1992, pro-am astronomers Terrence Dickinson and Douglas George, and optics maker Peter Ceravolo teamed up to do some optical quality field testing. Ceravolo fashioned a series of 6” f/8 reflectors of different quality levels (1/2 wave PV, 1/4 wave PV, 1/8 wave PV and 1/10 wave PV). Each pro-am astronomer tested each scope over several nights without being told which scope was figured to which level of quality, and recorded their impressions. Later, three of the scopes (1/2 wave, 1/4 wave and 1/10 wave) were trundled off to a BIG star party at Stellafane, and 103 different amateurs of different levels of skill observed through each scope and provided their rankings.
I think the mass amateur data is especially interesting, but it needs some qualification. It’s easier to determine which optic has a better figure by doing a star test (that is, centering a 2d magnitude star, ramping up the magnification, and examining the diffraction pattern for smoothness and symmetry on each side of focus. The problem is, we generally don’t observe the universe out of focus, so to most of us one scope is better than another similar scope only if it shows us more detail and a better quality in-focus image. Many of the amateurs star tested the target, which was Polaris. Accordingly, I’ll break the data out by all testers and then only those testers that did not star test and instead only studied the in-focus image.
Aggregate “all tester” data: 100% (all 103) correctly identified the 1/2 wave optic as the worst. 2/3 (66%) of all testers correctly identified the 1/10 wave optic as being superior to the 1/4 wave optic. Only 1/3 (33%) could not differentiate the 1/4 wave and 1/10 wave optics.
Data for tester who only observed Polaris in-focus: 100% correctly identified the 1/2 wave optic as sucking mightily. More than 50% correctly identified the 1/10 wave optic as being superior to the 1/4 wave optic on the basis of how well the companion star was rendered relative to the primary only. It should also be noted that seeing was rated as “poor” for these tests.
Based on this data, pretty clearly most people can identify a 1/10 wave optic as being superior to a 1/4 wave optic in-focus and in poor seeing. This means that 1/4 wave doesn’t satisfy our definition of “diffraction limited” in practice.
It’s a pity that the 1/8 wave optic wasn’t included in the Stellafane testing. While we’re pretty certain that 1/4 wave being diffraction limited is nonsense, we have no idea whether 1/10 wave is good enough or not. Would 1/8 wave be indistinguishable from 1/10 wave? Would 1/12 wave be distinguishable from 1/10 or 1/8 wave? Who knows?
Bottom line, there is no scientifically vetted, generally accepted minimum PV wave ratio that defines when and optic is diffraction limited. Therefore manufacturer and dealer claims than a given optic is “diffraction limited” are simply mumbo jumbo and promise you nothing you can bank on.
If you are interested in reading the Sky & Telescope article covering the 1992 testing, buy the DVD archive set and refer to the March 1992 edition of the magazine.
II. Glass Type Marketing
Glass type marketing takes different forms for different types of telescopes. Certain refractors use “ED glass”. The corrector on one line of SCTs is made of “water white glass”. Some reflectors have mirrors of “Pyrex” or “quartz” while others make do with “plate glass”. Should you care?
A. Refractors
In the late 70s Takahashi in Japan began experimenting with rare, expensive special dispersion lens materials such as Calcium Fluorite in an effort to reduce the amount of chromatic aberration seen visually and on film in refractors. Chromatic aberration (sometimes abbreviated “CA”) manifests as colored haloing or fringing around brighter objects in the field of view. Usually such haloing is violet to bluish in hue. White light (what we see) is comprised of combined light of different wavelengths. Different wavelengths of light are perceived as having particular hues to our eyes. The visible wavelengths are said to cover a spectrum. Think rainbows and you have the idea. Traditional achromatic doublets using glass types in the “crown and flint” classes generate this haloing because they are unable to focus all of the wavelengths of the visible spectrum (i.e., all colors of the rainbow) at the same focal point. The haloing, then, is really light of particular wavelengths that are out of focus.
Is CA bad? It can be. CA, like optical quality we discussed above, covers a range. Really bad CA (like a really bad 1/2 wave mirror) has a negative impact on image quality that is impossible to miss. On the other hand, moderate CA (like a borderline 1/4 wave mirror) may or may not be objectionable to an observer. The effect of CA on image quality is really beyond the scope of this thread, though it would be an interesting topic for a different, future thread. For present purposes, understand that in the 1970s telescope manufacturers started looking for ways of making commercially available refractors that reduced CA over earlier crown and flint achromatic doublets.
Around the same time as Takahashi’s pioneering work with fluorite, Roland Christen of Astro-Physics fame, began playing with newly available special dispersion glass types (used in the aerospace industry) and doublet and newer triplet lens designs with very much the same goal in mind – reducing CA in refractors. In a bit of a push-me, pull-you manner, the emergence of reduced CA refractor designs pushed glass makers to innovate in the area of low dispersion optical glasses, and the broader commercial availability of such glasses, in turn, prompted refractor makers to explore reduced CA designs using these new glasses.
Today, the most commonly available low dispersion optical materials include fluorite, Ohara FPL-51, Ohara FPL-53, LZOS OK4 and Hoya FCD1. Ohara and Hoya are Japanese glass makers. LZOS is a Russian glass maker. The other alphanumeric designations represent specific low dispersion (aka “ED”) glass types.
Recently there was a great brouhaha regarding FPL-53 vs. FPL-51 in the CN Refractors Forum. I was one of the principal “pot stirrers” in that particular glass-type skirmish. It’s not my intent to rehash those points and counterpoints here, and I will not acknowledge or respond to any post along the lines of “FPL-53 has better color correction that FPL-51”. Partly because it’s utter nonsense and partly because it’s irrelevant to the conclusion I offer in this post on glass type relevance from the beginning consumer’s perspective.
Here are a few glass-type independent rules of thumb that help explain the guidance I’m about to give. Longer focal length (larger focal ratio) refractors of a given design generally have less CA than shorter focal length (smaller focal ratio) refractors of that same aperture and design. In a given refractor, the amount of CA visible in the eyepiece generally increases as magnification increases. Some folks have eyes that are more sensitive to the typical CA wavelengths than other folks.
In my view, the first order of business is to learn to recognize CA in the image. Beg, borrow, buy or steal a cheap achromat like an Orion Short Tube 80 (80mm f/5 crown and flint achromat). Pump 40x per inch through that puppy and point it at Sirius or Vega. Purple haze, all in my brain! Now you know what CA looks like. But hold your horses. If you’re looking at Sirius at 120x, shift the scope down to open cluster M41. Whoah! Where’d the CA go? If your target was Vega, slide across to globular cluster M13 in the Keystone of Hercules. You’ll wonder where the purple went when you…er…you get the idea. But wait, there’s more. Go back to Sirius or Vega and change eyepieces. Instead of 120x (40x per inch) drop down to 30x (10x per inch). Bye-bye purple.
Answering the question of whether or not CA will make your life a living hell really requires you to give some thought about what kinds of things you like to look at and at what magnifications. You may decide that CA isn’t a big deal for your intended use. Alternately you may decide that CA isn’t great and you’d like to get rid of it, but it’s not so bad that you’re willing to pay a fortune to suppress it. Finally, you might hate it and decide that the only way you’ll ever own a refractor is if it is visually color free to your eye.
Wherever you come out on the CA tolerance question, special dispersion glasses and newer lens designs make it possible to produce refractors with little or no in-focus CA. It is possible to make a visually color free telescope using a doublet of crown and flint, a doublet that includes a low dispersion (ED) glass type, a triplet using common optical glasses and a triplet incorporating a low dispersion glass type. Real world data based on currently available refractors shows that the only generalities you can safely make are that refractors of a given aperture employing ED glass in a doublet or triplet design can be made at a shorter focal length and still remain relatively free of CA than those made using conventional optical glasses like crown and flint.
While the refractive properties of all of the glass types used (ED elements and mating elements) as well as the lens configuration (two elements or three, ED in front, in the middle, or at the rear) define the color correction properties of the optic, there are so many different mating glass types and subtle design details (lens curvature, spacing, placement), that generalizations beyond the statement above are opinion and not fact. Because you and I will rarely if ever know the radii of curvature of the lens elements used in our refractor, the glass type of the non-ED element(s) and the placement of the different glass types on the optical group, we lack the fundamental information we would need in order to predict color correction for a given scope even using sophisticated optical design software.
So, when you see one manufacturer or dealer advertising that it’s 4” doublet refractor uses “FPL-53” and another stating that it uses “ED” glass in its 4” triplet, the only safe and certain conclusion is that both of these scopes will have better color correction than any 4” achromatic doublet available using standard optical glasses. That is, the presence of some form of ED glass means better color correction than the lack thereof at a given aperture. For the additional data of specific ED glass type (FPL-53 or FCD1) to be meaningful to you, you’d require armloads of additional data and most likely some modeling software to reliably predict more about the relative color correction of the two telescopes using the different glass types.
In short, when you see “FPL-53” or “fluorite” in an advertisement, the odds are good that you are being sold to (charitably) or manipulated and conditioned (less charitably). Just say no to glass type nonsense. There is no reason to believe that refractor A from maker #1 using FPL-53 will be any better or worse in quality or any better or worse in color correction than refarctor B from maker #2 using FPL-51. Ignore the noise and puffery. Instead, read user reports on refractors you are potentially interested in. If you see someone on CN with that refractor in his or her signature, ask questions by PM. Ask them to point it at Vega and push 50x per inch, and report back to you. Most of us are more than happy to accommodate such requests. Star parties, too, are a great opportunity to look through different refractors. I do suggest that when comparing refractors at star parties, be sensitive to differences in aperture. Before deciding that 5” refractor #1 is better than 3” refractor #2, make sure that you’ve compared them both at the same magnification per inch of aperture and the same exit pupil. At low magnifications per inch all telescopes are loafing and deliver hard to distinguish optical quality. Magnification is what separates the wheat from the chaff.
In no event make the mistake of assuming that a refractor using FPL-51 is inferior to all others using FPL-53.
B. Mirrors
I know what you’re thinking: “The FPL-53 vs. FPL-51 marketing speak was a scam, so I bet the mirror substrate material advertising is also a marketing game.” Not quite. Or at least not quite to the same degree. Different types of mirror substrate do have different thermal properties. These differences have meaning both to the mirror maker when making the mirror and, potentially, the buyer who purchases the mirror. Glass types like Pyrex or quartz (fused silica) cool faster than other common glass types like BK-7 or plate glass. Faster cooling saves the optician time when grinding and testing the mirror. Typically the optician must wait unto the mirror cools to inspect the effectiveness of a given period of grinding. Faster cooling materials mean less wait-time between grinding and testing. Time is money.
Faster cooling can also be of benefit to the mirror owner. Faster cooling materials like Pyrex and quartz also tend to maintain their intended figure (shape) better while the cool down. This means that the mirror will a perform bit closer to the optimal level achieved when it’s fully cooled during cool down. But here’s the catch (you knew there was going to be a catch, didn’t you?
). While small mirrors of Pyrex do cool faster than small mirrors of plate glass, small mirrors of any glass type cool pretty quickly. Cooling speed is affected by mirror mass and material. Small mirrors don’t have much mass, and therefore cool relatively quickly. Think of it as a piece of steak. If I had a thawed 2” thick New York Strip from the butcher and put it in the freezer, it would be soft to the touch for a fairly long time. If, however, it was a 3/4“ thick steak, it would be frozen solid in a much shorter time. Same basic idea.
In my estimation 6” and 8” and to a lesser extent 10” mirrors all cool relatively quickly. I wouldn’t worry too much whether such mirrors (when used in a relatively exposed structure like a Newtonian or a Dob anyway) employed Pyrex or plate glass. On the 10” if I could get Pyrex easily, I might opt for it. For mirrors larger than 10”, it starts to make a greater difference as aperture and mass increase. Pyrex or quartz becomes a more significant selling point and benefit in 12” and larger mirrors.
So mirror substrate “kind of” matters. On the other hand, if given the choice between a 1/2 wave 12” quartz mirror and a 1/6 wave 12” plate glass mirror, I’d go with the better figured mirror any day.
III. Glass Brand Marketing
This is a variant of the same kind of marketing speak represented by glass-type marketing. Glass makers like Schott, Ohara and Hoya all make multitudes of different types of optical glass. They also make multitudes of different qualities of those multitudes of different types of optical glass. Going back to the butcher shop on this one, you butcher offers many different cuts and qualities of beef. You have USDA certified, USDA choice, USDA prime; hangar steak, top sirloin, New York and filet mignon. If you say “I’ll take a New York and your butcher hands you a USDA choice strip, are you really getting a better cut of beef than a USDA Prime grade top sirloin?
Glass brand is similar. Ohara has low quality FPL-53 and high quality FPL-53; low quality FPL-51 and high quality FPL-51. Schott has great quality BK7 and low quality BK7. You get the idea. Just because the glass in your telescope comes from Schott and Ohara, it doesn’t mean that the glass was good quality glass.
Glass maker brand, like glass type, is relatively meaningless in judging the quality and performance characteristics of a refractor using such glass.
IV. Lens Count Marketing
Same song, third verse. The latest refractor marketing ploy is to emphasize that a given refractor is a TRIPLET rather than a lowly doublet *sniff, sniff*. Three is better than two right? If you think so, I have some oceanfront property in southern Illinois to sell you. Like information about ED glass type and glass brand, knowing the refractor lens count doesn’t tell you much of value without a whole bunch of other data that scope makers do not routinely share.
In no event make the mistake of assuming that a refractor using three lens elements is automatically better than one using just two lens elements.
V. A Better Way to Shop
Optical quality is king. Telescopes are complex systems and many concealed variables beyond those advertised affect what you see at the eyepiece. Manufacturers and dealers want as much of your money as they can get, and to give you as little as possible in exchange. It’s called “margin” and it’s what keeps their lights on. Recognize this, and take their ad copy with a grain…strike that…a handful of salt.
Instead, do your own homework. Ask about scopes you may be interested in on the appropriate CN forums. Feel free to PM owners of particular scopes of interest and ask them for their candid opinion of those scopes. Read user reviews. Search the web for lab test data. Whatever you do, do NOT sit back and let the marketers spoon feed you. If you do, you’ll become fat on mediocre gear and thin on funds rather quickly.
There are certainly bargains to be had. Telescopes of high optical and mechanical quality that perform well, but do not break the bank. You just need to be diligent in seeking them out. That is your job, not the manufacturer or dealer’s responsibility. Their job is to make money to survive.
Whew! *That* was a mouthful!
Happy shopping!
