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Seeing the Heavens

Written by Iver P. Cooper

"The soul without imagination is what an observatory would be without a telescope," according to Henry Ward Beecher. In the seventeenth century, solar system astronomy lay at the center of the debates as to place of mankind in the universe, and the relationship of religion and science. The telescope played a decisive role in these debates.

It is true that the Church and the royal courts have access to the books from the future, and know what they say about the Solar System. Certainly, the Pope in 1634: The Galileo Affair is aware of those teachings and hence anxious to find a way to minimize the repercussions of the Galileo Trial.

It might seem that, with all the astronomy and physics books in Grantville, there is no dire need for improved telescopes. However, it is one thing to read something, and another to observe it for yourself.

Astronomers in Canon

None of the up-timers has a degree in astronomy. However, several have degrees in physics: John McDougal "Mac" Clements (M.S.), Charnock Fielder (1931–1634)(M.S.), James Michael "Jim" McNally and James Victor Saluzzo. Eve Zibarth was six semester hours short of a second major in physics, and Frederic Swisher studied some physics before he dropped out of college. There are also mathematicians in Grantville.

Any of the college grads could have taken an astronomy class or two. In fact, it is not exactly unusual for math-allergic liberal arts majors to satisfy their "science distribution requirement" by taking an astronomy class.

There are astronomy clubs in several West Virginia towns: Wheeling, Clarksburg, Charleston, Huntington, Athens, White Sulphur Springs, Tridelphia, and Morgantown.

The Morgantown (WVU) club presently has twenty-five members, and the club is affiliated with the WVU Physics department. It meets every other Wednesday (around the first and third quarter moons). On the Friday and Saturday nights near new moon the club often has a star party in Chestnut Ridge Park. The Physics department has the "Tomchin Planetarium and Observatory," with a Spitz A3P Planetarium Projector and a 14-inch Celestron telescope. This equipment is available to club members.

The other club reasonably close to Mannington (Grantville) is the Central Appalachian Astronomy Club in Clarksburg (south of Fairmont). Note that Fairmont State University has a satellite campus in Clarksburg. The CAAC owns the Good Hope Observatory, ten miles south of Clarksburg, which is equipped with a piggybacked Meade 16" F/10 LX 200 GPS Schmidt-Cassegrain with a Takahashi 4" FS-102 II, and a Williams Optics Zenithstar 80mm F/6. They also have a 14" F/10 LX 200 GPS Schmidt-Cassegrain, and, for solar observing, a Coronado Max Scope 60.

Unfortunately, I have not found any specific evidence that anyone from Mannington was a member of either of these clubs.

So far as light pollution goes, Mannington is in Bortle Class 4, "Rural/Suburban transition." ( http://www.novac.com/lp/wv.php ) But Morgantown and Fairmont are worse, and they have amateur astronomers.

Rick Boatright did an informal survey of his "middle-to-upper-middle class" neighborhood in 2000: "In the four blocks nearest my house, (16 house/street segments) there are 104 homes. In those 104 homes, there are ZERO reflecting telescopes, two sub-three-inch refractors, and 11 pairs of binoculars."

Still, it is now canon that at least one up-timer is an amateur astronomer of sorts: high school graduate Johnnie Farrell. See Peter Hobson's "Lessons in Astronomy" (Grantville Gazette, Volume 11).

At the end of 1634: The Galileo Affair, the new Cardinal-Protector Mazzare wangles the appointment of Father Christopher Scheiner (1573–1650) to Grantville. Mazzare tells the Pope, ". . . we have books on astronomy in Grantville, and are creating a great university nearby at Jena—but we have no astronomers. And he is a superb one."

Refracting Telescopes and Their Problems

Credit for the invention of the telescope is usually given to Hans Lipperhey (Lippershey?), who filed a patent application on Oct. 2, 1608 (Panek 25). This touched off a priority dispute, with Jacob Adriaenszoon and Saccharias Janssen. The States General solomonically decided that none of them deserved a patent, on the theory that a device which was simultaneously invented by several parties was probably obvious.

The Lipperhey spyglass combined a weak biconvex lens away from the eye (later termed, the objective lens) and a strong biconcave lens near the eye (the eyepiece). In 1609, Galileo created his own spyglass; it used a planoconvex objective ("plano" indicating one side flat) and a planoconcave eyepiece. By March 1610, he had published his first set of observations, in The Starry Messenger.

The two basic refractor designs are the Galilean and the Keplerian (the latter described in Kepler's Dioptrice, 1611). Both use a planoconvex or biconvex objective lens. The Galilean uses a negative (planoconcave or biconcave) eyepiece, and the Keplerian a positive (planoconvex or biconvex) one.

The Galilean creates an upright image, whereas the Keplerian image is inverted. The Galilean telescope is also more compact; the distance between the lenses is the difference between the focal lengths of the objective and the eyepiece, whereas for the Keplerian design it is their sum.

On the other hand, for a given magnification, the field of view (FOV) of the Keplerian telescope is much broader. The FOV for the 20x Galilean design was perhaps fifteen arc minutes (half the diameter of the full moon). Huygens' 1656 Keplerian refractor, 23 feet long, 100x, had a seventeen arc minute FOV. (Van Helden).

Also, in a Keplerian design, you can mount a micrometer at the focal plane, so you can measure the angular size of the object observed. (Pope GvK). The micrometer was first added by William Gascoigne (1620?–44) in 1638 (Bell 12).

Kepler was not an observer, and it appears that the first astronomers to use his design were Scheiner, and Francesco Fontana (1585-1656). In Scheiner's Rosa Ursina (1630), he noted that he had been using the new type for several years. Fontana, in Novae coelestium terrestrium rerum observationes (1646), claimed that he had put two convex lens into a tube back in 1608 (and thus to have priority not only over Scheiner, but also Kepler).

At the time of the Ring of Fire, most astronomers still had Galilean refractors. According to Van Helden, these typically had an planoconvex objective lens, with a focal length of 30–40 inches, and an aperture (stopped down) of 0.5–1 inch. Combined with an eyepiece of focal length 2 inches, that gave a magnification of 15–20x. As to the actual optical quality, he says, "The glass was full of little bubbles and had a greenish tinge (caused by the iron content of the glass); the shape of the lenses was reasonably good near their centers but poor near the periphery (hence the restricted aperture); the polish was rather poor."

However, Pope says that although Galileo's glass "suffered from many cosmetic defects," at the apertures he chose, his lenses "should have performed essentially as well as any modern lens of similar design."

The Keplerian design was dominant by the mid-seventeenth century. The Capuchin friar Antonius Maria Schyrlaeus de Rheita (1597–1660) added (1645) a reverting eyepiece to the Keplerian refractor, and also experimented with binocular telescopes.

The first telescopes were called refracting telescopes because convex lenses focus light by refracting (bending) it. The simplest convex surface to impart to a lens is a spherical surface. Unfortunately, if the lens has a spherical cross-section, then paraxial rays (rays parallel to the axis of the lens) passing through the lens near its center will not meet at the same point as those passing through the periphery of the lens. This problem is called spherical aberration, and results in a fuzzier image.

The problem of spherical aberration was recognized and mathematically analyzed by Rene Descartes, who published his analysis in 1637 (I don't know how much he had already figured by, say, 1634.) Spherical aberration increases as the square of the aperture (Thompson 5), which of course discouraged attempts to increase light-gathering power by increasing the aperture.

Descartes also pointed out how to overcome the problem with lenses having a non-spherical surface. But attempts to grind such surfaces in the seventeenth century were unsuccessful (Bell 12).

There was a second problem which could not be solved solely by use of an aspherical surface. A simple lens will refract light of different wavelengths (colors) to different degrees (this is called dispersion), so each color has its own focus distance. Focus on an object, and it will have a reddish or bluish halo. This problem is called chromatic aberration. Isaac Newton discussed it in his Optics, and commented, "'tis a wonder that telescopes represent objects so distinct as they do" (417).

The spherical aberration could be reduced by using a high f-ratio (ratio of the objective focal length to the aperture). Perhaps more importantly, the relative chromatic aberration (the chromatic blur relative to the size of the image) could also be reduced by that strategy (Bell 11).

In consequence, the focal length of the objective, and hence the length of the telescope, was greatly increased. The Huyghens brothers used a twelve footer in 1655. The tubes of these long-focus scopes were made of wood, which was stronger than the paper or leather used by the Galileo, and lighter than iron.

By the early 1670s, Johannes Hevelius had built a 150-foot telescope. This was called an "aerial telescope" because it was suspended in the air. Pity the Renaissance astronomer who was observing on a gusty night! It didn't have a tube, strictly speaking; an illustration reveals that it was a long spar, with the objective attached at one end, and the eyepiece at the other, and wooden diaphragms at intervals in-between (Bell Fig. 11).

Newton comments that "very long tubes are cumbersome, and scarce to be readily managed, and by reason of their length are very apt to bend, and shake by bending, so as to cause a continual trembling in the objects."(421). While I am sure the 140-footer was a great tourist attraction, the productive work was done on somewhat more modest contraptions, 25-35 feet long and with apertures of 2-3 inches (Bell 18).

The first major improvement in refracting telescopes came in the eighteenth century, with the invention of the achromatic lens (originally by Hall in 1733, but rediscovered by John Dolland in 1758). This combined a positive biconvex crown glass and a negative concave-convex flint glass lens in such a way that their focal lengths were inversely proportional to their dispersions. Flint glass exhibited much greater dispersion than crown glass.

The purpose, of course, was to minimize chromatic aberration. Specifically, the design caused two wavelengths (e.g., red and blue) to focus at the same place. There will still be chromatic aberration at the wavelengths not specifically corrected for, but the degree of aberration would be less than for a traditional lens. The overall chromatic blur is reduced by about a factor of 40 (Smith, Modern Optical Engineering, 399).

The "Achromatic Telescope" is described in "Telescope," 1911 Encyclopedia. Unfortunately, the essayist was more concerned with the merits of Dolland's claim to be the first true inventor than with communicating the practical details of how to construct an achromatic lens. The practical mathematics are in another encyclopedia article, "Aberration in Optical Systems," and see also "Light," "Dispersion (Optics)."

An achromatic lens may be a triplet; the encyclopedia mentions that Peter Dollond sandwiched a concave flint lens was sandwiched between two convex crown glass lenses.

In 1632, flint glass (a lead-rich glass of high refractive index) was unknown. Formulae for flint glass are in the encyclopedias, see Cooper, In Vitro Veritas (Grantville Gazette, Volume 5), but it will take time to bring flint glass on the market.

To make an achromat, you have to be able to accurately determine the refractive index of a glass. Willebrord Snell (1591–1626) was the first scientist to measure the refractive index with sufficient precision to deduce the law of refraction (Snell's law, unless you're French, in which case you call it Descartes' law.) So this was "leading edge science" when Grantville arrived in Thuringia.

The achromatic lens is a type of doublet lens, that is, one which consists of two simple (singlet) lenses which are attached to each other. Even a cemented doublet potentially yields a big increase in quality from a singlet, since its three independently specifiable surfaces allow one to achieve a particular focal length and at the same time correct for chromatic and spherical aberration (Levenson 14). Cemented doublets are limited in size to perhaps three inches by the differential thermal expansion of the two glasses.

In an air-spaced doublet, a fixture holds the two simple lens in close proximity, with an airspace in between. There are four surfaces which can be specified, so the design can more readily correct for coma, too (Levenson 19). On the other hand, there are more air-glass surfaces and alignment is more difficult (Smith, Modern Lens Design, 115).

There are two ways of making an apochromatic lens, that is, one which eliminates chromatic aberration at three different wavelengths (e.g., red, green and blue). (Levenson 21-23). One is to use a triplet with appropriately configured surfaces.

The other is to make a doublet in which one of the lenses is made of a very low dispersion material, such as fluorite crystal or certain rare earth glasses. The low dispersion glass is not going to be available until well after RoF. And nowadays, such apochromatic lenses cost about twenty times as much as a "mere" achromatic lens (antiquetelescopes.org).

Reflecting Telescopes

The first reflecting telescope was built by Niccolo Zucchi (1586–1670),a professor of mathematics at the Jesuit College of Rome, in 1616. He used it to observe the belts of Jupiter in 1630. His book Optica philosophia experimentalis et ratione a fundamentis constituta (1652–56) may have influenced the later, better known reflector designs of Gregory and Newton. Zucchi's telescope used a bronze concave mirror instead of a lens. He viewed the mirror image through a lens, possibly handheld. Some authorities say that he had to put his head in front of the mirror in order to make observations—which would have been something of a nuisance. Others say that the mirror was tilted to avoid obstruction by the observer. With a tilted mirror, the light would be reflected obliquely, and the observer could stand to one side of the telescope tube.(Wilson, 2).

In 1630, French astronomer Marin Mersenne (1588–1648) proposed using a second, concave mirror to reflect the light down through a hole in the center of the larger primary mirror. (He may also have suggested that these mirrors be paraboloid in shape.) Unfortunately, René Descartes persuaded him not to proceed, apparently because of the difficulty of securing high quality concave mirrors of sufficient size. But Mersenne explained his design in his l'Harmonie Universelle (1636). (Rybski; Hong)

A similar design was proposed by James Gregory (1638–1675), a Scottish mathematician, in his treatise Optica Promota (1663). The telescope was to have used both a concave parabolic and a concave ellipsoidal mirror(ZOOM). The image formed would have been right-side-up, so the Gregorian telescope could have been used in the daytime to observe terrestrial sights, not just at night to see the heavens.

One of the disadvantages of the Gregorian reflector design was that it featured an eyehole in the primary mirror, which reduced its light-gathering power. Another was that an ellipsoidal surface is hard to grind. Worse, "if an optical system contains two sequential reflectors, regardless of their shapes, the combined effect is to magnify any geometrical imperfections in either surface."(Zebrowski 113) Gregory commissioned craftsmen to build a working telescope according to his plans, but without success(White, 169).

The credit for actual realization of the reflecting telescope goes to one of the intellectual giants of world history. Sir Isaac Newton (1642–1727), declaring that "the improvement of telescopes of given lengths by refractions is desperate," adopted a radically different approach, employing reflection, and "using instead of an object-glass a concave metal."

His speculum had a diameter of about 2 inches, and a thickness of one-third of an inch, and it was ground to the shape of a sphere with a diameter of about 25 inches. While not mentioned in Newton's Optics, it may safely be assumed that he placed it in the bottom of a tube and caught the reflected rays on a 45° secondary mirror, which in turn redirected the light to a planoconvex eyepiece.

The first Newtonian reflector was only about six inches long and magnified about 35 times. Newton says that the primary mirror was made of copper(420–21), but more likely it was speculum metal, which was then an alloy of six parts copper, two parts tin, and one part arsenic(CYBRATIONS).

Unlike Gregory, Newton did not place his trust in craftsmen to reduce his design to practice. "I asked him where he had it made," recalled John Conduitt, "he said he made it himself, & when I asked him where he got his tools he said he made them himself & laughing added 'if I had stayed for other people to make my tools & things for me, I would have never made anything of it . . .'"(White, 168).

With his new scope, Newton saw "Jupiter distinctly round and his satellites, and Venus horned."(Id.) Newton displayed it at a meeting of the Royal Society of London in December, 1671, and shortly thereafter he was voted in as a Fellow(White 169–71, RICE).

The great advantage of reflectors (telescopes with mirrors) over refractors (telescopes with lenses) is that they do not refract light. When light is reflected, all wavelengths are redirected at the same angle, so chromatic aberration does not occur.

The original Newtonian design had a spherical primary mirror. Like a spherical lens, a spherical mirror cannot focus parallel rays of light down to a single focal point; it suffers from spherical aberration.

In 1723, John Hadley (1682–1744) replaced Newton's spherical primary mirror with a parabolic one, thereby avoiding this problem. There is no doubt that Newton was aware of the advantages of a paraboloid shape over a spherical one. In analyzing refractor (lens-based) telescopes, he declared, "the imperfection of telescopes is vulgarly attributed to the spherical figures of the glasses, and, therefore, mathematicians have propounded to figure them by the conical sections" (Newton, 412);those, of course, would include the parabola. But Newton calculated (erroneously) that the contribution of spherical aberration to the scattering of the rays was only 1/5449th that of chromatic aberration. Having solved the latter problem by replacing lenses with mirrors, Newton was no doubt of the opinion that the additional sharpness achievable with a paraboloid mirror was insufficient to justify the effort necessary to grind a mirror to that shape.

Hadley's greater contribution was that he devised a reliable method of monitoring the approach to a parabolic cross-section. First, he ground the mirror to a spherical shape. Then he ground the mirror more deeply in the center than at the periphery. Without his assay method, this would have been entirely hit-or-miss. But he "placed a tiny illuminated pinhole at the mirror's center of curvature and examined the reflected cone of light in the vicinity of the image. From the appearance of this cone, Hadley could infer the state of the mirror's surface and was thus able to pass, by successive polishings, from a spherical to a paraboloidal figure."(TRIPOD) Like Hadley's telescope, modern Newtonian reflectors use a parabolic primary mirror.

Problems in mirror grinding and in maintaining an untarnished surface discouraged the early adoption of reflector telescopes.

Other Telescope Designs

In the Newtonian reflector, an on-axis planar mirror moves the focal point of the primary mirror (spherical or parabolic in shape) outside the main telescope tube. The eyepiece tube is perpendicular to the main tube. In the older Gregorian design, it was parallel to the main tube, and aligned with it. Of course, while Newton avoided the need for an eyehole in the primary mirror, his secondary mirror would of course prevent some of the incoming light from reaching his primary mirror in the first place.

Herschel tried eliminating the secondary mirror altogether. That made it a "front view" telescope (like Zocchi's), and Herschel tilted the mirror so he could see the image without blocking the view. A "Herschelian" reflector, of 48-inch aperture and 40-foot focal length, was used by its "inventor" to discover Enceladus and Mimus. (Bell 33)

Herschel had eliminated the light loss due to the secondary mirror, which was rather high with speculum metal. Unfortunately, it wasn't easy for the observer to see into a large Herschelian reflector if the target were near the zenith—forty-foot ladders being somewhat rickety.

Also, the tilt created an astigmatic distortion, albeit one alleviated by the high f-ratio (Doherty 16).

Hence, we turn the clock back to the early seventeenth century to look at an alternative design. Laurent Cassegrain (1629–93), a Catholic priest, wrote a paper on the megaphone, published in the Proceedings of the Paris Academy of Sciences for 25 April 1672. An accompanying note described his telescope design. A Cassegrain telescope is a wide-angle reflecting telescope with a concave mirror that receives light and focuses an image. A second, convex mirror reflects the light through a gap in the primary mirror, allowing the eyepiece or camera to be mounted at the back end of the tube.

While not pointed out by Cassegrain, the combination of a concave mirror and a convex one tends to limit the adverse effects of geometric imperfections in either surface. Despite this advantage, the Cassegrain reflector sank into obscurity for almost three hundred years, under the weight of Newton's scathing criticism of it(Zebrowski, 114).

Catadioptric Telescopes

If the primary mirror of the Cassegrain reflector were spherical, it would suffer from spherical aberration. A correcting plate (a lens) was added (in front of the primary mirror) in 1930 by the Estonian astronomer and lens-maker Bernard Schmidt (1879–1935), creating the Schmidt-Cassegrain telescope(ZOOM). Since it uses both a mirror and a lens, it is called a catadioptric design. The Schmidt correction lens was flat on the front side, and had a complex curve on the rear side.

A. Bouwers of Amsterdam, Holland, in February of 1941 and Dmitry Maksutov of Moscow, Russia, in October of 1941 independently invented an alternative correction lens which was curved on both surfaces. It is called a meniscus corrector shell, and the overall telescope design which incorporates it is called a Maksutov-Cassegrain reflector. In 1957, John Gregory realized that the secondary mirror could be dispensed with if a small central portion of the rear surface of the meniscus corrector shell were silvered to make it reflective. The result was the "Mak" reflector.(Weasner).

In what is now called the "classical" Cassegrain design, the primary mirror is parabolic and the secondary mirror is hyperbolic. This avoids spherical aberration without the need for a corrective lens. It is unclear whether Cassegrain himself conceived of the hyperbolic secondary mirror; or whether it was a later development. Accurately grinding both parabolic and hyperbolic mirrors would have been extraordinarily difficult in the late seventeenth century.

Mirrors for Telescopes

The mirrors for reflecting telescopes were usually made of speculum metal, a mixture of copper and tin. The metal at best reflected only about 60% of the light (Swadha; Thompson 13),and less as it tarnished.

Newton recognized both the problem, and a possible solution: "because metal is more difficult to polish than glass, and is afterwards very apt to be spoiled by tarnishing, and reflects not so much light as glass quick-silvered over does, I would propound to use instead of the metal a glass ground concave on the foreside, and as much convex on the backside, and quick-silvered over on the convex side." In other words, he had conceived of a back-silvered glass concave primary mirror.

However, nothing came of this suggestion until the German chemist Baron Justus von Liebig devised (1835) the method of depositing a film of silver on a glass surface. This technological advance made large reflectors practical. The preparation of speculum mirrors was an esoteric art, while many nineteenth-century workers knew how to grind and polish glass.

Also, while glass was fragile, it was still easier to handle than speculum metal, which one writer has called "wilfully perverse." Speculum metal was also more than three times the density of plate glass (Texereau 25).

The story of the 1870 Melbourne Cassegrainian reflector is instructive. The Australians decided not to use the newfangled silvered glass mirror. They ordered a 48" (1.2 meter) speculum mirror from Dublin. It was only with the third attempt at casting that success was achieved. The mirror was shipped with a protective coating of shellac. When the Australians removed the shellac, they damaged the reflective surface. Rather than shipping the mirrors back to Ireland, the Australians decided to polish it themselves, with unhappy results. G. Ritchie wrote, "I consider the failure of the Melbourne reflector to be one of the greatest calamities in the history of instrumental astronomy."(Learner, 107–9).

Finally, while silvered mirrors, like speculum metal, will tarnish, the silver of a silvered glass mirror could be dissolved away and replaced with a fresh coating, leaving the mirror shape unaffected. (GEOCITIES)

The first silvered glass reflecting telescope, just 4 inches in aperture (smaller than the one I owned as a high school student), was built by Steinheil in 1856. Foucault made a 13-inch silvered glass mirror in 1857. Soon thereafter, a reflector with a 48-inch silvered glass mirror was installed at the Paris Observatory, but its performance was mediocre.

Big Glass

The problem with reflectors was that they were much more sensitive than refractors to temperature effects, to the flexion of the telescope tube, and to misalignment of the optics. Nonetheless, for large telescopes, they had substantial advantages.

As lenses were increased in size, they had to be made thicker, which increased their absorption of light. This was particularly a problem for astrophotography, as the film was most sensitive to violet and ultraviolet light, and flint glass strongly absorbed these radiations. The large lenses also had to be supported at the edges, and hence liable to warping. In contrast, silvered mirrors strongly reflected violet and ultraviolet light, and large mirrors could be supported all the way across the rear of the mirror "blank," rather than just at its edges (Learner, 110).

Toward the end of the 1930s, silvering was superseded by aluminizing. While aluminum is not quite as reflective as silver, it is much more durable. To recoat a mirror, it must be lifted out of its frame. The Mount Wilson mirror weighed four tons; obviously, the less it had to be played with, the better.

Another advantage of aluminum is that when it is oxidized, the resulting aluminum oxide coating is transparent, whereas silver oxide is black (think "tarnish").

Another important development in the history of telescope making was the invention of PYREX® glass. This glass was much less sensitive to temperature changes than plate glass. The first use of PYREX® glass in a large telescope was in the 76-inch reflector for the Canadian David Dunlap Observatory (Learner, 118).

Eyepieces

The purpose of the eyepiece (ocular) is to enlarge what is seen at the focal plane. The magnification obtained is the ratio of the focal length of the objective (mirror or lens) to the focal length of the eyepiece.

Unfortunately, the greater the enlargement, the smaller the field of view. Which is why big scopes often have guide scopes piggybacked on top of them. Raising magnification also makes the image dimmer.

The original Galilean eyepiece was a singlet. Since light can come into the eyepiece at steep angles, the designer most worry, not only about spherical and chromatic aberration, but also coma, astigmatism, and other distortions. The use of multiple elements (see table) allows the correction of one or more distortions. A compound ocular was first constructed (Augsburg, 1649) by Johann Wiesel(Dijksterhuis 60); the better known Huyghens eyepiece was made in the 1660s.

(Levenson, 48-52).

The 1911 "Telescope" article has figures showing the Kellner and Cooke eyepieces, and the Huyghen and Ramsden eyepieces are depicted in the "Microscope" essay. The Erfle, Orthoscopic and Plossl designs may be more difficult to recreate.

Mounting the Scope

The purpose of the mount is to support the telescope, with minimum vibration, yet permit it to be readily pointed at a target and to track the target's movements. And, of course, if the telescope is not in an observatory, the mount has to be light enough so the telescope is still portable. These goals are not easily reconciled.

The intricacies of predicting where a celestial object will be at a particular time, and the various coordinate systems in which its location can be expressed, are covered in my article, "Soundings and Sextants" (and I am very glad I don't have to explain them a second time).

Here, I will concentrate on the issue of how to mount the scope so you can point it in a particular direction. Telescope mounts are diagrammed and discussed in the 1911 "Telescope" article, and of course pictures of telescopes in various books on astronomy will inadvertently reveal additional variations on the theme.

General purpose telescope mounts, in order to point anywhere in the sky, must be freely rotatable on two axes, and for scientific use you have to be able to set the angle on each axis. The two basic mounts correspond to the two basic coordinate systems.

An altitude azimuth ("alt-az") mount has vertical and horizontal axes. The vertical axis allows the user to set the altitude and the horizontal axis, the azimuth. An alt-az mount is similar to a gun mount on a battleship. One form of alt-az mount is shown in Hevelius, Selenographia (1647).

One common implementation of an "alt-az" mount is a "rocker box"; the vertical axis is provided by a yoke, and the yoke is pivotably mounted on a horizontal base. Another is found on some camera tripods; one end of a C-shaped arm is attached to the base, which rotates on top of the tripod, and the other end provides a pivot for the telescope. This is only suitable for small telescopes.

In an equatorial mount, the base, instead of being horizontal, is tilted so it is parallel to the earth's equatorial plane. The advantage of this is that the apparent motion of the stars is the result of the rotation of the earth and, with an equatorial mount, one just needs turn the telescope on the one axis (which points toward the north or south pole), in order to keep a star in view. That in turn means that the telescope can be kept pointing at the star by hooking it up to a simple clock-regulated motor drive. In contrast, with an alt-az mount, the telescope must be adjusted on both axes to keep a star in view.

The two axes of the equatorial mount are the right ascension and declination axes, which are the tilted counterparts of the altitude and azimuth axes of the alt-az mount. The RA axis points toward the pole so turning the telescope about that axis tracks the star.

The first equatorial mount is believed to have been constructed by Scheiner (!) in 1638, for a helioscope (King 42).

There are many different ways of implementing the equatorial mount (Smith).

My home telescope of decades past used the German equatorial mount, which features a lopsided-T. The crossbar serves as the declination axis. The midpoint of the telescope tube is pivotably mounted to the short end, and a counterweight is slid onto the long end.

The central staff of the T is the RA (polar) axis. On a large observatory scope, it will probably be held on a fixed, slanted pier, customized so that the slant causes the RA axis to point toward the pole. On a portable amateur scope, it will be pivot-mounted on a tubular base, the base lockably hinged to a pedestal (so the angle can be changed if the telescope is moved to a location with a significantly different latitude), and the pedestal mounted on a tripod.

There are two problems with a German equatorial mount: the polar axis is subject to a lot of stress (since the tube hangs on one end) and the telescope sometimes has to be swapped from one side of the pier to the other ("meridian reversal").

Next we have the English cross axis. Instead of a T, we have something like a plus sign. Imagine that one bar of the "plus" is short and horizontal; that is the declination axis and the telescope tube and the counterweight are attached to opposing ends. The other bar (the RA axis) is long and threads through the declination axis; one end touches the ground and the other (the high end) is attached to two legs. Thus, the RA axis and its legs form a lopsided tripod, with the polar end of the RA axis high off the ground (high enough so the telescope can be rotated all the way around the declination axis).

In the English yoke, the declination axis is modified; instead of the telescope tube being attached to one end, it is mounted "inside" the center of the axis, in a rectangular yoke. At the midpoint of the long sides of the yoke there are opposed pivot points to which the tube is attached; these form the declination axis. The short ends of the yoke are attached to opposing pivot points, the ends of a large L-shaped base, to form the RA axis. This is the design which was used to mount the Hooker Telescope on Mount Wilson. It is stable but can't be used to see stars close to the pole because the tube would crash into the yoke (which is shorter than the tube) if you tried.

The "horseshoe" mount (Savard) is a modification of the English yoke. The higher of the short ends of the yoke is replaced by a C-shaped piece (the "horseshoe") which cradles the telescope tube when it points toward the pole. In essence, we have modified the piece of the yoke which the tube would otherwise crash into. The "horseshoe" itself is supported by the high end of the L-shaped base.

The English Fork combines the polar axis of the German mount with half a yoke mount (that's the "fork"). It doesn't require a counterweight. It may be thought of as the equatorial analogue of a rocker-box; the base axis of the fork sits on a wedge (pier) instead of lying horizontally.

There are still more mount designs . . . but it is clear from the foregoing that the astronomers of the new time line have plenty of choices.

The cost of building a telescope with an equatorial mount increases as roughly the 2.7th power of the diameter of the mirror (AST110).

In general, the equatorial mount is heavier and more expensive, and the modern trend in the old time line was to return to the alt-az mount. Computers can be used to drive movement, simultaneously, on both axes. Of course, in the decade following the RoF, and outside the vicinity of Grantville, computers are going to be hard to come by.

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There are special mounts which afford more limited movement (but are cheaper and more stable than the general purpose mounts).

A transit telescope, like a telescope with an alt-az mount, can be adjusted vertically, but it cannot turn horizontally. It always points somewhere on the half circle corresponding to the upper half of the local meridian (i.e., north or south). It is used to create star catalogues; when a star crosses the local meridian its altitude (declination) is measured and the time (right ascension) is noted. The transit telescope can also be used to accurately determine the time, based on when a star of known right ascension crosses the meridian.

A poor cousin of the transit telescope, a "meridian circle," was used by Tycho Brahe. It was a quadrant; the altitude was measured as with a transit telescope, but the star was sighted with the naked eye.

A zenith telescope is even simpler than a transit telescope; it points directly overhead. Such telescopes, which are cheap to construct even when large in diameter, were traditionally used in latitude and time measurements. There are now some very large zenith telescopes which use, as their mirror, liquid mercury. When spun, it naturally assumes a paraboloid surface.

Drives and "Gotos"

The equatorial mount made it possible to automate the process of tracking stars by using a weight-driven clockwork mechanism to turn the telescope at just the right speed about the polar axis.

At some point in the twentieth century, the clockwork drive was replaced by an electric motor.

Advances in electronics made it possible to "drive" a telescope with an "alt-az" mount, rotating it more or less simultaneously on both axes.

The next step was the "goto" telescope, which could not only track a star, but could also "jump" to one. Canon says that there is a "goto" telescope in Grantville. When the "goto" is turned on, it may ask the observer to enter the longitude and latitude, or try to locate itself from a GPS signal (the latter won't work after RoF, of course). The "goto" also needs the time and date. (Obviously, it is not going to accept a date of "1632.")

What date and time do you enter? Ideally, you enter a date in the twentieth century which is within the "goto" date range but is an integer number of sidereal days separated from the actual date of observation in 163x. (The sidereal day is the time interval between two successive crossings of the same celestial meridian by a star. Or, less precisely, the time for the star to return to the same apparent "place in the sky.") This will require a bit of calculating but once you have identified one correspondence you can just count days forward from there.

When the "goto" is turned on, the user has to calibrate the system by pointing the scope at one or more bright "alignment" stars in the unit's database. This will allow the unit to correct for errors in the mounting of the scope, or in the observing parameters.

The unit's database, of course, is going to be expecting the stars to be in the positions in the celestial sphere which they occupied around the year 2000. As opposed to 163x. The positions are going to be something like five degrees off.

Obviously, the "goto" isn't designed to correct for time travel-impelled precession, which is the revolution of the North Celestial Pole around the North Ecliptic Pole. However, it will think that the astronomer has failed to point the RA axis of the scope toward the NCP and correct for this "polar misalignment." Thus, it will unknowingly try to correct for precession. What I am not sure of is whether there is a limit on how much polar misalignment is correctible. At some point the "goto" may just "shrug electrons" ...