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New School vs Old School

by

Chris Lord

Rodger W. Gordon and I occasionally are accused of being "Old School", which is tyro speak for being out of touch with the hobby. We are dinosaurs unaware of how the hobby has moved on. New school evidently is where the hobby now is, and the implication of being labelled, "Old School" is that rather than moaning about the state of the hobby nowadays, we ought to play catch up.

Catch up to what though? From what we see, most new school astronomy consists of buying expensive kit, including a ccd camera, heading for the nearest star party at a new moon weekend, and learning how to take deep sky astrophotographs. The aim seems to get your pictures published in a national magazine. You can swank around on forums as a 'guru' handing out sage advice.

Or there is the new school deep sky observing, preferably with a big Dobbo, the bigger the better. Your observing prowess being determined by the length of the step ladder needed to reach the eyepiece.

It never seems to occur to new schoolers that there is more to amateur astronomy than deep sky astrophotography and deep sky observing. New schoolers seem to think the only alternative is solar system, variable and double stars, and comet hunting, or maybe supernove hunting. That's it, the full gamut. Might as well stick to getting better at taking ccd images of deep sky objects. If one becomes an adept, then you can establish a reputation and who knows, maybe a lasting legacy.

There is a well worn old school addage, "When everybody's somebody, nobody's anybody," and it applies to new school thinking all down the line. The only way to make your mark, if that is your long term goal, is to find a niche and exploit it. The field of astronomy is vast, it encompases not only visual and photographic observation and recording, but research, both physical and historical, telescope making, the history of telescope making and the history of instrument making.

What Rodger and I realise distinguishes new school from old school, is that old school has the benefit of acquired knowledge, and perspective. Most new schoolers know their own specialist field and have only a vague appreciation beyond. But worst of all new schoolers presume that because they are experts in their own field, they either know it all, or what they don't know isn't worth knowing.

A sweeping generalisation? New schoolers will think so, but we have a classic example of how new school subject awareness can lead the unwary new schooler into unfamiliar territory without their being the least aware of it.

Recently Springer published as part of their Patrick Moore practical astronomy series, 'Choosing and Using a Refracting Telescope' written by Neil English, a regular Astronomy Now columnist, who came to the fore as an astro-imager published in Astronomy magazine five years ago (2005).

Chris Lord contributed to the sections dealing with the strengths and weaknesses of achromatic and apochromatic refractors, and is cited in several chapters. The author's métier is telescope review and field testing. What is self evidently not his métier is the history of the telescope or telescope and atmospheric optics.

This is a review that would never get into print in any national magazine. A copy has been sent to Springer's editorial staff, just for the record. We hope, having read the review, you will begin to appreciate why old school is better than new school.


CHOOSING and USING a REFRACTING TELESCOPE
NEIL ENGLISH _ SPRINGER ISBN 978-1-4419-6402-1
2011
PATRICK MOORE'S PRACTICAL ASTRONOMY SERIES


Dr. Neil English is a review columnist for Astronomy Now magazine. This book is the province of his métier, telescope reviews and field tests. The book is in three sections, dealing with achromatic; apochromatic; and testing, refractors.

The sections describing currently or recently available refractors, and upon testing refractors, are interesting and informative. The author holds the reader's curiosity as he leads you through the entire field of refractors from the humble spy glass to the gargantuan observatory class affordable only to the well heeled.

His book is interesting, informative, and well illustrated. Most of the descriptions, explanations and arguments, readily understood, except for the rather esoteric so called Sacek effect. (You'll have to read the last section to see what I mean).

Unfortunately what clearly is not Dr. English's métier is the history of the telescope, or optical theory.

Chapters 1 & 2 cover the history of the refractor and the classic achromatic respectively. Chapter 1 especially is riddled with errors and inaccuracies, some elementary, some fundamental, that quite frankly spoil what is otherwise a helpful book.

If the numerous errors are not detailed they may in future be repeated by those too lazy to consult original sources. Dealing with each, in succession they are as follows:



Ch.1. p3, para 3, in dealing with Hans Lipperhey's invention......"Apparently, he or one of his children accidentally discovered that by holding two lenses in line with each other, distant objects appeared enlarged".

This tale and variants of it where his apprentice makes the discovery is a well attested mid C19th apocryphal invention. Prior to Grant's "History of Physical Astronomy" 1852, in which it is first mentioned, there is no source.

The earliest evidence for the existence of an actual refracting telescope is Lipperhey's patent application to the States General of the Prince of Nassau on 25th September 1608.

Ch.1. pp3-4, para 4, referring to news of the invention reaching Galileo in May 1609......"had reached the ears of a fiery Italian scientist, Galileo Galilei, whilst visiting Holland".

Where the author got this notion is anybody's guess. There is no evidence Galileo ever left Italy. There are many sources one can consult regarding Galileo's learning of Lipperhey's telescope. None state Galileo visited Holland. Galileo states quite clearly in his 'Siderius Nuncius' printed in March 1610, that he received confirmation of the Dutchman's spyglass in a letter written by a French nobleman, Jacques Badovere, residing in Paris, received ten months earlier. Galileo knew this man as Geralomo Badourre, a mathematics student of his in 1597. He first learned of it through Paolo Sarpi, a Servite monk and Procurator General to the order in Venice. Ref: See the Galileo Project @ Rice University <http://galileo.rice.edu/sci/lipperhey.html> An English translation of Galileo's Siderius Nuncius maybe downloaded from <http://hsci.ou.edu/images/barker/5990/Sidereus-Nuncius-whole.pdf>

Ch.1. p4, para 4......"That much was clear to the German astronomer Johannes Kepler, who received a Galilean telescope as a gift from a friend in 1610".

Kepler was leant a telescope by the Archbishop of Cologne , owned by Duke Emest. Galileo ignored an earlier request from Kepler to borrow a telescope. Ref: Galileo's Battle for the Heavens <http://www.scientus.org/Galileo-Battle-for-Heavens.html>

Ch.1, p4, para 4, referring to the benefits of the Keplerian telescope......."Another bonus was its ability to project images..."

Although Scheiner used a Keplerian to project Solar images, the Galilean can also be used for Solar projection, as was suggested to Galileo in May 1612 by his mathematics student Benedetto Castelli. It is testimony to the author's unquestioning reliance on common knowledge that has led him to repeat the incorrect notion that a negative eyepiece cannot be used to project a real image. Ref: Galileo's sunspot letters to Mark Welser <http://mintaka.sdsu.edu/GF/vision/Welser.html> I have built a replica of Galileo's refractor "Old Discoverer" and used it for Solar projection <http://www.cloudynights.com/item.php?item_id=1860> But you do not need a Galilean telescope to test this for yourself. All that is needed is a field glass or opera glass. You won't get a very big image, and it will be too bright to descry sunspots, but you can see for yourself that the image can be focused on a white card.

Ch.1, p5, para 2 & 3, dealing with chromatic aberration, the author asserts René Déscartes........."demonstrated that the image quality of convex lenses could be improved my (sic by) making the curvature of the lens as shallow as possible.....The strategy increases the depth of focus so that the eye can accommodate the spread of colours...:"

The author is claiming that Déscartes, in his treatise, 'La Dioptrique' published in Leiden, 1637, arrived at a theoretical understanding of chromatic aberration. He is also claiming that chromatic defocus, can be made to lie within the depth of focus. Both these claims are utter nonsense.

Déscartes' work dealt with spherical error and various ways to either eliminate or reduce it, using aspheric lens surfaces or by increasing the focal length. However even this was not the underlying motive for ever longer focal length refractors. It was tacitly misunderstood at the time that only by having a huge image scale could details be resolved to an ever finer degree. It was imagined that a telescope exceeding 100 feet could reveal details on the Moon only a few feet across. Chromatic error played no part in this understanding. Colour fringing was attributed as being due to Blebs (inclusions) in the glass, and an imperfect polish.

Depth of focus (DoF) depends on the circle of least confusion (CoC), a geometric optics concept. It is generally taken to be ±4x wavelength x F2 where F is the OG focal ratio. A traditional long focus refractor at the time, such as Huygens refractor, would be ~f/60, at which the DoF = ±8mm. Chromatic defocus for Huygens' refractor between the g & r lines would be ~57mm, and the CoC varies from 0.48mm in the yellow and twice that in blue and red. The Airy disc for Huygens refractor in yellow would be only 0.04mm. (This is explained in Sidgwick's "Amateur Astronomer's Handbook, Ch.4, p90 (Gambol Ed. 1971). There is no way the chromatic defocus of a single lens OG could ever be accommodated within its geometric DoF.

Ch.1, p5, para 4, on Christiaan Huygens..."Christiaan Huygens used a more modest instrument (with a 2.3-in. objective and 23-ft focal length) to elucidate the true nature of Saturn's ring system, as well as its largest and brightest satellite, Titan".

The object glass of the x50 telescope Christiaan Huygens used to discover Titan on March 25th 1665, was recovered in 1867 in the University of Utrech's collection, and is now treasured as one of the University's most important objects. It is 57mm diameter, only 3.4mm thick and 3367mm focal length. Huygens described it as having focal length 10 Rhineland feet, or 11 Imperial feet - not 23 feet. However the objective was made by his brother Constantijn, who dated around its circumference, 3FEBR MDCLV. <http://www.xs4all.nl/~carlkop/huyglens.html> Ref: 'Huygens's Ring Cassini's Division & Saturn's Children', Albert van Helden, 2004, Smithsonian Library, ©2006 & Ref: Christiaan Huyens and his telescopes', Peter Louwman <http://www.esa.int/esapub/sp/sp1278/sp1278p1.pdf>

Ch.1, p6, para 3...."With a similar telescope, the Danish astronomer Olé RØmer, witnessing a timing glitch in the eclipses of a Jovian satellite, incredibly deduced the speed of light - 300,000 km/s".

Yet another mangling of the historical facts. Olé RØmer observed eclipses of the Jovian satellites in an attempt to develop tables that could be used to solve the problem of the longitude. After 8 years observation, in 1676 he published his results, and revealed light had a finite velocity. The ratio of the speed of light to the speed with which Earth orbits the Sun is 365.246/22π = 7,600. The actual ratio is 299,792/29.8 = 10,100.

RØmer neither calculated this ratio, nor did he give a value for the speed of light. Christiaan Huygens did calculate the speed of light from RØmer's data. He deduced that light travelled 16 2/3 Earth diameters per second, ~70% of the currently accepted value. Ref <http://en.wikipedia.org/wiki/Ole_R%C3%B8mer>

Ch.1, p8, para 3, on the assumed achromatism of the eye,..."In 1695 James Gregory, then the Savillian (sic) Professor of Astronomy at Oxford University, refuted Newton's conclusion that dispersion of light always accompanied refraction. Gregory's inspiration was the extraordinary human eye."

The author is confusing James Gregory (1639-1675), who published 'Optica Promota' in 1763 describing the Gregorian reflector, with David Gregory (1659-1708), Savilian Professor of Astronomy at Oxford, 1691 to 1708, who was James Gregory's nephew. David Gregory printed 'Catoptricæ et Dioptricæ Sphericæ Elementa' in 1702, subsequently translated into English in 1713 (Castries) in which he suggested that it would perhaps be an improvement of telescopes, if, in imitation of the human eye, the object-glass were composed of different media. Encyc. Brit. art. Optics. Ref: ' A Philosophical and Mathematical Dictionary", Charles Hutton, LL.D., Vol.I, 1815, p603.

Ch.1, p9, para 2, concerning the invention and patenting of the achromatic doublet..."Peter Dollond applied for a patent. Moor Hall twice attempted to challenge the patent on the grounds that he was the inventor. The core of Dollond's challenge was predicated on the fact that his firm was the first to demonstrate it to the public and thus should be the first to profit from it. Dollond won his day in court..."

This is a muddled misrepresentation of the available evidence. There is scant evidence to support the contention that Chester Moor Hall actually succeeded in making a workable achromatic lens. The only evidence lies in Chancery Court and Kings Bench transcripts in which a lens jobber called George Bass presented lenses (not doublets) on behalf of the Spectacle Makers Company who were defending Peter Dollond's suit for breach of patent, and a letter published 20 years later in the 'Gentleman' magazine under the pseudonym "Veritas". Francis Watkins, in partnership with John Dollond, paid £70 for the patent application in return for a 50% share of the profits. Peter Dollond subsequently dissolved the partnership with Watkins in 1763 and bought out his patent right for the agreed sum of £200.

Ch.1, p11, para 1, referring to John Dollond's invention of the achromatic doublet in 1758......"Largely by trial and error, he managed to create a prototype triplet objective that saw first light in 1757, creating considerable interest from some of the most illustrious astronomers of the age. The then Astronomer Royal Neville Maskelyne was so impressed by one of Dollond's triplets - a 3.75in. instrument - that he had it mounted in a small room all by itself".

This is a frightful mangling of the historical evidence. There is an extensive set of notes related to the development of the achromatic lens together with a timeline on my Index page. To state John Dollond alighted upon it largely by trial and error ignores the thorough experiments he conducted and the paper presented on his behalf by James Short to the Royal Society in 1758 that led to his being elected a Fellow and awarded the Copley medal, their most prestigious award.

Neville Maskelyne was still at Trinity College, Cambridge, in 1757, sitting his MA. He was elected a Fellow of Trinity College in 1756, and became assistant to the then Astronomer Royal, James Bradley, the following year. Maskelyne succeeded Nathaniel Bliss as Astronomer Royal on 26th. Feb. 1765. The first prototype Dollond triplet was made in 1762 and Peter Dollond sold triplets from 1763-4. The triplet procured by Maskelyne was probably made in 1772, the same year he had achromatic lenses fitted to Bradley's instruments, (not 1757). Ref: <http://pinetreeweb.com/bp-nevil-maskelyne.htm>

There are numerous papers in various journals dealing with these matters. The author displays a lazy approach to historical investigation, repeating secondary instead of consulting primary sources. What one famous Oxford scientific historian refers to as the Enid Blyton school of history.

Ch.1, p12, para 2, on Fraunhöfer's development of the achromatic doublet......"Fraunhöfer carefully studied the Dollond doublet objective and introduced significant changes to its design. Fraunhöfer made the first surface more strongly convex. He then made the two central surfaces slightly different in shape and introduced a very small air-gap between them. The innermost optical surface was nearly flat".

The author creates the impression Fraunhöfer developed his achromatic doublet by examining and modifying a Dollond doublet. He does not state what type of Dollond doublet was examined, and it matters, because the Dollonds produced initially flint forward, and latterly crown forward doublets. Assuming the author has the crown forward doublet in mind, what he mentions about the two central surfaces, i.e. the second and third surface, suggests there was no air gap between the inner surfaces of the Dollond doublet.

Nothing could be further from the truth, and again we have a clear example of sloppy research. In July 1996 Rolf Willach published a paper entitled 'New Light on the Invention of the Achromatic Telescope Objective'; Notes and Records of the Royal Society of London, Vol.50, No.2, pp195-210. Willach describes his examination, measurements and optical analysis of several early achromatic doublets, including one made by Peter Dollond c1765-70. The diagram below is taken from Willach's paper, fig 4. p206., of a 2-inch f/31 achromatic doublet.


Note the air gap between the second and third surface.

In the Fraunhöfer doublet the first surface has a steeper radius than the second surface. The third surface is set close to the second but slightly shallower, and significantly, the fourth surface is slightly convex. What the author is describing is a lens that has never existed, a conflation of the Fraunhöfer and the Littrow design, which does have an almost flat fourth surface, usually slightly concave for ease of testing.

This diagram is from Roger Ceragioli's "Survey of Achromatics" Ch. 3b. Note there is in theory a minimal air gap between the second and third surfaces of the Fraunhöfer, it is maintained using edge spacers.


Henry King in his 'History of the Telescope' refers to Joseph Fraunhöfer examining a Dollond doublet, and concluding it was overcorrected. Fraunhöfer devised methods of accurately measuring the refractive indices and partial dispersions of optical glass. He designed his achromatic, coma free doublet using optical theory, as well as trial and error. He derived his lens prescriptions using the previously published works of Alexis Clairaut and Jean le Rond d'Alembert.

Ch.1, p12, para 2,....on aplanatic doublets........"Fraunhöfer's so-called aplanatic refractors became the new standard by which all future refracting telescopes were measured for more than a century to come".

Why 'so-called'? Fraunhöfer's doublet was aplanatic, i.e. corrected for spherical aberration and coma. However at the time (late second and third decade C19th) Fraunhöfer's was not the only aplanatic achromatic doublet. In 1821 John Herschel published a mathematical account of an aplanatic doublet with a set of tables, "set down for the convenience of those who may be inclined to make trial of this construction." Ref: J.F.W. Herschel, "On the aberrations of compound lenses and object-glasses", Philosophical Transactions of the Royal Society of London (1821), 222-67, p 222 & p261. Sir James South commissioned one of the leading English opticians, Charles Tulley, to make him an achromatic objective according to Herschel's prescription in 1822, an objective 3.25-inch aperture and 45-inch focal length.

Ch.1, p13, para 3, on the emergence of Thomas Cooke, the author implies the only rival to Merz & Mahler was Thomas Cooke. This is a gross over simplification, and a highly misleading passage. At the time Fraunhöfer was heading the Reichenbach Institute, English telescope makers, Edward Troughton, William Simms, George Dollond, and Charles Tulley were flourishing. Furthermore the first maker of large achromatic refractors, on a similar scale to Merz & Mahler, was Thomas Grubb, who established his firm in 1833. Thomas Cooke established his telescope making business four years later, and it was to be at least a decade before he was regularly making refractors bigger than 6-inches aperture. The author makes no mention of Thomas Grubb.

Ch.1, p15, para 1, on the Newall telescope...."Today, the 25-in. has found a new home at Penteli Observatory, just north of the city of Athens, Greece. It's been there since 1958".

The Newall telescope was acquired by the Athens observatory in 1957. Ref <http://www.hasi.gr/instruments/ast73>

Ch.2, p21, para 2, on Fraunhöfer making his achromatic doublet........"The year 1824 marks a very special year for the telescope. That was the year in which Joseph Fraunhöfer created the first recognizably modern refractor,...."

Joseph Fraunhöfer completed the object lens for the 9.5-inch Dorpat refractor (now Tartu, Estonia) in 1819, and it was delivered in 1824. However this was not the first doublet Fraunhöfer made. His first recorded object glass, a 7-inch was completed in 1812. Ref <http://www.plicht.de/chris/35fraunh.htm><http://www.earlytech.com/shop/view_item/1995319911>

Ch.2, p30, para 2, on de-focus aberration......."This aberration is more commonly referred to as "depth of focus." Depth of focus (ΔF) measures the amount of defocusing that can be tolerated before the image looks noticeably impaired to the eye and is calculated using the following formula: ΔF = ±2λF2 where λ is the wavelength of light and F is the focal ratio of the telescope".

Depth of Focus is a geometric construct. Defocus aberration is a phenomenon of diffraction. The Conrady equation for DoF is ΔF = ± 4λF2. The author's error is repeated in the Appendix C p271, where also the equation is wrongly written: ΔF = ±2ΔF2 where Δ is the wavelength of light ... Ref: Applied Optics and Optical Design, A.E. Conrady ©1957, Vol.1, Ch.3, [38] p137

Ch.2, p30, para 5, on the effects of defocus aberration....."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".

The author conflates defocus aberration with depth of focus, and strays into the so called "f/ratio conjecture" which I have covered in a paper published in September 2010 ATM Letters Journal. Conrady DoF is calculated from an assumption the optic is Rayleigh limited (1/4 wave P-V). Defocus aberration depends on the optical correction, and ultimately the Strehl ratio. A system with a Strehl ratio of 1 has 4.13λF2 defocus range, and a 1/4 wave P-V system zero defocus range. Furthermore seeing induced defocus caused by wavefront retardation causes a focal shift of equal degree in any telescope regardless of its f/ratio. Focusing an f/5 refractor is more critical than focusing an f/10 of equal aperture, but once acclimated, both will react to seeing induced defocus in the same way. To argue, as the author does, that a long focus classical achromatic provides a steadier viewing experience, than a short focus apochromatic of the same aperture, is simply not so.

Ch.2, p30, para 4, on the effects of depth of focus, the author compares two photographs captured at f/5.6 and f/11, commenting on the increased depth of field in the latter. Depth of Field is not the same as Depth of Focus. The two are related, but Depth of Field has no relevance to astronomical telescopes because the object distance lies at ∞. Ref <http://en.wikipedia.org/wiki/Depth_of_field>

Ch.2, p27, para 1, on Seidel errors......."Chromatic aberration is just one of a group of optical aberrations ........These aberrations are known as the Seidel aberrations...."

Chromatic aberration is not a Seidel aberration. The author lists the five Seidel errors, Spherical aberration, Coma, Astigmatism, Field Curvature & Distortion. Seidel aberrations are monochromatic aberrations. Field curvature and distortion have no relevance to visual observation with a refractor because the field of view is small, typically less than 1°. Chromatic aberration has two parameters, longitudinal and lateral. Ref <http://www.quadibloc.com/science/opt0505.htm><http://en.wikipedia.org/wiki/Chromatic_aberration>

Springer need to improve their proof reading and ensure reference books of this nature are written with intellectual rigour. Otherwise Patrick Moore's practical astronomy series will become synonymous with mediocrity.


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