Georgius Panayiotou became famous under what name both George - MBA - 217 View Full Document Georgius Panayiotou became famous under what name (both) George Michael 85 Capers are pickled flower seeds of what plant Nasturtium 86 In 1925 at Windsor Bookies went on strike - against what Betting Tax 87 What is a doucet A Stags Testicle 88 What is the worlds largest sand island northeast of Brisbane Fraser island 89 FT (London) Dow Jones (USA) what is Japans Share Index called Nikkei 90 Who was the leader of the wolf pack in The Jungle Book Akala 91 What's the difference between sleeping gorillas and men Gorillas don’t snore 92 Nobody Does it Better was sung in which Bond film The Spy Who Loved Me 93 Halcyon is the poetic name for which bird Kingfisher 94 Who had a hit with Sylvia's Mother Dr Hook 95 Who composed the music for the opera The Tales of Hoffman Jacques Offenbach 96 In the Bible from whom did David steal his wife Bathsheba The Hittite warrior Uriah 9

7 The Cassegranian, Gregorian and Schmidt are types of what Telescopes 98 Shakespeare - Antony, Romeo, Othello - what in common Suicide 99 What instrument is sometimes called the clown of the orchestra Bassoon 100 What world capital city is heated by volcanic springs Reykjavik (Iceland) Page 33 This is the end of the preview.

Sign up to access the rest of the document. TERM 10000 general knowledge questions and answers No Questions Quiz 18 Ans 10000_questions 36 Starizona's Telescope Basics My Basket Above: Optical layout of a typical Schmidt-Cassegrain Schmidt-Cassegrains telescopes (SCTs) have become one of the most popular types of telescopes. This is due to the versatility and compactness of the optical design. SCTs are only about twice as long as their aperture , making them highly portable. They are also well-suited to a number of applications including photography and terrestrial observing as well as stargazing. Most new SCTs are computerized, adding to their ease of use and portability. How Schmidt-Cassegrains Work Schmidt-Cassegrain telescopes are a catadioptric design, meaning they use both lenses and mirrors. SCTs are primarily reflecting telescopes, but they use a corrector lens to eliminate aberrations that would result from the mirror design alone. In an SCT the incoming light passes through the Schmidt corrector lens (also called a corrector plate) at the front of telescope.

It is reflected from a concave primary mirror at the back of the scope which focuses the light to the front of the telescope where it is reflected again by a smaller, convex secondary mirror . Finally, the light travels back through a hole in the primary mirror to the rear of the scope where an eyepiece is located for visual observing (or a camera for photographing). By folding the light in this manner, an SCT can be made much smaller than an equivalent Newtonian or refractor . At first it seems that an SCT should have a focal length twice as long as the length of the tube, since the light travels twice the length of the tube. However, SCTs actually have focal lengths about 5 times longer than the tube length. This is due to the convex curvature of the secondary mirror. This effectively magnifies the focal length, making the scope act much longer than it is. The diagram below shows how a Cassegrain type telescope (such as an SCT or similar design) works to increase the effective focal length. The primary mirror of an SCT usually has a focal length only about twice its diameter. For example, an 8" telescope would have a primary mirror focal length of about 16".

This focuses the light to a point 16" in front of the mirror. By placing a convex secondary mirror between the primary mirror and this focal point, the light is reflected back toward the primary mirror but at a less steep angle. If you trace this light cone back to where it reaches the full width of the telescope's aperture you can see how long the effective focal length is. Most SCTs have secondary mirrors that provide a 5x magnification factor. In this example that would make the effective focal length 80", or ten times the diameter of the primary mirror, in a telescope only about 16" long. Above: How a Cassegrain telescope creates a long effective focal length in a short tube. The actual length of the telescope is barely longer than the separation between the two mirrors. Varieties of Cassegrain Telescopes So what makes a Schmidt-Cassegrain different from a Maksutov-Cassegrain , or just a plain old Cassegrain ? The answer has to do with the corrector lens at the front of the scope. In an all-reflective Cassegrain telescope (usually called a Classical Cassegrain), there is no corrector lens.

To eliminate aberrations, this type of telescope must have aspherical mirrors, which are difficult and expensive to manufacture. By placing a corrector lens in front of the telescope, these aberrations can be minimized or eliminated while using spherical mirrors which are cheaper and easier to make. As the name implies, Schmidt-Cassegrains use a Schmidt corrector lens, while a Maksutov-Cassegrain uses a Maksutov corrector lens. Schmidt corrector lenses have an aspherical shape. The curve on the lens is very slight, and the glass looks perfectly flat to the eye. The curve is drawn greatly exaggerated in the diagram at the top of the page. With both mirrors left spherical, an SCT suffers from coma , an aberration affecting sharpness of the stars at the edge of the field. By making the secondary mirror aspheric, coma can be eliminated. Meade's Advanced Ritchey-Chrétien telescopes are of this latter design, simulating the effect of true Ritchey-Chrétien optics in an SCT configuration. The Popularity of Schmidt-Cassegrains Why are SCTs so popular? One word: versatility.

Most amateur astronomers enjoy many different aspects of our hobby, from observing planets to hunting for deep-sky objects to photographing the heavens. SCTs are good for all of these things. While certain telescopes might be better suited for one particular application, SCTs can do it all. For the money, a Dobsonian will provide more aperture and hence be better for deep-sky observing, but it cannot be used for photography. A high-quality refractor can give some of the best planetary views of any type of telescope, but a large-aperture refractor capable of competing with an 8" or 10" SCT in light-gathering ability is a huge, painfully expensive telescope. SCTs combine a compact telescope with good moderate-aperture optics which are well-suited to visual observing and imaging. Goto SCTs Almost all current Schmidt-Cassegrain models are computerized goto telescopes. SCTs are typically the flagship models of a manufacturer's product line and usually incorporate state-of-the-art technology. Goto mounts also lend themselves to compact design and add to the portability of SCTs. Schmidt-Cassegrain Prices As mentioned above, most SCTs are goto telescopes.

They come in a variety of sizes from about 5" to 16". Within each size there are often several models with varying features, mostly in terms of mount features such as tracking capabilities. The mount will determine the telescope's photographic abilities. Some less-expensive designs eliminate the high-precision motors necessary for photography; they are still suitable for visual observing but will not be as good for imaging. Most SCTs are fork-mounted, making them easy to setup and use, but requiring an equatorial wedge for long-exposure imaging. Some high-end SCTs are sold instead with heavy-duty German equatorial mounts , which are typically more stable than fork-mounted telescopes, but are also more expensive and require more setup time. Some inexpensive SCTs are also sold on German equatorial mounts, but these smaller mounts are not particularly well-suited to long-exposure photography. The most popular SCTs are 8" in aperture. An 8" SCT on a goto mount starts around $1400. The same telescope with a mount capable of tracking accurately for long-exposure photography will start around $2000. Smaller SCTs start around $1000.

Larger SCTs can cost up to around $10,000. SCTs are also sold as optical tube assemblies (OTAs) without any mount for those observers who already have a German equatorial mount. 8" OTAs cost about $1000, with 14" to 16" OTAs costing $4000-6000. Is a Schmidt-Cassegrain Best for Me? For observers who want to be able to do a little of everything, from observing to photographing, SCTs are a great all-around telescope. They are very portable (at least in 11" and smaller sizes), and easy to setup and use. They often have the latest and greatest features in terms of computer systems and other high-tech gadgetry. If your only interest is in visual observing, you can get a larger-aperture Dobsonian for the price. But most observers have varied interests and that makes the Schmidt-Cassegrain a very popular choice. Cassegrain Primer - Characteristics of Cassegrains What does it all mean? And does a curved field really matter?

We make Convex Hyperbolic Optical Elements This service is not meant to supply amateurs with Cassegrain secondaries. It is primarily for scientific and military institutions looking for secondaries for long-wavelength high-energy scientific laser systems as a less expensive alternative to fabrication than using Hindle spheres or shells. Substrate material is fused silica and these secondaries can be made up to 12" in diameter. Secondaries are tested through the back using a unique embodiment of the Schmadel test and can be very accurately made but the effectiveness of the unique methodology is only as good as the characterization of the primary and to a degree the fabrication quality of the primary itself. Please note that we are ITAR compliant and prefer customers that are US nationals such as universities and scientific institutions. Continue Cassegrain Primer I get a lot of queries regarding the purchase of Cassegrain telescope optics of some form or another. Many of these questions indicate there is a considerable amount of misunderstanding as to what the various types of Cassegrain telescopes are designed to do and what they are capable of doing. Most of the confusion seems to revolve around the true purpose of a Ritchey-Chretien telescope.

Other misconceptions surround exactly what a Dall-Kirkham is and its advantages as well as its limitations. And then there is the classical Cassegrain. What makes it classical? And what are its advantages and disadvantages? Hopefully, the following will help clarify some of these questions. The concept of the two mirror telescope began to emerge in the mid 1600s with the postulation of both the Gregorian and Cassegrain type instruments. The two are primarily differentiated by their secondary mirrors, the Gregorian utilizes a concave mirror placed beyond the focal point of the primary mirror and the Cassegrain utilizes a convex secondary mirror placed somewhat inside of the focus of the primary mirror. Both of these mirrors are so formed as to cause a final focus to take place usually somewhere behind the primary mirror, which has been perforated so as to allow the light to pass through. Both the Cassegrain and Gregorian types are extremely useful instruments. Gregorians were initially the more popular of the two because the concave secondary allowed primitive 18th century shop testing at the center of curvature to take place while the convex Cassegrain secondary could not be shop tested at the center of curvature.

The Cassegrain telescope did not assume significant popularity until after the 1850s, and really even later than that, when more sophisticated shop tests were developed. The testing of a Cassegrain system really requires the use of an autocollimating flat mirror equal to the size of the primary mirror. Alternative tests have been devised to test the secondary mirrors separately, but it is almost always best to test the entire telescope as a completed system. Prior to the use of autocollimating shop tests the only way to test the telescope was to actually assemble the optics in a completed instrument and perform a star test; something that is problematic, being influenced by atmospheric turbulence and weather dependant. It has been the advent of modern shop testing that has allowed the Cassegrain to assume the role that it has in modern optical astronomy. The original postulation for the Cassegrain telescope was to have a primary parabolic mirror focusing its rays onto a convex hyperbolic secondary mirror; the classical Cassegrain. This combination of aspheric curves yields excellent correction over moderate fields both for visual and photographic astronomy. The typical Cassegrain was designed to work at focal ratios ranging from f/12 to perhaps f/30.

Such geometry results in a fairly small field view, and small fields were readily accepted in early visual and photographic work. It was not until interest in wide field photography that the usefulness of the two mirror system began to be expanded. In 1906 Martin Schwartzchild, a German physicist and optical scientist, published a set of equations that defined the characteristics of two mirror telescopes. Applied to the Cassegrain design these relatively simple formulas began to reveal the fact that by juggling the asphericity between the two mirrors off axis aberrations could be controlled; practically speaking, essentially reduced where possible and appropriate. This was the first time in history that the concept of optimization in the reflecting telescope was understood in a truly scientific sense. Optimization had been developed in the early 1800s by Josef Fraunhofer, and carried forward by Steinheil and Abbe, for aplanatic refractor objectives but it had not been well understood when applied to reflecting systems. Schwartzchild changed all this. History is fuzzy here so there is some speculation to be made, but it is likely that through these equations George Willis Ritchey may have worked out the original designs for what came to be known as the Ritchey-Chretien system.

He had established a long distance relationship with the French optical scientist Henry Chretien and Chretien may have made Ritchey aware of these equations and prompted Ritchey to do some exploratory work. All of this is extremely conjectural but the fact is this design emerged just after the publication of Scwartzchild's equations. George Willis Ritchey had a particular interest in astro photography, and a developing interest in the infant area of wide field astro photography. The fundamental concept of the Ritchey-Chretien is that the off axis aberrations resulting from wide field photography are reduced to a minimum through aspheric optimization. In a classical Cassegrain there are two aberrations that are problematic: astigmatism and coma. The astigmatism component is quite small while the coma component is comparatively large. It turns out that by making the primary mirror slightly more prolate, that is, going beyond a parabola into a hyperbola, and compensating by making the secondary mirror even more hyperbolic, one can correct coma at the expense of astigmatism. But since astigmatism is so small it can be raised considerably before it becomes larger than the coma. The result is a two mirror system optimized over a specific field of view.

We now see that the Ritchey-Chretien camera was developed for a specific purpose, wide field photography, not high resolution imaging over a small field. Ritchey-Chretien systems, therefore, are usually made quite fast for a two mirror Cassegrain type system. While classical Cassegrains for visual and high-resolution imaging are in the arena of f/15 to f/25, Ritchey-Chretien systems are usually at about f/8 or f/9 and often as fast as f/6. Such systems have primary mirrors a short as f/2.5 or f/2 and require massive aspherization of their strongly curved primary and secondary mirrors. It must be understood that the final optical performance in terms of wavefront of such fast and highly aspherized systems is usually a lot lower than what is found in slower less aspherized Cassegrain systems. From a practical perspective, extreme aspherization and the desire for a high quality wavefront is never a good combination.

As to how much sky area a Ritchey-Chretien can cover it is relatively small when compared to other wide-angle systems such as the Wright, Houghton or Maksutov systems and the ultra-wide-angle Schmidt type systems. While a Schmidt system might cover three or four degrees, a Ritchey-Chretien might cover one degree. Something else to consider with the Ritchey-Chretien design is that in its original embodiment (Ritchey's specific designs) it would form round stellar images somewhat aberrated (out of focus) rather than optimized asymmetric comatic images (which we tend to like for wide field photography). While this is of no particular interest to the majority of amateur astronomers, early wide field photography interests were often concerned with the science of astrometry, the precise measurements of stars in relation to each other within a given field, not beautiful pictures. As such, the early Ritchey-Chretien systems were optimized (pushed) even further than we would likely do today to the extent of forming perfectly round stellar images, even if they were a little aberrated. The important thing was to define the very exact center of these images, not ogle over their beauty.

When discussing the Ritchey-Chretien it is often mentioned that this type of telescope has a curved field, 'like a Schmidt system', so the phrase usually goes. While it is true that it has a curved field it really is of no practical consequence for amateur size instruments. In the spot diagrams given below I have optimized the systems to a flat field - like you get on your CCD imager. Attempts to optimize to a curve field offered nothing in the way of improvement. For example, a 12.5 inch system working at f/8 as a curvature field of approximately 67 inches radius convex toward the viewer. If one analyzes the sagitta over a focal plane area of 1 inch diameter we find that it has a depth of 0.0075". It should be noted that all Cassegrains of a given geometric design have the same curvature field whether they are a Ritchey-Chretien, classical, Dall-Kirkham or any other aspheric combination. The last type of commonly encountered Cassegrain to consider is the Dall-Kirkham.

This type of Cassegrain appears to have been developed in the 1930s by Horace Dall of Luton, England and uses a spherical secondary mirror combined with a primary mirror aspherized somewhat less than a full paraboloid. To understand exactly what this means, a sphere is defined as having an aspherization constant of 0 while a paraboloid is defined as -1. An ellipsoid is defined as something less than a paraboloid (and greater than a sphere) and an hyperboloid would be defined as something beyond a paraboloid or greater than -1. (The anomaly of referring to, say, -1.2 as greater than -1 and -.80 as less than -1 is accepted for this purpose.) Typically, a Dall-Kirkham might have an aspherization constant somewhere in the area of -.70 to -.80. A Ritchey-Chretien, by contrast, might have an aspherization constant of -1.12. The advantage of the Dall-Kirkham lies in that the spherical secondary is fundamentally easier to accurately construct.

Figuring a convex prolate (ellipsoidal, paraboloidal or hypboloidal) surface is one of the more difficult things in optics to accomplish well. Combined with the spherical secondary and lightly aspherized primary, the system be more accurately fabricated than any other compound two mirror design. If properly designed and constructed a Dall-Kirkham can deliver the finest images of any Cassegrain type telescope. The principle problem with the Dall-Kirkham design is that it does not correct for comatic off axis images. The extent to which this is a problem is largely connected with the speed of the primary mirror; the faster the primary mirror, all other things remaining equal, the greater the off axis coma. Realistically, the Dall-Kirkham performs extremely well with primaries of f/4 or slower. Slower primaries, however, increase the physical size of the secondary obstruction. Overall focal ratios are typically not lower than 15 with the best systems at f/18 or above. A Dall-Kirkham is best viewed as a strictly narrow-field lunar, planetary and double star instrument, to which use it is admirably suited.

Now that I have discussed the basics let's look at some spot diagrams produced by various types of Cassegrain systems of various focal ratios. I have arranged these in descending order according to effective focal ratio. All spot diagrams were made assuming a mirror diameter of 12.5 inches and a back focus of 15 inches. The spot diagrams for each equivalent focal ratio were optimized as a classical system, Ritchey-Chretien system and Dall-Kirkham system as appropriate. All diagrams are optimized to so as to form the best image over the field, rather than form the best focus at the center. All of the configurations given here will form precise on-axis images. Not all systems were considered appropriate for optimization has a Ritchey-Chretien system due to their large focal ratio, there being essentially no difference between classical Cassegrain and Ritchey-Chretien spot diagram results. Conversely, the faster systems would fail completely as a Dall-Kirkham. Spots are given for each type: on-axis, 1/8 deg, 1/4 deg, 1/2 deg and (for smaller ratios) 1 deg. All are given for flat fields.

(In the accompanying spot diagrams the angles given are half-angles.) In practical terms, the higher focal ratio systems will have only 1/4 and 1/8 degree working fields, but 1/2 degree was used and a standard outside reference.

In Paris there are two islands Ile de la Cite and what Ile St - IT - 402 View Full Document In Paris there are two islands - Ile de la Cite and what Ile St-Louise 68 Mapother IV is the real surname of what film star Tom Cruise 69 Jean-Christopher Denner invented what musical instrument Clarinet 70 Mendavoy and Martinez are characters in which TV show NYPD Blue 71 Fredrick Bulsara was the lead singer of what pop group Queen - Freddie Mercury 72 The Titanic has a sister ship - name it The Olympic 73 What was sharkskin once used as Sandpaper 74 What does an otologist study The ear and its diseases 75 What is the essential ingredient in a Mornay sauce Cheese 76 Sardines are the young of which fish Pilchard 77 Who wrote Oedipus Rex Sophocles 78 The Camorra was the forerunner of what organisation in the USA The Mafia 79 What is a Havana Brown A Small Rabbit 80 What did Simon of Cyrene do in The Bible Carry Christ's cross 81 In what sport is a stimpmeter used

 Golf - measure greens pace 82 Gene Hackman sheriff Big Whiskey - got Oscar - What film Unforgiven 83 What countries flag red circle on green background Singapore 84 Georgius Panayiotou became famous under what name (both) George Michael 85 Capers are pickled flower seeds of what plant Nasturtium 86 In 1925 at Windsor Bookies went on strike - against what Betting Tax 87 What is a doucet A Stags Testicle 88 What is the worlds largest sand island northeast of Brisbane Fraser island 89 FT (London) Dow Jones (USA) what is Japans Share Index called Nikkei 90 Who was the leader of the wolf pack in The Jungle Book Akala 91 What's the difference between sleeping gorillas and men Gorillas don’t snore 92 Nobody Does it Better was sung in which Bond film The Spy Who Loved Me 93 Halcyon is the poetic name for which bird Kingfisher 94 Who had a hit with Sylvia's Mother Dr Hook 95 Who composed the music for the opera The Tales of Hoffman Jacques Offenbach 96 In the Bible from whom did David

 steal his wife Bathsheba The Hittite warrior Uriah 97 The Cassegranian, Gregorian and Schmidt are types of what Telescopes 98 Shakespeare - Antony, Romeo, Othello - what in common Suicide 99 What instrument is sometimes called the clown of the orchestra Bassoon 100 What world capital city is heated by volcanic springs Reykjavik (Iceland) Page 33 This preview has intentionally blurred sections.

Sign up to view the full version. View Full Document 10000 general knowledge questions and answers No Questions Quiz 17 Answers 1 What is the main ingredient of a Maron glace Chestnuts 2 Only one woman's lifespan is given in the Bible - Who Sarah Wife Abraham 127 Genis 23 3 What causes the tangy smell at the seaside Rotting Seaweed 4 In 1976 in USA 23 people got swine fever and died from what The Treatment 5 Who said "If a lie is told in the Whitehouse Nixon gets a royalty" Richard Nixon 6 In the 1976 Olympics who were the Yellow Bananas Officials (cos of uniform colour) 7 Amuhea Princess of Medes was the wife of who Nebuchadnezzars 8 What Roman Emperor was killed by an overdose of laxative Nero - by an aunt 9 How did folk singer Roy Harper catch Toxoplasmosis Kiss of life - to a sheep 10 What was Louis 14th born with two of - that amazed everyone Teeth 11 This is the end of the preview. Sign up to access the rest of the document.

TERM Kenyatta University IT 402 - Spring 2015 1 2 3 4 5 Sampling In Research What is research? According Webster (1985), to researc HYPO.docx Cassegrain Notes. Cassegrain Notes. Written by Kenneth Novak, BS. Second Edition First, I'd like to thank the readers of the first edition of Cassegrain Notes that took the time to write and offer suggestions for improvements and also those readers purchasing the first edition. Many improvements have been made in the second edition within the limits imposed by the demand for the booklet, unfortunately, telescope design and construction books have a relatively limited market, otherwise more material could be added. Maybe more can be added in the next printing. References are given at the end of each Section for additional material. No attempt is made to redo already published material. All parts of a Cassegrain telescope can be made by some skilled amateurs with sufficient equipment and materials. However since most Cass telescopes are built using commercial parts the emphasis is on using them. Commercial parts are carefully designed to fit each other and to minimize assembly, adjustment and use of a Cass telescope.

Many special designs are possible requiring special parts but they require special designs to properly support the optics and to use available equipment and materials unless you have the skill, materials and equipment to duplicate special designs the information is of little use. Detailed information on machining Cass parts including the tube and corrector cell are given in "Construction of a Maksutov Telescope" by W. I. Fillmore, Sky Publishing Corp. 1961. Copywright 1974, 1976, 1978 by Kenneth Novak. Index of Chapters Testing Secondaries Secondary Tests Bob's Note: The above index is maintained with page numbers. There are no page numbers on the following web pages as different text sizes will produce a different number of pages. However, each chapter heading has a web page associated with it. Enjoy! End of Bob's Note. Cassegrain Optical system Designs The classical Cassegrain telescope design was originated in 1672 by Guillaspe Cassegrain. At the time it could not be used in real telescopes because both the primary mirror and secondary mirror required aspherical surfaces. surfaces. Optical making was limited to spherical surfaces.

At about the same time Isaac Newton developed the Newtonian telescope using one aspheric primary mirror and flat secondary mirror. A few of these telescopes were constructed. A few of these telescopes were constructed with spherical surfaces but without the aspheric mirrors the results were not encouraging. It wasn't until 1723 that the first telescope with an aspheric mirror was built - a Newtonian of about F10.5. Today we think of mirrors as pieces of glass with a thin coating of metal. but in the early development of reflectors the mirrors were pieces of metal ground to the proper curve and polished until very shiny. The shiny surface lasted only a few weeks before it had to be re polished and corrected again With all of the problems with the aspheric surfaces and nearly constant re polishing required the effort to develop the reflector telescopes might seem out of place. Refractor lens telescopes that had been in use for decades. However, the refractor lenses in use at the time were uncorrected for color distortion in particular plus many other optical problems. Reflectors were developed to correct most of these optical distortions but especially color. Newtonian and Cassegrain telescopes collect light without passing it thru glass, thus avoiding the color problem.

Refractor lenses of very high F-ratio reduce color defects while reflectors can be made fully corrected at any f-ratio including very low f-ratios. Reflector telescopes did not achieve extensive use until methods making aspheric surfaces were developed mainly after Foucault perfected his test and methods of coating glass with metal were developed (silver coatings at first and aluminum coatings for longer life). Early this century reflectors could effectively compete with refractors. Development had greatly improved reflectors. However reflectors could be more easily made in large apertures, very low f-ratios and at low cost. Since a Cassegrain primary mirror is a low f-ratio with a fairly high effective f-ratio gained by the magnification of the secondary, both Newtonian and Cassegrain telescopes, could use much shorter and lighter tubes. The classical Cassegrain telescope optical system uses a parabolic primary mirror and a hyperbolic secondary. This gives good image correction, except for coma, and a flat field. As the reflector telescope gained popularity early this century technology for making the optics also improved. And variations of he Cassegrain system were developed to improve image corrections and/or make producing the optical surfaces easier.

The Gregorian telescope was invented even before the classical Cassegrain. It used a parabolic primary and elliptical secondary. The optical performance is about the same as the Cassegrain is but needs a longer tube. The Gregorian system is a workable optical system but is rarely made as it offers no special advantage over the Cassegrain system and has the disadvantage of a longer tube. Bernard Schmidt developed the Schmidt principle in the 1930's, taking a spherical primary mirror and usually with a thin aspheric corrector lens but the focal plane is not flat. It has a quite strong curve. The design corrected for coma very nicely. The Schmidt principle has been applied to make Schmidt cameras (both with and without the corrector lens) and to Schmidt-Cassegrain telescopes. The major advantage to the Schmidt principle is to give well corrected images to very low f-ratio cameras and telescopes. In Schmidt cameras the film is usually curved to match the curve of the focal plane. Other designs are made to reduce coma such as the Ritchey-Chretien with hyperbolic curves on both the primary and secondary mirrors.

The focal plane can be made fairly flat so when used for photography the film is sometimes only curved slightly to suit. The Maksutov-Cassegrain uses a thick corrector lens over the upper end of the tube but usually all surfaces are spherical. In some cases, one or more of the surfaces are made aspherical in order to get improved correction, especially if the f-ratio is not very high. The coma is not as well corrected as the classical Cassegrain, Ritchey-Chretien, or Schmidt-Cassegrain. In more recent years these Cassegrain designs have been further modified, often with the help of computers. The Dall-Kirkham-Cassegrain uses an ellipsoidal primary and a spherical secondary mirror to simplify making the optics. Both mirror surfaces are easier to produce by amateurs and optical firms but the aberrations get bad off-axis. The Buchreder-Cassegrain uses all spherical mirrors and a number of spherical refractor relay lenses. The large five all spherical surfaces with the secondary discourage construction. The Magee-Cassegrain has two lens of the Mangin style (rear surface aluminized).

Unlike full aperture lenses the refractor secondary lens is easier to make and does not use large pieces of optical glass which is quite expensive. It is limited to fairly small apertures but is usually made with an effective f-ratio of about F8 to give a quite wide field of view for a Cassegrain in a very short tube. The Seigler-Maksutov-Cassegrain has four all spherical surfaces; two in the corrector lens, one in the primary, and one in the secondary mirror. It also is usually made with an effective f-ratio of about F8 to use a very short tube. The Seigler-Maksutov has a special advantage with a well corrected field of 2 degrees and can be made in apertures larger than possible with the Magee-Cassegrain. Many of these Cassegrain variations are cadioptic, that is, they use a combination of both refractor and reflector optics to collect the light and bring it to the focal plane. This may offer special advantages but tends to discourage construction especially by amateurs.

Cassegrain telescopes not ready made are usually classical Cassegrain or Dall-Kirkham either from home made optics or purchased commercial optical sets. The Ritchey=Chretien design could become more popular if optical sets were commercially available. For amateurs with experience making classical Cassegrain optics it might be well to consider the Richey-Chretien since it offers improved corrections over most other optical systems and can be made in large apertures. a = (F + b) / (X+1) Focal length of sec. = B / (X + 1) A = a * X = B + b Cass effective f-ratio f-ratio = F * X I / X = image size at prime focus f = F / M Cassegrain telescopes are laid out in a kind of juggling act to get the telescope to work well, seduce the problems making the optics and give a telescope that is comfortable to use - all at the same time. The f-ratio of the primary can be just about anything but above F6 the total effective f-ratio becomes too high. On the other hand it is difficult especially for amateurs to make the primary much less than about F4.

Similarly the secondary can be made for any magnification but above 6x the total effective f-ratio becomes too high. It is possible to make the Cass secondary 2x but for \most amateurs 3x is about the lowest magnification used. Because of these limits, most Cassegrain telescopes made by amateurs have an effective f-ratio of F12 to F20 with F16 being the most common. If you are buying a ready-made optics then the job of making the optics are left to the optical ably and are more expensive. The formulae for designing Cass telescopes are given here for amateurs making their own optics and amateurs having custom optics made to suit. Stock Cass optics are available at reasonably low cost but most of the above formulae are not needed because there is little choice of all the many variables. You may want to know the magnification of the secondary (X) so you can find the focal length of the primary (F). Use formula (1) X = (E - b - B) / B (2) F = E / X (1) first then formula (2) if (F) and (X) are not known.

In some cases a Cass telescope is in hand but the optical specifications are not known. Formulae (1) and (2) allows you determine optical specifications knowing the total effective f-ratio and measuring the two spacing dimensions directly. This avoids removing the optics from the telescope and testing the optics for focal length. For example, if you wanted to add a baffle, most of the dimensions shown above must be known. Each variable is important even though you are not making optics so most will be considered here. Some variables are a matter of personal choice much as mirror aperture size and total effective f-ratio. Back focus (b); the back focus on a Cass telescope is the distance from the face of the primary mirror to the Cass focus. The back focus must be long enough to bring the final image to a comfortable eyepiece position for the observer , in most Cass telescope the back focus is used up by the mirror thickness (T); Mirror mount (CL distance); height of the focusing mount when fully racked "in" (H); and optical path length of the star diagonal, (OPL). If you have optics made the back focus distance can be selected to match the parts you use.

For ready-made optics the mechanical parts might not make up the full back focus distance. The excess can be taken up with focusing mount extensions (placed between the focuser base and the face of the end cap) or by focuser tube extensions (placed between the focuser tube and the eyepiece. To find the proper length of the focusing mount extensions for focuser tube extensions, take the back focal length (b), subtract the distance used by the mirror thickness (T) subtract the CL distance of the mirror mount, subtract focuser height (H) and optical path length used by the star diagonal (OPL). If the remainder is positive, select the next shortest extension within one inch of the remainder. If the remainder is negative you will probably have to use a shorter focusing mount. STAR DIAGONALS; Star diagonals use up part of the back focus. The light beam from the Cass secondary to the final image is in the same direction as it would be in a normal refractor telescope. Without a star diagonal an observer head must face the same direction as the primary mirror or lens which is up toward the sky. While this can be done; it is very hard on the observors neck especially for objects overhead.

To avoid this problem most refractors and Cassegrain type telescope use a star diagonal to allow comfortable observations. Not only does it reflect the light beam near the focus by 90 degrees but since most star diagonals can be rotated 360 degrees in the focusing mount it gives maximum telescope flexibility and observer comfort without rotating the tube. A Cass telescope with its short tube and rotatable star diagonal make it one of the most comfortable and easy to use telescopes available. In some small Cass telescopes the standard star diagonal is replaced by a diagonal mirror, usually mounted on a single vane holder, positioned in front of the primary mirror. This allows the use of a regular Newtonian type focuser mounted on the side of the tube and avoids the need to cut a hole in the center of the primary mirror times this arrangement must be used because the back focal length is too short for mounting the focuser at the end of the tube. In these cases the tube should be made to rotate as a Newtonian type telescope to allow comfortable eyepiece position. Star diagonals can be divided into two general types - mirror type and prism type. Since prisms are not color corrected they can cause color problems.

The color problems related to prism diagonals vary greatly with the effective f-ratio of the telescope. Prism star diagonals are recommended for use on refractors and Cass telescope of F14 or more. Color distortion is not a problem, the fluorite coating has a very long life, and light loss is less than with a mirror type. Mirror type star diagonals should be used at F12 or less and are recommended at F14 or less to avoid color defects in the images. The mirror coating does not last as long. It is sometimes recoated as needed with the primary and secondary mirrors. Despite these slightly less desirable featured, mirror type star diagonals are very valuable for lower f-ratio Cassegrains and for large star diagonals. Prisms are not generally available to make star diagonals for 2" focusing mounts because of the high cost. Mirror type star diagonals are available in the 2" size when 2" eyepieces are desired to get lower magnifications and wider fields of view. FOCUSING MOUNTS: The eyepiece focuser also uses up some of the back focal length. a focusing travel of 3" is generally sufficient.

If more is needed a focusing tube extension can be added. In most Cass telescopes a standard 1 1/4" eyepiece size is sufficient. A 2" focuser is used only for low f-ratio, wide field, or large aperture Cass telescopes (2" aperture or more). SEPARATION (B): The distance between the Cass primary and secondary mirrors is called the separation distance. The exact correct separation is determined by the focal length of the secondary. Once the optics are made the separation distance is fixed. Errors in separation can cause deterioration in performance. Some Cass telescopes use a variable separation either by moving the primary or the secondary mirrors to change the back focus. This in turn can focuses the telescope without the use of a focusing mount and allows a telescope to be focused on objects a few feet from the telescope (for bird and animal observations). Some optical performance is sacrificed to get very long focusing range. For an astronomical telescope the objects being observed are effectively all at infinity. The separation should be fixed accurately to match the optical specifications. If you want to use your telescope for observing birds, animals, and other more nearby objects you may need additional 'out' travel for the focuser.

It is a very simple matter to add focuser tube extensions as needed between the eyepiece and the focuser. In this way none of the optical performance is sacrificed. IMAGE SIZE (I): The final Cass image size is formed at the focal plane. Your eyepieces will magnify this image so changing the eyepiece will change the magnification of the telescope. Used here this image size refers to the image size of the full 100 percent illumination. Beyond the circle of 100 percent illumination the image brightness decreases. the 100 percent illumination circle size is selected to be what can best be used by the eyepieces. The size of the 100 percent illumination image circle best to use has been debated by a number of experienced people and varies from 1/4 inch to over a full inch. Selecting an excessively oversized image will increase the size of the secondary. On the other hand a 1/4 inch image size will not bring 100 percent illumination to the full field of view of all eyepieces but usually there is at least 50 precent illumination at the edge.

The best practice is to select image size from your own experience to suit your personal preferences and eyepieces. Lacking that here are some guidelines. A 1/2 inch image size is suggested when you plan on using eyepieces mostly of less than 24mm focal length; an image size of 3/4 inch when you plan on using eyepieces often of 24mm to 60mm focal length; and an image size of one inch for prime focus or Cass focus photography for large, extended objects. Refer to "Newtonian Notes" for more discussion on image sizes and to "all about Telescopes" for a chart on the linear field size limits of various eyepieces. The linear field size is what will fit into the eyepiece. The image size at the Cass focus is what you make available for all eyepieces. Only the smaller of the two sizes all match your eye to be used. CASSEGRAIN SECONDARY SIZE (C): The most commonly used formula for determining the Cass secondary size has been in "Amateur Telescope Making", Book one in the original format. The author, Hohn H.

Hindle points out that the formula he gives is approx9imate and yields an undersized answer with respect to the actual required size. The formula given here was derived differently and is exact. Actually the first term MF + Mb / FX + F is equivalent to the formula in his article. The second term BI/FX compensates for the final image size. The larger the desired final image size the larger the secondary must be. It is incorrect to assume the image at the focal point is also a point. The image has real size so if you assume it is a point to image size of 100 percent illumination will be zero. (3) Cassegrain Secondary Mirror Size C = (MF +Mb / FX + F) + (BI / FX) where Page 10 | Cassegrain Telescopes & Equipment for Sale - Astronomical Telescopes | OPT Telescopes How a Cassegrain Works In the classic Cassegrain telescope a parabolic primary mirror has a hole placed in its center. While this may seem like a very strange thing to do, the incoming electromagnetic waves are actually captured by the parabolic mirror’s edges and re-directed to the hyperbolic secondary mirror where they converge.

The light (or wave) is then refocused back towards the hole where it converges to a focal point – the eyepiece. This “folding” technique makes for a very compact design which delivers longer focal lengths and permits higher magnification factors. Cassegrain Designs In staying true to classic configurations, two types follow the original Cassegrain design – the Ritchey-Chretien telescope and the Dall-Kirkham Cassegrain telescope. When optician Bernhard Schmidt developed an astronomical camera that employed the Cassegrain to direct its light onto a photographic plate, another idea was born. Using a corrector plate, or lens, at the front of the Cassegrain telescope further improves its focusing abilities. Although many believe it was Schmidt who developed the Schmidt-Cassegrain telescope, it was actually James Gilbert Baker who created the first working models. Other variations on the Cassegrain design include the Maksutov-Cassegrain telescope and the Argunov-Cassegrain telescope. Cassegrain Uses The Cassegrain telescope is highly prized for its wide field photographic abilities and high magnification performance. Many believe it produces unrivaled views of Solar System studies and double stars.

For astrophotography, a camera, or a light detector such as a CCD, is placed at the focal point instead of an eyepiece. The mirrors in this type of Cassegrain telescope are designed and arranged to share a single focal point. Still other Cassegrain models make use of a tilted arrangement called “off axis”. This allows the image to focus without the shadow of the secondary mirror. Many radio telescopes are off-axis Cassegrain designs. A key selling point is their extreme portability and "best of both worlds" views. On the average, they are about twice as expensive as a comparative reflector and yet cost far less than an equal aperture refractor. Their sharp, high contrast images on any subject make the Schmidt-Cassegrain Telescope (SCT) or Maksutov Telescope (Mak) a good choice for the intermediate to more experienced observer. Maksutov Cassegrain Design The Maksutov-Cassegrain is a specific telescope design invented in 1941 by Russian optician Dmitri Maksutov.

It is patterned somewhat after the Schmidt-Cassegrain telescope as far as general appearances go, but the Maksutov-Cassegrain is an entity unto itself. Unlike the traditional catadioptric style which uses a flat lens across the front (the meniscus), the Maksutov utilizes a positive lens arrangement for the meniscus in conjunction with a spherical primary mirror. This large, full diameter lens structure is great at correcting off-axis and chromatic aberrations. While the Maksutov stops coma and false colors, it gets very expensive and is prone to lens sag because of the size and weight of the lens itself. It simply doesn’t apply itself well to large apertures for precisely the same reasons that large refractors aren’t common. The folded optical path in conjunction with a secondary silvered “spot” on the convex side of the meniscus facing the primary mirror makes an excellent choice for a durable and portable telescope. It has a simple construction. By incorporating the silvered spot instead of a secondary mirror, it gives it the advantage of fixing the alignment of the secondary and eliminates the need for a ‘spider’ that would cause diffraction spikes.

Maksutov Cassegrain Astronomy Applications With its long, folded optical path, the Maksutov-Cassegrain offers up ridiculously high focal ratios, making it a superb instrument for high magnification studies. While they offer a very restricted field of view, they also excel at lunar and planetary imaging. This makes the Maksutov-Cassegrain telescope especially well designed for certain astronomical applications. Since the optical train is “fixed”, the telescope can be sealed away from exterior conditions. This makes it well suited to tracking, remote viewing, and radar calibration and boresighting where instruments are subjected to severe environments and high g-forces are encountered. The Early Reflecting Telescope:Cassegrain, Mersenne, and Cavalieri Historian Bias The Early Reflecting Telescope:Cassegrain, Mersenne, and Cavalieri Isaac Newton is often credited as the inventor of the reflecting telescope. In fact, a crude reflecting telescope had been built and used more than half a century before Newton's invention. Other activity before Newton's birth resulted in the publication of a reflecting telescope design that is still used today, the Mersenne telescope. Answering the question of who invented the reflected is complicated. As is the early history of the reflecting telescope.

The builder of that first crude telescope, Nicolo Zucchi, and the scientist who published the Mersenne telescope design, Marin Mersenne, were both Catholic priests. The involvement of church scientists in the early development of both the reflecting and refracting telescopes is often missed (see Fathers of the Telescope ). Newton:Inventor or Innovator Isaac Newton developed the design for his reflecting telescope in 1668 (see Timeline of the Telescope ). Newton's idea of building telescopes using mirrors instead of lenses was not new. The magnifying effect of concave mirrors had been put to practical use as reading aids by medieval monks centuries before (see Timeline of the Telescope: Year 1300 ). Leonardo da Vinci had used concave mirrors to study the planets more than a century earlier (see Timeline of the Telescope: Year 1513 ). The earliest known attempt to build a reflecting telescope was by Father Niccolo Zucchi, an Italian Jesuit, around 1616. This was only a few years after refracting telescopes started being used for astronomy. Zucchi stated that he used it to view objects both "celestial and terrestrial".

Zucchi was not happy with the result so he went back to building and using refracting telescopes. There was a good reason for this early interest in building telescopes using mirrors. There were many serious problems with early refracting telescopes. The lenses of the early telescopes were not made in billion-dollar computer-controlled Japanese factories. They were hand-blown by artisans. By today's standards the glass used was very poor for optics, being plagued by both bubbles and various types of distortions (specifically chromatic and spherical aberration). Several of Galileo's contemporaries proposed designs that put mirrors in place of lenses. Even Galileo recognized this possibility. Two of these contemporaries, Father Bonaventura Cavalieri and Father Marin Mersenne suggested designs for constructing telescopes using parabolic mirrors instead of lenses. It was well-known that a parabolic mirror could have the same effect as a convex lense...but it wouldn't have the problems associated with chromatic aberration. Most modern research telescopes use a combination of lenses and mirrors where a large (primary) mirror reflects light onto a smaller (secondary) mirror which then reflects the light back through a hole in the primary mirror to an eyepiece.

This type of telescope is named after Laurent Cassegrain, the Catholic priest who proposed the design shortly after Newton proposed his design. A diagram of this type of telescope is shown below on the right . The Hubble Space Telescope site has another diagram of the Cassegrain in the Hubble Space Telescope. Both Cavalieri and Mersenne proposed designs that combined perforated primary reflectors and smaller secondary reflectors. The image on the left is a figure from Bonaventura Cavalieri's Specchio Ustorio 1632(modified from the Max Planck Institute Digital Library ). Cavalieri's work may have been missed because he proposed the combination to amplify sound. Marin Mersenne proposed a similar design in Harmonie Universelle (1636). Specchio Ustorio Plate Cassegrain Telescope Design In Specchio Ustorio, Bonaventura Cavalieri also proposed a telescope design using a flat secondary mirror angled at a diagonal, just as Newton had. Since Cavalieri did not provide a figure for this design , it is difficult to know for certain how close his design would have been to Newton's. [_1_] .

Newton would have had easy access to both Cavalieri's and Nicolo Zucchi's work on telescopes early in his career. His friend and mentor, Isaac Barrow, had a copy of the Cavalieri's Specchio Ustorio in his private library [_2_] as well as a copy of Zucchi's Optica Philosophica. Marin Mersenne and the Telescope You don't expect to find major advances in the design of telescopes in a book on music. In 1636, Father Marin Mersenne published Harmonie Universelle, a mathematical study of music. It also proposed configurations of mirrors that could be used to produce either telescopic effects or burning mirrors. These configurations were prototypical forms of the Gregorian and Cassegrain telescopes. The image below shows Mersenne's diagrams of his telescopes (image was taken from [_3_] ). There is a side story to Mersenne's and Cavalieri's works. Harmonie Universelle was a book about music but it is important to the history of telescopes. Lo Specchio was a book about mirrors and telescopes but is probably more important to the history of classical mechanics.

Lo Specchio contained the first published description of the parabolic nature of projectile motion. Cavalieri's publication of this important concept even caused a short-lived rift in his friendship with Galileo Galilei. Both Galileo Galilei and Thomas Harriot had described projectile motion in private notes but had never published. Mersenne's work on reflecting telescopes was very advanced. Today it is doubted that either he or any of his contemporaries (including Descartes and Galileo) understood the full significance of his work. A full understanding of the advanced nature of Mersenne's work would have to wait until the twentieth century. An indication of this is that the Mersenne telescope, still being produced today, is largely a development of the twentieth century. Mersenne, went further than simply presenting configurations that are used in modern telescopes; his designs featured the strong telephoto effect critical to modern photographic lenses. This all happened 30 years before Newton's telescope [_4_] . Mersenne actually never did build telescopes to his designs. Oddly, he was dissuaded from building them by Rene Descartes. Descartes felt that reflecting telescopes were impractical. Given the technology of the day, he was correct.

It would be more than a century before reflecting telescopes were competitive with refractors. The mirrors of the time were made of polished metal which tended to tarnish. Also the tolerances for mirrors is four times more critical than for lenses [_5_] . Why are Mersenne's contributions ignored? It is difficult to find his name in popular histories of the telescope. Mersenne was widely considered to be the inventor of the reflecting telescope in the nineteenth century. Several nineteenth century encyclopedias, including the Encyclopedia Americana , identified Mersenne as the inventor of the reflecting telescope. Others, if they did not credit him with the invention of the reflecting telescope, did mention his contribution to its early development ( see Google Books ). Mersenne fell from grace in the twentieth century. Mersenne was not even mentioned in a 1943 survey of the early development of the telescope by the historical journal, Isis [_6_] . The importance given to a historical figure sometimes depends as much on the dominant biases of the day as on their contributions (see Sarton-A Case for Bias ). M.

Cassegrain - Man without a Name On April 15, 1672 , the Journal de Scravans published an excerpt of a letter from a M. de Berce describing a telescope design proposed by a M. Cassegrain. It had a large (primary) concave mirror which reflected light onto a smaller (secondary) convex mirror which then reflected the light back through a hole in the primary mirror to an eyepiece. The diagram that was used to illustrate the device is shown below. Variations of this type of design would dominate the construction of research telescopes from the start of the twentieth century onward. In spite of the importance of the design, the true identity of this M. Cassegrain would not be known for more than three centuries, when it was discovered in 2000. There was a good reason for this obscurity. By the end of 1672, there would be no-one who would want to be this M. Cassegrain or take ownership of the design. This author has the dubious honour of receiving a very public humiliation for his design from two of the greatest scientists of all time, Isaac Newton and Christian Huygens.

Christian Huygens was especially caustic. Isaac Newton predicted that the design would never be of any use. You see therefore, that the advantages of this design are none, but the disadvantages so great and unavoidable, that I fear it will never be put in practise with good effect [_7_] . Both the great scientists were wrong. The Cassegrain design is the basis for many of the most famous twentieth century telescopes including the Hubble Space Telescope and the 200 inch Hale Telescope on Mt. Palomar. The identity of M. Cassegrain was finally discovered in 2000. He was Laurent Cassegrain, a French Catholic priest from the region around Chartres, France. He was a teacher at a lycee (high school). There is an interesting irony here. A common theme in popular literature is the clash between religion and science. The history of the telescope does not do much to support this. Most of the history of astronomical telescopes has been dominated by either Keplerian refractors or Cassegrainian reflectors. The Cassegrain telescope was proposed by a priest.

The Keplerian telescope was first proposed by Johannes Kepler, but first built by the Catholic priest, Christopher Scheiner. Many of the important early advances in refractors were also contributed by priests (see Timeline of the Telescope ). Copyright Joseph Sant (2016). 16-inch Meade Schmidt-Cassegrain Double Stars 16-inch Meade Schmidt-Cassegrain In 1999 some members feared we were putting an increased strain on our 8¼-inch Clark with all the public open nights. Despite having a 12-inch Meade LX200 and the Patton Telescope, it was thought an additional 16-inch Meade LX200 telescope could be used not only for public nights, but also for digital imaging and internet access. A grant was written, in part, to acquire such an instrument. The request went like this: The second part of this proposal concerns the purchase of a Meade 16-inch computer-controlled telescope. This new instrument will help Skyscrapers to accommodate public observing sessions and groups in a timelier manner.

It would also enable us to provide K-12 science students digital astronomical images, via the Internet, for which our historic Alvan Clark telescope (vintage 1878) in the main observatory was not designed to accomplish. Together these instruments will complement one another, giving the visitor a well-rounded view of the universe. We were quite surprised, but very happy, when we were notified in November 1999 that the scope acquisition had been approved. The 16” Meade telescope was ordered in February 2000, with an expected delivery time of six months. The custom pier arrived in about two months. Well, the scope itself was ready a few months early and it was shipped from Meade on May 8 and received by Skyscrapers soon thereafter. That was the good news. The bad news was, not a single one of us who helped write the grant ever thought about where we were going to put the scope should we get it. In other words, we hadn’t planned on a permanent observatory to house it. But in true Skyscrapers fashion, we rallied and built a 12 foot by 16 foot roll-off roof observatory.

Once again it was entirely constructed by our members. Construction began in July 2000. The basic frame was completed before the end of the year and the onset of cold and snowy weather. The wiring had to wait until the 2001 spring thaw, since the power and computer cables were going to be run underground. By July 6 all the work had been completed and the 16-inch Meade soon settled into its new home. The 16-inch roll-off roof observatory was officially turned over to the members at the October 13, 2001, AstroAssembly.

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Question: Cassegranian, Gregorian and Schmidt are all types of what?
Answer: Telescope