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Recognizing Marketing Speak!

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#1 jrbarnett


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Posted 05 August 2011 - 04:23 PM

The truth will set you free (or at least potentially save you some dough).

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. :grin:

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? :grin: ). 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! :grin: Happy shopping!

#2 rdandrea



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Posted 05 August 2011 - 04:33 PM

Optical quality is king.

But mechanical quality is at least the Crown Prince. It doesn't do much good to put a first-rate optic in an assembly that's going to fall apart.

Fortunately, if most builders are going to invest time and expense on one, they're more than likely to invest time and expense on the other.

#3 UmaDog



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Posted 05 August 2011 - 06:30 PM

Wow, Jim. Yet another detailed article on the beginner's forum. The mods should collate them all into a single sticky. Thanks for your work.

#4 James Paulson

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Posted 05 August 2011 - 06:39 PM

This is an excellent article and I enjoyed reading it Jim. Thanks for writing it.


#5 Vondragonnoggin



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Posted 05 August 2011 - 06:50 PM

Great post Jim! All of that was and still is (with the amount of debate and emphasis or de-emphasis placed on these terms) quite the mystery. I have gone to just getting an impression of how one likes the view and how much they use it to ascertain the quality of a scope, rather than buzzwords or hyperbole surrounding materials. it's the "whole" scope that counts in everything. Doesn't do much good to throw a Zambuto in a rusted ol tube that shimmies and shakes when you apply the 50 lbs per square inch of pressure to bludgeon it along an alt-az motion.

The whole scope and mount needs to sing to me.

#6 Midnight Dan

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Posted 05 August 2011 - 08:05 PM

Holy cow! Where DO you get the time! :grin:


#7 germana1


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Posted 05 August 2011 - 08:23 PM

:bow:Holly cow that's a great post. If You go to NEAF this coming year I'd really like to meet you, All Your posts kinda make My head spin, very
Enlightening! Pete

#8 roscoe



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Posted 05 August 2011 - 09:09 PM


Good batch of info there! I agree this one needs to be stickied or added to 'articles'.
It took me a few years to figure all this out, and after all that lurking and reading, was lucky enough to ask the right questions, and lucky enough to get good advice, so both my 80 and 120 are good performers. Neither came to me by way of a full-page glossy ad............. both came by way of multiple good recommendations.

Thanks for taking the time to put this information together!


#9 jeff heck

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Posted 05 August 2011 - 09:45 PM

You are starting to scare me now, Jim. :grin: Still waiting on your first light report from a dark site on the 16" Teeter.

#10 jrbarnett


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Posted 05 August 2011 - 09:55 PM

Friday, August 26, 2011 and Saturday, August 27, 2011. Pinnacles National Monument near Paicines, California. There will be a well-illustrated trip report submitted to CN for publication soon after the trip. Keep an eye peeled for the dark skies themed article based on our April trip to the Mojave National Preserve. No Teeter/Lockwood, but nice skies and a few lessons to boot.



#11 Ed Wiley

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Posted 05 August 2011 - 10:27 PM

Great post, Jim! I really enjoyed reading it.

Clear skies,

#12 magic612



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Posted 05 August 2011 - 10:40 PM

Wow, Jim. Yet another detailed article on the beginner's forum. The mods should collate them all into a single sticky. Thanks for your work.

I was wondering why he doesn't just write a book instead. :)

(And I mean that in all seriousness.)

#13 Bruce Wengryn

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Posted 05 August 2011 - 10:41 PM

Jim, nice article. I find most of your postings interesting to read, when I have the time. I had some idea about the 1/2 wave vs the 1/10 wave, read it on an optical website somewhere and had a trusted optomitist confirm it. One questions still remains, do I get the APO triplet or the ED doublet as a small grab and go refractor?
Thanks for taking the time to post.

Clear skies

#14 jhirsch2001


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Posted 05 August 2011 - 10:49 PM

Wow, Jim. Yet another detailed article on the beginner's forum. The mods should collate them all into a single sticky. Thanks for your work.

I was wondering why he doesn't just write a book instead. :)

(And I mean that in all seriousness.)

I'd buy it !!!!!

#15 GlennLeDrew


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Posted 05 August 2011 - 11:33 PM

A very fine treatise for the tyro!

I'd recommend you make one change. Glass types used for mirrors don't have any significant difference in cooling time (for given dimensions, of course). Rather, it's the difference in their coefficient of thermal expansion which matters. In the simplest sense, for a given difference in temperature between a mirror and its surrounds, a low CTE results in a smaller distortion of the mirror's figure as it cools toward equilibrium.

#16 jrbarnett


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Posted 06 August 2011 - 12:05 AM

Thanks for the clarification Glenn. Right answer from me, but for the wrong reason partially. :grin:

- Jim

#17 FlorinAndrei



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Posted 06 August 2011 - 12:35 AM

Jim, you should really collect all these great posts and put them all in one place, on a blog or something.

There are many astronomy-related forums where people struggle with fairly basic notions - it would be great to have all this material in one place and then one could go "hey, here's a link to good reading stuff, have at it".

Like, for example, your piece about the Intelliscope from a while ago, that was pretty good too, I passed it along several times now. Just gather everything in one place. Write a new one? Post it here, then copy/paste it on the blog. Done.

That would help a lot of people.

#18 Maverick199



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Posted 06 August 2011 - 01:17 AM

[quote name="magic612"][quote]
I was wondering why he doesn't just write a book instead. :)

(And I mean that in all seriousness.) [/quote]

This thought occurred to me as well. I would sure buy this book.

#19 BillFerris



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Posted 06 August 2011 - 10:33 AM

Here's a good starting point for first-time telescope shoppers: How to Start Right in Astronomy. In addition to being informative, there are several useful links and some good advice in the article.

Bill in Flag

#20 jrbarnett


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Posted 06 August 2011 - 03:05 PM

Thanks Bill!

- Jim

#21 auriga


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Posted 06 August 2011 - 04:06 PM

Hi, Bill,
I know you are a highly accomplished and respected observer and I always look forward to your posts.

I would like to propose an alternative to the advice in Alan MacRobert’s post in the link you gave.

I think his advice puts needless barriers in the path of the newcomer to amateur astronomy.

Specifically I would not do the delaying actions and obsessive-compulsive stuff he recommends.

I would not “learn the sky naked eye first.” It’s much better to find one or two important constellations first, say Scorpius and Sagittarius or Cygnus in the summer and Orion and Perseus in the fall and winter, and concentrate on them at first. Then look for the adjacent constellations when you feel ready, and not before. It is not necessary to serve a naked eye indenture before proceeding to telescopes.

Find these constellations by having someone point them out to you at a local star party.

Do this stuff the easy way.

Don’t feel you must “use binoculars first.” Binoculars can be used at any time and are nice for surfing the Milky Way but it is not necessary to serve a binocular indenture before proceeding to telescopes.

Do not bother “ransacking the public library” and "diving into books and atlases" unless this turns you on. For many people this advice is pedantic and overkill and is a counsel of perfection. Instead, buy the Cambridge Sky Atlas, Deep Map 600, and either Nightwatch or The Backyard Astronomer’s Guide, by Terence Dickinson. That’s plenty for the newcomer.

Do not “lose your ego.” Be proud of yourself. Amateur astronomy is a great adventure and it speaks well for you that you are undertaking it.

To choose a telescope, attend several local star parties, look through the scopes, talk with the owners, watch them set up and take down the scope, try to lift the scope yourself, and make your decision on that basis.

To find objects, the first step is to have other amateurs at the star party show you where they are and help you find them. I still remember the person who showed me how to starhop to M27, and the person who showed me how to find M5. The second step is, if permitted at the local star party, use a laser finder obtained from Howie Glatter, or if you prefer, use a Telrad.

To learn collimation, have other amateurs show you how to do it, repeatedly.

Make amateur astronomy a social enterprise, not scholarly research or an extended indenture, unless that turns you on as it apparently did MacRobert.

I have had very good experience with the approach I suggest. I try to return the favor by helping other people learn to choose scopes and to observe, just as people have been so helpful to me. Amateur astronomers are very generous and enthusiastic people.

Bill Meyers
Observing since 1943

#22 auriga


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Posted 06 August 2011 - 04:11 PM

Excellent post! Lucid and comprehensive. The data you present are compelling.

Bill Meyers
Observing since 1943

#23 Scott in NC

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Posted 06 August 2011 - 09:29 PM

Great post, Jim!

#24 tecmage



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Posted 06 August 2011 - 09:59 PM

Jim, that's another outstanding post. I think we need to realize that marketing often has very little to do with design (engineering).

I used to work for a telcom company, and over several months, I read and gathered as much competitive information as I could. I learned that marketing material has very little to do with design and is mainly about market positioning (did I mention I have an MBA specializing in Marketing). The other thing I picked up is journalists to some extent depend on their sources in forming an article.

#25 David Knisely

David Knisely

    James Webb Space Telescope

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Posted 07 August 2011 - 02:41 AM

Hi there Jim. You posted:

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.

True, so true. I first heard the term "diffraction limited" in my Astronomical Instrumentation Techniques course when I was working towards my Physics degree. Dr. Don Taylor asked, "What do the Space Telescope and a 2.4 inch refractor have in common? ANSWER: They are both "diffraction limited"". At that point, he indicated that the optics only needed to be good enough to produce an image that was as small or smaller than the local seeing permitted. If the seeing at a major observatory site never got better than 1 arc second, the telescope would only be required to give that kind of star image "spread", since going significantly finer in quality would not produce much of a gain in performance (and would be much harder (and expensive) to do with very large optics). This was the way "diffraction limited" tended to be looked at early-on, but somehow, that changed. Someone kind of linked the term to the use of the Rayleigh 1/4 wave p-v wavefront error figure (1/8th wave on the mirror surface), while later it also was linked to the Marechal 1/14th wave RMS wavefront error figure. I don't know who did the 1/4 wave link, but the 1/14th wave RMS figure I traced back at least to Daniel Schroeder (ASTRONOMICAL OPTICS, p. 191). At least he says that *one* convention is to consider a system "diffraction limited" when the Strehl ratio is greater than or equal to 0.8 (average wavefront error (RMS) of 1/14th wave or smaller). Still, it is only one convention, so there we are. It is unfortunate we don't have an easy to understand (and independently measured) standard for optical quality in the amateur astronomy equipment producers, although a guaranteed Strehl figure of greater than 0.8 might go a ways towards that goal. Clear skies to you.

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