You may remember me giving a talk, back in February, on some recent correspondence I had with a doctoral student in California by the name of Justin O'Jack. Mr. O'Jack studies comparative religion, and he was able to shed some light on the extraordinary legend of "Spectre of the Brocken" or "Glory", a sky phenomenon related to the rainbow. Mr. O'Jack's dissertation was on religious suicides in China, and he was able to confirm hints I had discovered, that certain revered mountain top sites in China are known both for views of the Spectre, and also for religious suicides. It is stretching things to say, as Henry Miller claimed in one of his books, that on seeing the Spectre, "a devout follower of the Buddha will throw himself from a mountain peak into the arms of Buddha"  — but there is at least a grain of truth in the assertion.
One gratifying aspect of our correspondence was that it resulted from Mr. O'Jack seeing my web page, on astronomical talks - I give a talk entitled "The Arms of Buddha" on the Glory. It is difficult to imagine the two of us ever knowing of each other's existence before the Internet age. In this article, I'd like to tell you about another Internet-based correspondence I have had with an academic in California, shedding light on another of my favourite sky phenomena — the Green Flash.
Jules Verne and the Green Flash
By Mike Frost
The green flash is an elusive phenomenon associated with the setting Sun (you can see it at dawn too, but it isn't as impressive). To see the green flash, you need to watch the Sun setting over a clear, unobstructed horizon that is lower than you are — sea horizons are good, also sunsets viewed from airplanes or skyscrapers. As the very final segment of the Sun sets, sometimes the very final ray of light from the Sun will flash a vivid green. Occasionally the flash is blue, or violet, but usually, unfortunately, you don't see any flash at all. I have been watching sunsets whenever I can for years, and I have only seen the green flash once, from the west coast of Grand Cayman in the Caribbean in 1988.
I also give a talk on the green flash — in fact it's my favourite and most popular talk . The physics of the green flash is fascinating. Basically, the green is caused by refraction in the atmosphere, which splits sunlight into its constituent colours, and atmospheric scattering, which gets rid of the blue end of the spectrum. The effect is to differentiate the colours of the setting sun: red at the bottom, orange and yellow in the middle, and a thin rim of green at the top. The green flash is the light from the rim, which of course is the last part of the Sun to set. To see the rim, it helps if there is an inversion layer in the atmosphere, distorting the disc of the Sun and extending the duration of the flash (this is why it is so elusive). Finally there is a physiological aspect. If you stare at the setting Sun, there is an after-image effect which enhances the green colour. It is possible to photograph the green flash, but it isn't as impressive as it appears to the eye.
At this point I should insert the compulsory health warning. Staring at the Sun can permanently damage your eyesight, especially if you use magnification. The green flash is a phenomenon that only occurs in the last few seconds of the sunset, after almost all the Sun has already set, so it can be observed safely. But it isn't necessary to start watching until well into the sunset. Please take care!
Interesting though the physics is, what make the green flash such an appealing subject to lecture on are the wonderful legends associated with it. These are reputed to go back a long way. Most references on the subject start by claiming that the green flash is represented in Egyptian mythology. I am not completely convinced by these claims. But here is one source who appears to have found a mention of the green flash in Egyptian mythology; Norman Lockyer, Rugby's very own astronomer and founder of archaeo-astronomy: -
"Shu is also the Dawn, or sunlight. Tefnut represents the coloured rays at dawn. Shu and Tefnut are the eyes of Horus. Shu was also called "Neshem", which means green felspar, in consequence of the green colour observed at dawn. The green tint at dawn and sunset are represented further by the 'sycamore of emerald.' Sechet is another goddess of the Dawn, the fiery Dawn." 
The first incontrovertible description of the green flash was by an obscure Italian academic, Pietro Giuseppi Maggi, in 1852. The first description in English was by James Joule, in a letter to the Manchester Philosophical Society in 1869. However the green flash was brought to the attention of the general public a few years later, in 1882, not by a scientific paper or newspaper article, but by a novella, "Le Rayon-Vert" (the Green Ray) by the French author, Jules Verne.
I'm sure that everybody reading this article will have encountered Jules Verne's work in some form; usually one of four works, "Twenty Thousand Leagues Under the Sea", "Around the World in Eighty Days", "Journey to the Centre of the Earth" and "From the Earth to the Moon". Yet Jules Verne wrote dozens of novels and novellas, most of which have been long forgotten, for example "Measuring the Meridian", "Tigers and Traitors" and "Among the Cannibals". "The Green Ray" certainly falls into the forgotten category, with some reason. It is one of the slightest of Verne's novels. I managed to acquire a copy: first of all borrowed through Rugby library, later, after much Internet searching, purchased from a bookshop in Pennsylvania .
The Green Ray is set, like most of Verne's stories, in the English speaking world — in this case, in Scotland. The heroine is the beautiful but vapid Helena Campbell, who lives with her indulgent uncles Sam and Sib in Glasgow. At the start of the novel, she announces that she cannot marry until she has seen the green flash, for she has heard of an old Scottish legend
"... this ray has the virtue of making him who has seen it impossible to be deceived in matters of sentiment; at its appearance all deceit and falsehood are done away, and he who has been fortunate enough once to behold it is enabled to see closely into his own heart and read the thoughts of others."
Helena, Sam and Sib and their entourage, set off on a tour of the west coast of Scotland, in a series of increasingly desperate attempts to see the green flash, from Oban, Iona and finally Fingal's Cave in the Hebrides. En route they encounter my favourite character (and role model), the nerdish Aristobulos Ursiclos, who annoys Helena intensely by continually attempting to explain the green flash.
"The Green Ray" raises several interesting questions. First and foremost — is the legend of the green flash genuine? By this I don't mean, "Does it really endow all those amazing abilities?" — remember, I've seen the green flash, I know! Rather, is the legend of green flash authentic? And if it is authentic, how did Jules Verne get to hear of it? (bearing in mind he was almost the first person to mention it in print). Did Verne ever visit Scotland? And did he ever see the green flash for himself?
Up until the end of 2001 I could only answer one of these questions for certain. Yes, Jules Verne did visit Scotland. He wasn't a great traveler, although he did own a series of small steam cruisers in which he pottered around the French coast. But it turns out that he did make two visits to Scotland. In 1859 he made a short visit to Glasgow, Edinburgh and the Trossachs (traveling by train from Liverpool, and returning to London). Verne wrote about these travels in a short, lightly fictionalized memoir entitled "Backwards to Britain" ("Voyage a reculons en Angleterre et en Ecosse") . In 1880 he made a return visit to Scotland in his own yacht, taking a cruise round the Western Isles, during which he undoubtedly visited many places that later featured in Le Rayon-Vert
Then, in the November 2001 edition of Sky and Telescope, I made a delightful discovery. Sky and Telescope has a short feature every month called "The Near Sky" on sky phenomena, which often features the green flash. In both the November and December editions, the sky phenomena feature extolled the many virtues of a web site, put together by an American physicist called Andrew (Andy) Young, of San Diego state university, California. This is the address of the web site.
http://mintaka.sdsu.edu/GF ("a Green Flash page")
This site is wonderful! You would be hard pressed to imagine a more comprehensive resource on a single subject — especially one so dear to my heart. "A Green Flash page" has detailed and painstaking analyses of the physics, including animations demonstrating the various types of mirages which can form green flashes; a photographic gallery; links to web sites featuring the green flash. Best and most wonderful of all is a 559-page bibliography, annotating everything ever written in print about the green flash.
Of course I downloaded the bibliography! Then I set to work answering my many questions about the green flash. The second name that I looked for  was Jules Verne — and when I found him in the bibliography, I got quite a surprise. "The Green Ray", which I had been telling people for years was almost the first time the green flash had ever been mentioned in print, wasn't even the first novel in which Verne had written about the green ray! That honour belonged to an 1877 novel called "Black Diamonds", or "Les Indes Noire". And that novel was also set in Scotland.
You can guess that I immediately set out to acquire a copy! This time I was able to purchase the book over the Internet from a bookshop in Cincinatti. Black Diamonds is set in the coalfields of the Trossachs, which, you'll remember, Verne visited on his first trip to Scotland. Astute readers of this article may have spotted that Black Diamonds, translated into French, does not come out as "Les Indes Noir", which actually means "The Black Indies". The French title is rather contrived and was pretty obscure even in Verne's day. The idea is that the British Empire was built on three separate sources of wealth — the fabulous riches of the East Indies, the flourishing trade in slaves and cotton with the West Indies, and finally the mineral wealth of the British coalfields, the Black Indies. It seems that few publishers liked the original title. The first American edition named the novel "Children of the Caves" which largely misses the point of the story. The English edition (and later American editions) settled on "Black Diamonds", and I think that's a good choice — it's a title that catches the spirit of the novel.
The novel is the story of the largest coalmine in the Trossachs, owned by one James Starr . At the beginning of the story the mine, it seems, has been worked out; however there are still people in it — the mine foreman Simon Ford, and his son, Harry, who live in an underground house, but also mysterious others, who never show their faces. Harry Ford is convinced that there are still unworked seams to be found in the mine, and in the first half of the book, he and James Starr make a series of remarkable discoveries. First of all the unconscious body of a young girl, Nell , who, it turns out, has lived her entire life underground. Second, and thrillingly, the discovery of not just a new seam, but also an entire new immense cave system, stretching for miles beneath the Trossachs and promising untold mineral riches. [I paraphrase a lot of novel!] 
As the second half of the novel begins, several years later, the coalmine is once again thriving — indeed there is a virtual underground city, which Verne gives a most appealing description of. On the romantic front, you won't be surprised to learn that young Nell and Harry Ford have fallen in love and want to marry. Harry, however, has his reservations. How can he possibly marry a young woman who has never seen the world above ground? How could it be fair for her to commit to him without seeing the infinite possibilities of the world outside the mine?
They decide that Nell should travel above ground, and the chapter that follows is my favourite of the whole book. Harry and James decide that it would be least disconcerting to Nell if she went topside initially during the hours of darkness, so as not to overwhelm her eyes with the light of the Sun. They take her to the surface and she sees for the first time the beauty of the night sky.
"Nell gazed on the myriad of shining stars which swarmed overhead.
'But how is it,' she asked, 'that if they are suns, my eyes can bear their brightness?'
'My child,' Starr replied, 'they are suns it is true, but suns at an enormous distance. The nearest of these millions of stars, whose rays can reach us, is Vega, that star in the Lyre which you can see there — almost in the zenith, and that's fifty thousand millions of leagues away. . ." 
Harry and James have decided that Nell should see her first sunrise from the city of Edinburgh. So they take Nell to Stirling, to catch a boat down the Firth of Forth to Leith. Nell, not surprisingly, is captivated by the journey. Dawn is approaching. They land at Leith, travel rapidly through the Auld Leekie and up the volcanic plug of Arthur's Seat, overlooking Edinburgh.
"The range of colours appeared one by one, in the order given them by the solar spectrum. The red on the early mists shaded into the violet of the zenith. From second to second the palette became more distinct: pink became red, red became fire. A kind of ashy light spread through space. At length one solitary light caught the girl's eye. It was that Green Ray which, at morning or evening, shoots upwards from the sea when the horizon is clear"
So — the very first ray of sunlight that young Nell ever sees is green! Is that impressive or what!! I'll leave the denouement of the novel for your pleasure — there are one or two surprises still to come (including the unexpected disappearance of Loch Katrine) but I think I'm not giving too much away to say that Nell and Harry live happily ever after.
Can we draw any conclusions about the Jules Verne and the green flash? I think we can. First of all Verne's description of the green ray in Black Diamonds lends weight to my suspicion that Verne never did see the green flash. The description simply doesn't ring true — the green ray does not shoot up from the horizon, it is the ray of light from the horizon to the eyeball. However, the fact that Verne mentions the green flash in both the novels he sets in Scotland — and no others — would suggest that he heard about the elusive ray in one of his visits to Scotland.
But what about the legend of the green flash's fantastic properties? Tellingly, I think, Verne makes no mention whatever of the "old Scottish legend" in Black Diamonds. And Andy Young's thorough investigations have so far managed to uncover not a snippet of evidence that the legend existed prior to the writing of "Le Rayon-Vert". Verne, it would seem, simply made the whole thing up.
One can guess that the mention of the green flash in Black Diamonds prompted lots of enquiries along the lines of "what is this green ray you are talking about?" Verne, remembering his earlier cruise in the Western Isles, decided to write a novel featuring the green ray, and needed to generate a good reason for Helena and her entourage to go chasing off after it. Hence the legend. 
So — the legend of the powers of the green flash is Jules Verne's concoction. But, like many a good story, it has subsequently acquired a life of its own. Almost all articles on the green flash tend to mention it, and most tend to assume that Verne was passing on an authentic piece of Scottish folklore.
However; all is not lost! A little more dipping into Andy Young's comprehensive bibliography did uncover an authentic Celtic legend of the green flash! I had previously come across hints of it in Marcel Minnaert's wonderful "The Nature of Light and Colour in the Outdoors" which was one of my original sources. A footnote in some editions of this book makes tantalizing reference to a legend associated with the green flash called "Living Light". And this legend comes not from Scotland but from the Isle of Man .
Living Light is the English translation of the Manx "Soilshey-Bio". Manx, like Cornish before it, is now a dead language; the last native speaker, Edward Madrill, died in 1974 . The reference to the Soilshey-Bio legend comes from the 1930's, when there were still Manx speakers but the language was already in trouble. A Manx writer called Mona Douglas was interested in recording Manx Folklore. Andy Young managed to uncover an article by Ms Douglas in the Times of September 10th, 1929.
". . .something like the green flash appears occasionally in Manx folklore. The old Manx name for it was soilshey-bio, or 'living light'; and I have gathered the impression, without having been actually told so, that it was thought to be an emanation of the sun's life. In several fragments taken down by me from Manx fisher folk, the 'flash' was seen at sunrise on the morning preceding the wreck of one or more boats. . . the 'flash' certainly also had its beneficial side in popular belief. . . I had this belief directly from a very old man who was, I should think, about the last of the 'charmers'. . . if any person could find what he called 'the herb of life' at the moment when it was touched by the soilshey-bio, death would never touch him or anyone to whom he gave a portion of the herb to eat."
So — seeing the green flash may not enable you to see into the hearts of others. But it might let you live forever!
I would like to finish by telling you about some more recent connections between Jules Verne and the green flash. Le Rayon-Vert has inspired a number of more recent works. In my talk on the green flash, I mention one such book that I came across serendipitously, "The Green Flash" by Winston Graham . You'll no doubt be astonished to hear that the book features a lovelorn heroine, Shona, who longs to see the green flash. However, in Winston Graham's book, the green flash forms a very small part of the story; it's by no means the central theme. Nonetheless the book is a pretty good yarn.
There is an even more recent retelling of the green flash myth. In 1996, more by luck than judgment, I had the great fortune to witness a sunset eclipse of the Sun, setting into the Aegean Sea southwest of Olu Deniz, Turkey. I had a clear unobstructed horizon which I observed from a beach bar a few metres above sea level, so it occurred to me that I might be lucky enough to see not one but two green flashes, as the two sides of the Sun, split by the eclipsing Moon, set into the Ocean. Of course, I saw not two green flashes but zero, as usual (but I have subsequently heard that a double green flash has been seen in similar circumstances). Also watching the eclipse with me was a young English couple. They were first of all bemused by my behaviour, then amused, then intrigued, and finally as enthralled as I was by the spectacular end to the eclipse (it was very beautiful). I was about to expound my green flash theory to them when they said, "Oh, we know about green flashes, we've seen the movie".
I was gobstruck. I asked them "what movie?" but all they could remember was that it was French and subtitled. Not much to go on! However, the bibliography from the green flash web site came up trumps yet again. The movie is called "Le Rayon Vert" (surprise, surprise), translating to "The Green Ray", although in America it was entitled "Summer". It was made, in 1988, by the veteran French director Eric Rohmer. It received very good reviews — my movie guide gives it 3Ω stars out of 4 — and it won the Golden Lion award at the Venice film festival.
Now here's a funny thing. Eric Rohmer is still directing movies, and his latest film was out earlier this year. Film Four did a retrospective of his career, and showed, amongst others, "The Green Ray". Unfortunately I don't get Film Four on my cable channels, and Sky Sports won't be doing a Rohmer retrospective season any time soon . So I thought that I had missed my chance — until I spotted that the National Film Theatre were also doing a season of Eric Rohmer films. And so, one Saturday in May, I took a train down to Euston, a tube to the South Bank, found the NFT, and settled down to watch "Le Rayon Vert" .
It's a charming movie. Not a tremendous amount happens, but I gather that Mr. Rohmer specialises in observational comedies made with a regular company (Mike Leigh springs to mind as a British equivalent). Le Rayon Vert features a strong central performance by Marie Riviere, who plays the lovelorn heroine Delphine, a secretary from Paris. Poor Delphine is really in a mess — she can't admit to herself that her fiancé has dumped her, none of the many suitable men who try to chat her up get a moment's attention, and worst of all she has nowhere to go on holiday for the summer. In desperation she joins friends in Cherbourg, and spends her afternoons moping along country lanes. A visit to La Plagne in the Alps lasts barely an afternoon before she gets fed up and takes the train back to Paris. Another friend lends her the keys to an apartment in Biarritz, and, whilst sitting alone by the beach there, she overhears a group of people discussing a remarkable book, "Le Rayon-Vert" by Jules Verne.
She listens, intently, as they describe the green ray and the miraculous powers it bestows on those who see it. One or two of the party have seen it, but others describe how they watched and saw nothing. The most nerdish of the party, a professor, gives a brief, half-accurate, explanation of the physics of the green flash, and one of the others (to my huge delight) compares him to my hero Aristobulos Ursiclos. Delphine listens, and moves quietly on.
Tired of Biarritz, our heroine returns to the railway station, to catch the next train back to Paris. But in the waiting room she falls into conversation with a fellow traveler, a carpenter from the country. He's on holiday just down the coast in St Jean de Luz. Perhaps she'd care to join him for a few hours? Delphine considers his offer carefully, then accepts, but with one condition. Before they go out for dinner, she would like him to come and watch the sunset with her.
They climb up to the headland above St Jean, to watch the sun set over a clear unobstructed horizon. Our two characters watch intently — Delphine in keen anticipation, her new boyfriend wondering what the hell is going on. It's a perfect sunset, and as the disk slips beneath the horizon, we see the final ray of light flash green .
"I saw it!" says Delphine, and we know that things are going to get better.
Cut to the final credits.
•1 The quote is from Henry Miller's "The Big Sur and the Oranges of Hieronymus Bosch" (Heinemann, 1958).
•2 So far this year, I've given the talk to Liverpool AS, Birmingham AS and Knowle AS.
•3 The quote is from "The Dawn of Astronomy" (JN Lockyer, 1896), which Vaughan was good enough to lend me a copy of. I sent Andy Young this quote, as it didn't appear in his bibliography. He thinks that it isn't green flash related, the use of the word 'tint' suggesting that it applies instead to the "greenish color often seen in the sky when the Sun is low". I'm not sure I agree, as Lockyer also mentions rays at sunrise.
•4 I love the Internet! Along with the Green Flash website, one of my favourites is www.bookfinder.com. Try it some time! Visit the site with the name of a book you have always wanted to own — you may surprise yourself. For "The Green Ray", I searched every few days for eighteen months before locating a copy.
•5 Thanks to David Cook, a Jules Verne enthusiast, for telling me about "Backwards to Britain". I have recently acquired a copy (through the Internet, of course, from a bookshop in Ontario). There is no mention of the Green Flash in it.
•6 The first name I searched for, needless to say, was "Mike Frost", as I wanted to see if Andy Young had found my seminal article "Blue Sky, Red Sun, Green Flash" in the November 1996 Astronomy Now. He had found the article. His summary of my article read, in its entirety "Many small errors." Sigh.
•7 Verne names the mine Aberfoyle, which sounds Welsh rather than Scottish to me. I suspected that Verne's usually assiduous research had failed him, but "Backwards to Britain" mentions that the valley between Loch Lomond and Loch Katrine is called Aberfoyle.
•8 There's a note in the book pointing out that "Nell" is short for "Helena". So, curiously, the heroines of Verne's two green flash novels have the same first name.
•9 Jules Verne loved parading his knowledge. He knew full well that the world's largest cave system was Mammoth Caves in Kentucky, so he compares the Scottish cave system in detail to Mammoth Caves. This appealed to me particularly as I have visited Mammoth Caves - they are very impressive (but contain no Mammoths).
•10 When Verne wrote Black Diamonds, astronomers were only just beginning to measure distances to stars. Not surprisingly they concentrated on the brighter stars. Vega is 26 light years from the Sun, which puts it in the galactic neighbourhood, but it is by no means the closest star. That honour currently belongs to Proxima Centauri, 4.2 light years away.
•11 Verne wouldn't be alone in concocting Scottish history. In 1762, a Scottish schoolmaster called James McPherson published a volume of Scottish mythology, featuring the legends of the warrior king Finn McCool, as recounted by his poet son Ossian. It turned out that McPherson made up the whole tale — McCool, Ossian and all (he did loosely incorporate genuine Celtic mythology). Verne mentions Ossian in The Green Ray.
•12 When I first heard about the Manx legend of "Living Light" I called the reference library in Douglas to see if they had heard of the legend. They hadn't.
•13 I remember reading in my childhood about the very last Manx speaker, and feeling sorry for him. Imagine not even being able to ask for a cup of tea! I hadn't realised at that time that he probably spoke English as a second language, and I'm sure that there are still people who speak Manx as a second language, just as there are still Cornish (Kernewek) speakers even though Cornish has been dead for 200 years. But thinking again many years on, Mr. Madrill still has my sympathies. Imagine knowing that nobody else dreams in the same language as you.
•14 Winston Graham also wrote the Poldark novels, however "The Green Flash" is set not in 18th century Cornwall but among the British aristocracy during the 1970's.
•15 Moreover, www.classicmovies.com were unable to supply me with a copy.
•16 I wrote to the NFT and asked if I could have the poster for "Le Rayon Vert" on display in the lobby. They said no.
•17 According to one reviewer, "Rohmer banked the entire movie on being able to film the green flash. He filmed sunsets for months and months." In fact, the cinematographer, Philippe Demard, was able to film the sunset shots without difficulty, although he did go to the Canary Islands rather than St Jean de Luz.
"The Green Ray" finished at 10.24 P.M. — and, at the NFT, nobody leaves their seats until the final credits have finished! So I was left with 26 minutes to get back to Euston before the last train to Rugby departed. I am no longer in London Marathon condition — nevertheless, I still made it with three minutes to spare.
By Steve Payne
Question When a flame comes out of a lighter on earth, it points up; which way would it point in a zero g environment? Asked by: Chad Fultz
Answer The short answer - It wouldn't point up, it would just form a burning ball right on the end of the lighter. The "why" is fairly simple upon reflection, but not particularly intuitive. The burning of gas is it's combination with an oxidant and the release of energy raises the temperature of the gas (hence "heat rises"). The warm gas has a lower density than the air around it and thus flows toward a region of lower density, i.e. - away from the gravitational pull of the Earth. In orbit, the net effects of gravity are balanced by the acceleration around the Earth. Thus, the warmed gases expand equally in all directions and consume oxygen from the atmosphere equally in all directions. The fire will burn in a ball, outward in a growing sphere. It will usually burn faster than it can supply itself with oxygen and burn out in a short period of time.
Answered by: Frank DiBonaventuro, B.S. Air Force Officer, Tinker AFB, OK
Further reading: Not Just Another Old Flame by NASA Science
By Ray Goodison
Ray has been writing to fellow amateurs who had letters in Sky & Telescope about his interest in observing and his problems with his Barlow lens, this is an edited version of his correspondence. Ed.
Two Letters from Sky & Telescope
A New Breed
I chuckled to myself as I read David Buffington's commentary about a new breed of "Go To" telescope observers. He warns veteran observers about computerized-telescope toting, impertinent amateur astronomers. I am a veteran observer who has been navigating the universe with star charts for about 40 years. During that time I have seen many newcomers appear beside me in the dark with the latest and greatest equipment. Most aren't there anymore. They quit for many reasons. Their complaints included the cold, mosquitoes, or just being too tired. When their batteries die or the gears in their telescopes grind to a halt, they shouldn't be afraid. My breed will still be out there to take their hands and show them around the night sky the old-fashioned way. A lot of us old-timers don't even need charts for many objects. Star patterns and constellations are emblazoned on our brains. Our relationship with the night sky is perhaps more intimate than they will ever understand and that makes me just a little sad for them. When these upstarts disappear in the dark, never to return, I will be waiting patiently for the next "unstoppable stampede?" I will respectfully suggest to them, as I have to all others, that the universe cannot be purchased. As Ken Fulton says in his book The Light-Hearted Astronomer, "You must approach the universe on your knees. She demands humility, respects perseverance, and abhors being taken for granted?'
MICHAEL E. ALLEN
I believe these new Go To telescopes have the potential to kill the best part of amateur astronomy, namely, the thrill of the hunt! It is enormously satisfying to bag a rare deep-sky object using only the skills and knowledge lovingly acquired over time. Go To scopes trivialize these skills, the way calculators trivialize math skills. Let's be honest: astronomy's principal attractions as a hobby are its difficulties. Its key rewards, beyond the quiet riot of personal discovery and nights of contemplative solitude, include the confidence and satisfaction that follow mastery of a difficult skill, as proved time and again by amateur telescope makers. If you just want to "see stuff" and require instant gratification, it would be better to buy a book of Hubble Space Telescope photographs than to purchase a Go To telescope - a book is cheaper and easier to carry around. Also, the quality is far better than anything you could experience through an amateur telescope.
I read with interest these two letters and wrote to them in support of their views to both contributors and also asked them some questions about my 2x Barlow lens and what would be a good choice of eyepiece for my telescope. (See MIRA 55 Ed)
I received these replies and some notes on barlow lens. . .
From Scientific American, Sept, 1943:
In Amateur Telescope Making, is the following on the Barlow lens; there is a formula for separated lenses, viz., F = (fl x f2) / (f1 + f2 -d), where F = focus of combination, fl = focus of telescope objective, f2 = focus of Barlow lens, and d = distance between the two lenses. Suppose we had a Barlow of focal length -18" (minus because the lens is diverging or negative) used with a 6" reflector of 48" focus. Suppose further that the Barlow is placed 2" inside the focus of the mirror. What is the new focal length? What is the magnifying power? We get out our pencil: Substituting in the formula:
F =(48x-18)/(48-18-46) = -864/ -16 = +54.
Note that the eyepiece has to move 6" out to the new focus. As the Barlow moves in toward the mirror the eyepiece moves out and the magnification increases. The magnifying power of this combination may be found by dividing the length of the cone cut off by the Barlow into the length of the new cone formed, the cone cut off is 2" long and that the new cone is 8" long. The quotient of these is 4. Thus the magnifying power is equivalent to that of a telescope having four times the focal length of the original. A magnifying power of four is about the maximum desirable. The Barlow lens must be of sufficient diameter to include slightly more than the diameter of the cone of light from your objective when the Barlow is placed as far within the focus as it will be used. 3/4" would be about right for the example above, since a 1/2" diameter field is about this size 2" inside of focus in a 48" focus telescope. As in any system which magnifies, the intensity of illumination goes down. The advantage of the Barlow is that the cone of light from it has a more acute angle at the vertex than the cone from an f/8 reflector. Thus, cheap eyepieces will perform satisfactorily with it and will be comparatively free from the bad color and spherical aberrations which they produce when fed the wide angle beam from a large aperture-ratio objective. Despite the imperfect achromatism of the Barlow, the overall result will be an improvement over the common eyepieces. Most of us can't afford good eyepieces needed to give good color correction on reflectors, so the Barlow is an easy way out o an achromatic lens of not too rigorous limits of figure and achromatism.
From Scientific American, Sept. 1944:
I thought some of your regulars would rise to the occasion and point out the errors but, as they haven't. . . He proposes a Barlow of -18" focal length, to be placed 2" inside the focus of a telescope of 48" fl. He figures correctly that the effective fl of the combination will be 54", but then goes on to assume that the eyepiece will move out 6", and that the magnifying power will be quadrupled. Actually, the eyepiece will move out only ¼" and the amplification will be only 12.5 percent. He is just dealing with the tip of a theoretical cone of light 54" long. The ratio is 2 1/4 :2::54:48. As a check, although the formula used is entirely adequate, I have just traced the paths of the rays trigonometrically through a hypothetical -18" Barlow and it came out 2.249". I have used a Barlow for years with sundry eyepieces. If you had a Barlow that would amplify x4 at 2" inside focus, the fl of the Barlow would be 2 - 2/3". If you then moved it to 1" inside focus, the amplification would be x1.6. If you moved it 6" inside focus you would see nothing at all through it, because the rays would then be divergent. At 2 - 2/3" inside focus they would be parallel, the secondary focus at infinity. Any particular Barlow has the same amplifying power when placed at the same distance inside focus, no matter what the telescope.
For a primary focus at 48", Barlow of 2 2/3" fl, 1" inside focus, amplification is, as already stated, x1.6 and power of telescope with 1" eyepiece is 76.8. Corresponding figures for 2" inside focus are x4 and 192; for 2 1/2" inside focus, x16 and 768; for 2 - 5/8" inside focus, x64 and x3072.
Suppose that a Barlow lens of focus -F is placed p inches inside the focus, A, of a telescope. The new focus, B, will be at a distance q from the Barlow. The relation between these distances is 1/q + 1/p = 1/f. The ratio q/p = magnifying power.
If we have a Barlow of 2-2/3" focus, placed 2" inside the focus of the telescope objective, we get 1/q - 1/2 = - 1/2 - 2/3 and q therefore = 8. In the above 1/2 takes the minus sign, in accordance with the direction of light from A, both images on same side of lens. The magnifying ratio then is q/p = 8/2 = 4.
Caution: In using a Barlow with reflecting telescopes, avoid high amplifications, else the color correction will be poor. A 3" focus Barlow to give an amplification of not more than x1.5 or x2 and will be the top useful limit. This works out to p = 1 1/2" (inside focus), q = 3" (new back focus) and q/p = 3/1 - 1/2 = 2 (amplification).
The result of moving the lens within reasonable limits is very small, as I had understood that one could produce any amplification by moving the Barlow back and forth.
I enjoyed reading your letter, and I encourage you to share your thoughts with Sky & Telescope or Astronomy. My mail is running about 50/50. About half think I am a moron for not embracing computerized 'scopes. They seem to hold this idea, that whatever puts telescopes in people's hands is good for astronomy. I'm not so sure. Certainly, such people could aid in group-participation observations, like timing the exact moment of some event... but should we trust data gathered by dilittantes?
Having been blessed with chronic back pain, I prefer small, light, Iong-focus telescopes. My Meade ETX-90, (the first generation: not computerized!), is very nice. It's a 90mm Maksutov, with an 11" tube and a four-foot focal length! The moon and planets look fabulous in it, even at high power! I use 26mm, 15mm, and 9.7mm eyepieces, and a 2x Barlow lens. I also regularly use a 60mm Celestron refractor, whose zoom lens provides 20-60x, and pair of Meade 8x42 wide angle binoculars. Speaking of Barlow lenses, when you say your Barlow "does not reveal anything," I assume you mean that when using it, the view through your eyepiece appears black, as though obstructed. This being the case, I can think of a few possibilities. First, remember that the "exit pupil" (the cone of light you are peeking into, in the eyepiece), is now half the size it was, making it harder to find. That is, it is possible to look though an eyepiece and see nothing, because you're not looking where the cone of light is. Sometimes my wife simply cannot see the close up of Jupiter I've set up, because she can't find the light. I find it helps to look for the dot of light on the eyepiece from a distance, and keep your eye on it as you move in closer. Try the Barlow on the 23mm eyepiece and see what happens.
Another possiblilty is that somehow the lens has been put together wrong. This is not uncommon, (think Hubble). Indeed, my Meade ETX came out of the box with its finderscope assembled backwards! Of course, another possibility is that there is an actual physical obstruction inside the tube. A dead bug would not be an impossibility. If it turns out a lens cap was on, though, you'd be wise not to admit it to anyone.