"Choosing and Using a Refracting Telescope" by Neil English - book

[A McEwan]

Hi all,

Having recently obtained my copy of this work, I thought I'd share a little review of it with you.

"Choosing and Using a Refracting Telescope" by Neil English

ISBN 978-1-4419-6402-1

Springer

This book is part of the "Patrick Moore's Practical Astronomy" range and deals exclusively with refractors. The copy I have is first printing softback, with 283 pages - it's a thick book for a single type of telescope. However, there is plenty of information to fill those pages. In fact, it's probably the only purely equipment oriented book that I have literally read cover to cover, and then used as a resource.

The book is split into three sections, each with numerous chapters within. They are: I) The Achromatic Refractor, II) The Apochromatic Refractor and III) Using Your Refractor. There are also four appendices including a glossary.

The first chapter is The Refracting Telescpe: A Brief History. It's one of the most interesting texts I've read on refractors! Succinct yet full of interesting facts. The rest of the section goes on to reveal how classical achromats are constructed and their optical attributes. Then a discussion of various rich-field and long-focus achromats is made, with reference to many current and not-so-current models. It's nice to see that the manufacturers of these telescopes are fully accredited too, making it easy to find information on particular models that the reader might be interested in.

It has to be said, and if you know Neil then you'll probably be aware of this anyway, that there does seem to be maybe an ever-so-slight favouring of the long-focus achromats! I personally found this interesting and relevent, as there is a resurgence of interest in long-focus achromats at the moment, with the Skylight f15s, Antares Elites, D&Gs, Istars etc proving very popular for amateurs interested in telescopes with a classical pedigree.

The second section, on Apochromatic Refractors, is also informative, showing how ED and other exotic glass objectives have evolved and become more available and affordable to the mainstream amateur astronomer. A very nice touch is that throughout the book, rather than giving just his own opinion, Neil has made use of contributions and opinions from refractor enthusiasts from all over the world! I am included in this group, along with many contributing members of the Cloudynights and SGL forums! The contributions are for the most part seemingly complete and intact, showing that what is printed in the book are genuine and worthwhile opinions of the various telescopes given by genuine interested owners, rather than merely a glossed over "these are nice" summary. Yes, even the negative points are still included!

The third section discusses additional accessories for your refractor of choice, including mounts, eyepieces, diagonals, focusers etc, and discusses how to test your refractor to discover the true quality of the optics. Neil makes mention of all the various aberations that your lens may have, including CA, spherical aberation, chromospherical aberation, coma and others. His notes on collimation and adjustment are very good, and to be honest this section contains enought practical information to keep a neophyte refractor owner happy and capable for a long time. It is obvious when reading this that a lot of Neil's comments are based on personal experiencce.

I've skimmed over these sections somewhat to show exactly what they contain, but they really have to be read for all the little facts, figures, demonstrations and anecdotes to be discovered. It's this collection of lttle tidbits, not just the plain facts and figures, that provide so much information to the reader. It must have taken a long time to collate all these little bits and pieces.

The final chapter, Looking Back, Looking Forward seems almost like a sermon on the virtues of the long-focus achromatic refractor! This single chapter contains a history of Neil's personal f15 achromat and discusses the comparative virtues of such instruments against the current batch of mid-aperture short focal length APOs. Especially, the question is how will refractor design progress? Where will the future take us? It's a fascinating chapter, bound to stir up emotions and get you thinking long and hard about future refractor purchases!

The presentation of the book is a little behind the quality of its actual content. Some of the editing seems to have been done by someone with no experience with astronomical equipment phraseology, and many of the pictures have been reduced and/or darkened so as to make them a little hard to make out the detail. This is the only letdown though, as the content itself is absolutely first class and well worth reading if you have any interest at all in refracting telescopes.

Ant

[F15Rules]

Ant,

Thanks so much for this really interesting preview of Neil's long awaited book. I've been waiting to find this on sale for almost a year, and it sounds as though it has been worth waiting for:hello2:.

Are we allowed on this forum to know where you can obtain it from/price etc?

Wonder if I can point one of my kids in that direction in time for Santa's visit:)..

Hat's off to Neil for what I know was a labour of love for him...whichever type of frac you prefer, I know Neil just loves this type of scope. I hope he sells a good number and I for one will be placing my order asap.

cheers

Dave

[A McEwan]

I got my copy from Amazon, and I see it's in stock there now! (£34.19)

Ant

[F15Rules]

Just ordered mine from Amazon site, but not direct from Amazon...other sellers are offering a bigger discount and I got mine ordered from a 98% rated book supplier for just under £25 posted :-)

[A McEwan]

As did I actually, F15!

[astroimpulse]

Nice review It's persuaded me to get it

Always enjoy reading what you write. A good style that lends itself to you writing something, one day

By the way...Book Depository is a great company and ships worldwide...free! Bought all my recent books from there.

£24.82

[ChrisLord]

Neil English, in his recently published book, "Choosing and Using a Refracting Telescope" SPRINGER ISBN 978-1-4419-6402-1 2011 <http://www.springer.com/astronomy/book/978-1-4419-6402-1> argues that defocus aberration affects the telescopic image...."think of a bout of bad seeing. During such moments, you'll find it very difficult to find the best focus position. Telescopes with a shallower depth of focus will be more affected by this focussing inaccuracy than instruments that enjoy a greater depth of focus. When the bad seeing subsides, the short focus scope will be found to require more corrective focussing than the long scope. So a F/5 refractor will have to work four times harder to 'chase the seeing,' as it were, compared to a F/10 instrument of the same aperture. As will be explained in the final chapter, depth of focus is a greatly overlooked aid to attaining a steady, comfortable viewing experience, especially when observing the Moon, planets, and double stars." (Ch/2 pp30-31, para 5)

English appears to be under the misapprehension that this conjecture is relatively recent (within the past decade or so); it isn't, it results from a misreading of a paper presented by Bob Cox to the Astronomical League, way back in 1963. He also appears to be of the opinion that seeing induced defocus somehow produces a fixed focus shift, dependent on the telescope's aperture, so telescopes with a greater depth of focus can more readily accommodate it. This is also incorrect. It is very easy to prove, using basic high school algebra, that a given seeing induced defocus, produces exactly the same defocus of the image plane, regardless of aperture or focal ratio. I have published my proof of this (I believe I am the first person to set down a formal algebraic proof of this effect) in the AUG/SEP 2010, ATM Letters Journal. I also acquainted the author with my proof before his book went to press in August 2010.

The first two chapters of English's book are riddled with historical and optical errors. I have listed some of the more egregious examples on my website <http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/oldschool_vs_newschool.html#TOP>. I consider the above the most egregious error of all his many errors. Being printed in an astronomy book, part of Springer's Patrick Moore Practical Astronomy Series, and thereby placing it on the public record for future generations of astronomers to read, compounds the error.

When I became aware of this so called 'F/Ratio Conjecture' back in the 1960's, I instinctively knew it was fundamentally wrong, but I couldn't prove it. You only have to compare several different types of telescope, common to the 60's and 70's, to know something was amiss. I had by the mid 70's used a 6 Paris Inch Cooke refractor c1870, 6.5-inch f/13.5, and my own 3-inch f/15 Littrow achromatic refractor, and a 6-inch f/8 Newtonian, and a 10-inch f/6.4 Newtonian. I had also made a 6-inch f/4 flat field Schmidt-Cassegrain, and used a classical 10-inch f/5-f/20 Newtonian-Cassegrain. I'd used all these telescopes to observe the Sun, Moon, planets and double stars. From my personal experience I knew the only thing that seemed to materially affect focus was aperture. The bigger the aperture the more focus correction required in poor seeing. However when I started using the 15-inch f/12 Grubb refractor at the Wilfred Hall observatory, I noted I could no longer see the Airy Disc, but a Seeing Disc, and infrequent focus correction was required.

It took me decades to figure out what was really going on. After several e-mails and a lengthy telephone call from Neli English, I decided to set my thoughts on the subject down on paper, and the result was the paper submitted and published by the ATM Letters Journal last August.

Neil English and his followers argue that depth of focus is what matters when it comes to a telescope providing a steady image in poor seeing. I KNOW this is wrong, because of my simple yet elegant mathematical proof. My theory is based on how defocus aberration is linked to system Strehl ratio. I'm not going into a detailed description here, if you want to know more you are going to have to download my paper <http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/THE%20LONG%20%26%20the%20SHORT%20of%20IT.pdf> and read it for yourself. You will probably have to re-read it, because most of it is technical, awkward to follow despite my best efforts to make it as straightforward as possible, and some of it is counter-intuitive, as is so often the case in science.

Any theory worth its salt has to be testable by observational experiment. To put my theory to the test I purchased and refurbished a classical 3-inch f/16 Fraunhöfer refractor, and conducted a series of observations, comparing focus correction between three small to mid-aperture refractors. An article can be found on my website's Logbook page <http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/PageMill_Resources/Fratio_conjecture_test.pdf>.

The result of this recent observational test supports my theory, and most definitely does not support Neil English's contention. The observational test I carried out is not difficult, and can be done by any experienced double star observer (or anyone accustomed to examining the Airy Disc at high power).

Contrary to English's assertion, bad seeing does not necessarily make ......" it very difficult to find the best focus position". Anyone who has used a high quality apochromatic refractor will be familiar with its "snap-in focus". Anyone who has used a classical long focus achromatic will be familiar with its broad range of focus, and the uncertainty as to where the optimum focus lies. Poor seeing, contrary to what English claims, makes judging best focus of a long focus achromatic harder than a short focus apo. What matters is the Polychromatic Strehl Ratio. Most apo's achieve a poly Strehl ~0.9 to 0.94. Most classical achromatics achieve a poly Strehl ~0.8. There is, to put it bluntly, no comparison.

Anyway, follow the links, download, and read. The record needs to set straight about this so-called F/Ratio Conjecture. Whereas a classical achromatic can give a quite useable image, it is not going to outperform a well made apo of similar aperture, not as far as visual high power work is concerned.

Chris Lord

[sixela]

Chris, it looks like you have at least Bryan Greer on your side of that argument (and me, but that's a quantité négligeable.)

Telescope Optics Topics

It's the second time I've seen Neil utter a "every telescope that didn't exist when I was born is worse" comment in a week. In his upcoming book on Dobs, he is set to do the same thing (dissing fast scopes), and in a thread on another forum, he uses questionable argumentation (first, he posits "sub f/3 scopes" are something real, whereas fast Dobs are actually f/3.3-f/3.66 except for one technology demonstrator by Mike Lockwood --a clear use of the straw man if there was ever one--, then somewhat later he claims his use of the word "fad" didn't imply any kind of negative connotation).

That all struck me as strong on opinion more than on a level assessment of the facts. Neil certainly knows a lot; but in the tone of his posts he strikes me as someone who thinks he knows even more, and that's always dangerous.

For an excellent satire, you just must read John Isaac's post:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/reflectors/Number/4257225

John's got a very long and very large reflector --one long enough to have caused wailing and gnashing of teeth on ladders-- so hes allowed to post something like that.

My view on the subject? Fast scopes need a very good dual speed focuser a lot more badly than slow scopes, and they're a pain in the behind if you want to do planetary imaging and don't have a Bahtinov mask. As for the rest, to quote Mike Lockwood: 'generally the people I hear complaining or pointing out the "negatives" are those who have not used a quality fast instrument.' And seeing effects do not depend on f/ratio.

[AndyH]

Ah good, so the book has been published.

I'll need to ask the family to get me a copy for my birthday.

Cheers,

Andy.

[Neil English]

Dear All,

Firstly, I am very grateful to Antony McEwan for writing the review. Thank you very much.

I am astonished that my work has generated so much heated debate.

RE: Stranger than Fiction

In my defence, the work that I carried out was refereed and rubber stamped many times by several bona fide academics. As a former university lecturer, I am acutely aware of the importance of this process. You can see these acknowledgements at the end of the "Stranger than Fiction" text on Cloudy Nights. The findings in that essay are absolutely valid.

Re: Lord's 'Review' of my Book

Chris Lord is an enthusiastic amateur and so his opinions must be seen in this light. He has no formal training in optics, despite the high school mathematics he attempts to cultivate on his website. Nor is he a historian.

Lord's only real gripe, so far as I can discern, is actually based on a sloppy mis-reading of the findings. The essay on CN openly acknowledges that the atmosphere is F/ ratio independent and the Greer model is absolutely true. It's the many other things that add up in long scopes that I was talking about, and, in that capacity, he has completely missed the point. Indeed, it appears that Lord has developed an unhealthy obsession with my published work, for reasons that are, frankly, quite beyond my comprehension! The book is, after all, a buyer’s guide to commercial refractors.

Lord is right though. The book does contain a few errors that have inadvertently crept into the text and I have earnestly tried to address them in an online errata which I can provide a link to if so required. Mea culpa.

Thank you for reading this.

Merry Christmas Everyone!

Neil.

[ChrisLord]

Re: "Choosing and Using a Refracting Telescope" by Neil English - book

Its not an obsession. Now the corrections have been aired, I'll let the matter rest. If it were just a few mistakes I'd have not bothered to raise them in the first place.

Most of the mistakes (I call them errors because they reveal misunderstanding rather than astro faux pas) I have seen in other books in the past, but not all in a single book!

Prior to the growth of the world wide web, tracking down original source material, or direct references to it, could be very time consuming and expensive. Not so bad if you had security of tenure at a University and could use the department's account to pay. The www enables you to go to original (or close to original) sources with ease. Yes, it can take some time, but not the months or years it used to take.

Every error I have pointed out, could have been easily avoided. All that was needed was the will to check (at the time of drafting). Instead of relying on what you think you know (& I am as guilty of this as the next person so please do not presume I'm holding myself up as some saintly exemplar), "Google" it, and check. You'll be amazed at how many misimpressions we hold. The reason is because they are so often repeated in pot boilers, and coffee table astro books, and magazine articles.

I'll give you couple of instances -

•1) the tale of Lipperhey's children playing with lenses one day and accidently alighting on the telescope. There are two ways to check if this tale has any basis in reality, and neither takes very long (by which I mean a few days). You can go back through authoritative histories - if you do, you get stuck at Grant's History of Physical Astronomy, first published in 1852. Grant does cite a book published in 1655 (Telescopii Inventore - by Petro Borello), but when you download the digitized translation, there is no match. And when you check the first book of the telescope, published by LaGalla in 1617 (De Phoenomenis in orbe Lunae novi Telescopii), its not mentioned there either.

The other way to see if it likely to be true is to read about why the telescope was invented when it was, and not beforehand. The answer, believe it or not, lies not in the optical quality of the objectives, (they were just as poor in 1608 as decades earlier) but Lipperhey's use of a 10mm stop (Yes his first telescope was stopped down from 30mm to just 10mm!). Rolf Willach describes this in his seminal work, "The Long Road to the Invention of the Telescope" published 2008. Without a stop the image was not just blurred, it was a total mess, there was no recognizable image without it. So if his children had discovered the telescope by accident, they would have needed three arms!

•2) you can't project an image of the Sun with a Galilean refractor. I used to think that. Why I don't know, perhaps because I read it somewhere. However in 2008 I decided to make a shellaced cardboard replica of Galileo's telescope "Old Discoverer". His telescope magnified x14, mine magnified x13. I wanted to know how his telescope might have performed. The field of view is irritatingly narrow. Curiously it grows in the dark as your eye pupil expands - bizarre. I began trying to replicate his observations. Not easy. I did observe the Sun direct at sunset, and then out of curiousity, tried projecting it, not believing it would work, but just to see what would happen. And, hey presto, I could focus the image. Except, instead of withdrawing the drawtube, I had to push it in. I then made a x20 Keplerian eyepiece, and compared the projected images. They were equally sharp. It was afterwards I read about Galileo's mathematics student and companion, Benedetto Castelli having recommended the method to Galileo in 1612.

If Neil English's book is a buyer's guide, why do I regard these seemingly trivial details important? Simple, because, to quote an infamous propagandist, when you repeat a lie often enough it becomes accepted truth. Mistakes (or errors) aren't lies, but they are tantamount to lies. They are untrue versions of what happened. Being in print, in a book that because of the internet will live on in perpetuity, the erroneous notion of how the telescope was invented is repeated, reinforced, substantiated, given credence, --- get the message? Likewise, you need a positive eyepiece to project a real image. Clearly you do not, otherwise how would a Barlow lens work?

I'm not in a position to criticise the guide section of the book, the meat. I'm not a gear head, and quite frankly I couldn't care less. Today's must have is tomorrow's has been. I select a telescope not on recommendation, or popular acclaim, but my own discernment.

What I do care about is thoroughness. If you're going to all the trouble and effort of writing a non-fiction book, you owe it to yourself, to your readers, and to posterity, to be as painstakingly thorough as you possibly can be. Pedantic - I hope so. Pedantry is the heart of sound authorship. Scholarship ought to be the watchword. There is nothing to be lost by being scholarly. That does not imply the writing style has to read like a dissertation.

Anyway, that's my point, I think it is pertinent to Neil English's book, even if the purpose of his book is a buyer's guide. I don't think I have missed the point. What I do think is there is a moral here. The moral is, look before you leap. If any of you out there feel there is a book inside you just straining to get out, please make sure you get it proof read chapter by chapter, by those sufficiently au fait with that particularly chapter, to check it, and possibly make editorial alterations or amendments. Springer ought to have an editorial team able to do so, but evidently they do not. But there are other publishers equally culpable.

___________________________________

===================================

Chris Lord

Brayebrook Observatory

Brayebrook Observatory

===================================

___________________________________

[Earl]

"is an enthusiastic amateur and so his opinions must be seen in this light"

Not in context to this thread for which i have no clue but

Regardless of who comments like that is insulting to amateurs, many so called amateurs know far more than pro's mainly for having far more experiance due to time served not letters at the end of there name.

[admin]

What is it about refractors that attracts such passionate debate... (Please don't answer!)

Seriously, keep it civil please. Discussion and debate is good but we expect it to be done in a respectful manner.

[Kef9]

This thread is mental! Why can't we all just be astro buddys

[Psychobilly]

I thought I was on CN and not SGL

[John]

This thread is mental! Why can't we all just be astro buddys

Here here

I'd like to read Neil's book and decide for myself.

Chris Lord has made 3 posts on SGL - all 3 to have a go at another SGL member - how sad !

[Steve]

By the way...Book Depository is a great company and ships worldwide...free!

Just bought myself a copy

[ollypenrice]

There are the issues and there is the psychology here. By comparison with the psychology the issues appear rather dull!

It might be worth remembering that refractors (wonderful things!) are best looked through and not at...

Olly

[Tim]

Thank you for the original review, and for the interesting opinions and responses which followed. There is a lot to digest there if one is so inclined, and probably much to discuss, but let's show respect to the original poster by remembering the gist of the post. We can always start a separate thread to discuss relative merits of whatever, if needs be.

Hopefully as the astro-community gets more established online, those who are gifted enough to understand the complex issues involved behind the workings of any given telescope, will be able to pool their resources and findings to help produce yet superior instruments, that we can all benefit from. Teamwork makes the dream work, as they say

Cheers

Tim

[ChrisLord]

Chris, it looks like you have at least Bryan Greer on your side of that argument (and me, but that's a quantité négligeable.)

Yes quite so, Bryan Geer's webpage on-line from 2000, <http://www.fpi-protostar.com/bgreer/seeing.htm> as you know used OSLO raytracing software to show what happens. Because the software is black box its not evident why it happens the way it does. But you would think that given the information the misunderstanding would be cleared up. Obviously not.

Mind you, one needs to be numerate in order to follow my mathematical proof.

If you are unable to follow either argument you're not in a position to judge are you? But never allow ignorance of the facts prevent you from expressing an opinion (or criticism of the messenger)!

The irony is hard to miss, when a book has been allegedly verified by, "several bone fide academics" yet found wanting by someone with no formal training in optics, nor a historian. What does this say about these academics? The Kings' suit of clothes springs to mind.

The inference is that only those with formal, recognized academic qualifications can have their comments given credence. A case of shoot the messenger me-thinks.

I fail to understand how any academic, versed in the history of the telescope, or astronomical optics, could have failed to miss the numerous technical errors.

The CN article you referenced takes the F/Ratio Conjecture argument to "Reductio Ad Absurdum", "A method of proof which proceeds by stating a proposition and then showing that it results in a contradiction, thus demonstrating the proposition to be false. In the words of G. H. Hardy, "Reductio ad absurdum", which Euclid loved so much, is one of a mathematician's finest weapons." (Coxeter and Greitzer 1967, p. 16; Hardy 1993, p. 34).

The implication is that Hevelius' 150 foot refractor @ f/225 ±113mm depth of focus would never be effected by seeing induced defocus. I have no intention of attempting to put that to the observational test! However Alan Binder has built a smaller replica and used it for double star measurements. <http://www.jdso.org/volume6/number4/binder47.pdf>

[ollypenrice]

'The irony is hard to miss, when a book has been allegedly verified by, "several bone fide academics" yet found wanting by someone with no formal training in optics, nor a historian. What does this say about these academics? The Kings' suit of clothes springs to mind.'

It says two things. It says that academics get things wrong, but I think most of us already knew that, Chris. One of my university tutors said with a smile, 'You know Olly, the worrying thing about doctors is that they don't know any more about their subject than you know about yours.'

It also says, to me at least, that your posts have another agenda.

Olly

[russ]

It also says, to me at least, that your posts have another agenda.

Just what i was thinking.

[sixela]

Now the corrections have been aired, I'll let the matter rest.

...

I think that we both made our point the first time around.

I don't think Neil's appeal to authority is a valid argument in reasoned debate any more than you do, but it's unnecessary to harp on it for long. Just voice your disagreement, state your reasons and move on. If people are interested, there's enough information around to make their own judgement.

On a wild tangent, this is a fascinating article (connected to astronomy) that shows that even good academics indeed sometimes make common mistakes --even in their own fields. I certainly did (in a former life, before I got lured into working for the Evil Industry), and if peer review didn't exist then I would have committed things to paper that would have made me blush for the rest of my life...

http://www.mso.anu.edu.au/~charley/papers/DavisLineweaver04.pdf (see Appendix .

[F15Rules]

I received my copy of Neil's book today, Christmas present to self:)..

Being only a keen visual amateur and refractor nut (mainly achros, but nothing fundamentally against apo's except their price and sometimes rather stubby ugliness:) (IMHO), I love reading anything about refractors and that's why I bought the book. It's also why, having started occasional correspondence with Neil following his "Going Retro" review of several vintage refractors, I have got to know him somewhat, never yet had the pleasure of meeting him in person.

What I do know is this. Neil is a charming, knowledgeable and passionate advocat of Refractors (of ALL types), who just happens to prefer long focus achromats, for all the reasons he states, not least the "depth of focus" argument which I for one subscribe to from my own visual experience.

I have to say that I was horrified to read the vitriol against Neil and his book which poured out of Chris Lord's posts:mad:. I do not know Chris, so have no axe to grind, other than the fact that he has clearly made some very personal attacks on someone who is guilty of no more than writing a fireside companion book which I, and I suspect many others, will thoroughly enjoy reading. Maybe there ARE some factual errors in the text - I am not informed well enough to say, and am certainly not an academic (I went to the University of Life!)..

..but I for one would far rather read an accessible, enthusiastic and interesting book like Neil's, warts and all, rather than the humourless, sanctimonious and patronising techno-rants posted on this thread by Chris. Talk about Bah Humbug!

Keep up the good work Neil, you are an inspiration and a thoroughly decent chap;). And your book is a great read. And thanks for the kind acknowledgement of what was a simple visual observational opinion I offered on a couple of my own scopes performance.

Happy Christmas to everyone, and clear skies for using those refractors - whether Apochromats or Achromats!

Dave

[sixela]

not least the "depth of focus" argument which I for one subscribe to from my own visual experience.

Whatever you may think of Chris Lord's style of debating, though, this is just plainly wrong if you mean that it helps those scopes beat the seeing better (of course the depth of focus is indeed larger, almost by definition, and that means that a very good dual speed focuser is needed once a scope becomes too fast to focus comfortably with a single speed focuser).

And I don't say that because I have an axe to grind with Neil, but just because it doesn't agree with physics once you get to the bottom of it. It's very tempting to "prove" something "from your own visual experience", but it's often very dangerous too.

There are, of course, very valid reasons to prefer a long f/ratio scope --indeed, if they are practical (but that's often a big if), there are actually almost no reasons to prefer a short f/ratio scope except field of view considerations, at least for visual observation-- and the ease of focusing is one of them, but the scopes don't beat the seeing any more than other scopes of similar aperture do.