Astronomy fanatica
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Astronomy Picture of the Day
2005 May 26
A Beautiful Trifid
Explanation: The beautiful Trifid Nebula (aka M20), a photogenic study in cosmic contrasts, lies about 5,000 light-years away toward the nebula rich constellation Sagittarius. A star forming region in the plane of our galaxy, the Trifid fittingly illustrates three basic types of astronomical nebulae; red emission nebulae dominated by light from hydrogen atoms, blue reflection nebulae produced by dust reflecting starlight, and dark absorption nebulae where dense dust clouds appear in silhouette. The bright red emission nebula, roughly separated into three parts by obscuring dust lanes, lends the Trifid its popular name. In this gorgeous wide view, the red emission region is also surrounded by the telltale blue haze of reflection nebulae. Light-year long pillars and jets sculpted by newborn stars - visible here below the center of the emission nebula - appear in Hubble Space Telescope close-up images of the region.
Credit & Copyright: Dean Jacobsen -
Robotic telescope
A robotic telescope is an astronomical telescope and detector system that makes observations without the intervention of a human. In astronomical disciplines, a telescope qualifies as robotic if it makes those observations without being operated by a human, even if a human has to initiate the observations at the beginning of the night, or end them in the morning. A robotic telescope is distinct from a remote telescope, though an instrument can be both robotic and remote.
Design
Robotic telescopes are complex systems that typically incorporate a number of subsystems. These subsystems include devices that provide telescope pointing capability, operation of the detector (typically a CCD camera), control of the dome or telescope enclosure, control over the telescope's focuser, detection of weather conditions, and other capabilities. Frequently these varying subsystems are presided over by a master control system, which is almost always a software component.
Robotic telescopes operate under closed loop or open loop principles. In an open loop system, a robotic telescope system points itself and collects its data without inspecting the results of its operations to insure it is operating properly. An open loop telescope is sometimes said to be operating on faith, in that if something goes wrong, there is no way for the control system to detect it and compensate.
A closed loop system has the capability to evaluate its operations through redundant inputs to detect errors. A common such input would be position encoders on the telescope's axes of motion, or the capability of evaluating the system's images to insure it was pointed at the correct field of view when they were exposed.
Most robotic telescopes are small telescopes. While large observatory instruments may be highly automated, few are operated without attendants.
History of professional robotic telescopes
Robotic telescopes were first developed by astronomers after electromechanical interfaces to computers became common at observatories. Early examples were expensive, had limited capabilities, and included a large number of unique subsystems, both in hardware and software. This contributed to a lack of progress in the development of robotic telescopes early in their history.
By the early 1980s, with the availability of cheap computers, several viable robotic telescope projects were conceived, and a few were developed. The 1985 book, Microcomputer Control of Telescopes, by Russel M. Genet and Mark Trueblood, was a landmark engineering study in the field. One of this book's achievements was pointing out many reasons, some quite subtle, why telescopes could not be reliably pointed using only basic astronomical calculations. The concepts explored in this book later became fully articulated in the telescope mount error modeling software called Tpoint.
Since the late 1980s, the University of Iowa has been in the forefront of robotic telescope development on the professional side. The Automated Telescope Facility (ATF), developed in the early 1990s, was located on the roof of the physics building at the University of Iowa in Iowa City. They went on to complete the Iowa Robotic Observatory, a robotic and remote telescope at the private Winer Observatory in 1997. This system successfully observed variable stars and contributed observations to dozens of scientific papers. In May 2002, they completed the Rigel Telescope. Each of these was a progression toward a more automated and utilitarian observatory.
The Lincoln Near-Earth Asteroid Research (LINEAR) Project is another example of a professional robotic telescope. LINEAR's competitors, the Lowell Observatory Near-Earth-Object Search, Catalina Sky Survey, Spacewatch, and others, have also developed varying levels of automation.
In 2004, professional robotic telescopes were characterized by a lack of design creativity and a reliance on closed source and proprietary software. The software is usually unique to the telescope it was designed for and cannot be used on any other system. Often, robotic telescope software becomes impossible to maintain and ultimately obsolete because the graduate students who wrote it move on to new positions, and their institutions lose their knowledge. On the other hand, professional systems generally feature very high observing efficiency and reliability. There is also an increasing tendency to adopt ASCOM technology at professional facilities.
History of amateur robotic telescopes
In 2004, most robotic telescopes are in the hands of amateur astronomers. A prerequisite for the explosion of amateur robotic telescopes was the availability of relatively inexpensive CCD cameras, which appeared on the commercial market in the early 1990s. These cameras not only allowed amateur astronomers to make pleasing images of the night sky, but also encouraged more sophisticated amateurs to pursue research projects in cooperation with professional astronomers. The main motive behind the development of amateur robotic telescopes has been the tedium of making research-oriented astronomical observations, such as taking endlessly repetitive images of a variable star.
In 1998, Bob Denny conceived of a software interface standard for astronomical equipment, based on Microsoft's Component Object Model, which he called the Astronomy Common Object Model (ASCOM). He also wrote and published the first examples of this standard, in the form of commercial telescope control and image analysis programs, and several freeware components. He also convinced Doug George to incorporate ASCOM capability into a commercial camera control software program. Through this technology, a master control system that integrated these applications could easily be written in perl, VBScript, or JavaScript. A sample script of that nature was provided by Denny.
Following coverage of ASCOM in Sky & Telescope magazine several months later, ASCOM architects such as Bob Denny, Doug George, Tim Long, and others later influenced ASCOM into becoming a set of codified interface standards for freeware device drivers for telescopes, CCD cameras, telescope focusers, and astronomical observatory domes. As a result amateur robotic telescopes have become increasingly more sophisticated and reliable, while software costs have plunged. ASCOM has also been adopted for some professional robotic telescopes.
Meanwhile, ASCOM users designed ever more capable master control systems. Papers presented at the Minor Planet Amateur-Professional Workshops (MPAPW) in 1999, 2000, and 2001 and the International Amateur-Professional Photoelectric Photometry Conferences of 1998, 1999, 2000, 2001, 2002, and 2003 documented increasingly sophisticated master control systems. Some of the capabilities of these systems included automatic selection of observing targets, the ability to interrupt observing or rearrange observing schedules for targets of opportunity, automatic selection of guide stars, and sophisticated error detection and correction algorithms. None of these were features of any documented robotic telescope, professional or amateur, prior to 1999.
Significance
By 2004, robotic observations accounted for an overwhelming percentage of the published scientific information on asteroid orbits and discoveries, variable star studies, supernova light curves and discoveries, and comet orbits.
Issue #94 -
Astronomy Picture of the Day
2005 May 27
Titan's Odd Spot
Explanation: Titan's odd spot could be a cloud, but if so, it's a persistent one. Peering into the thick, hazy atmosphere of Saturn's largest moon, cameras on board the Cassini spacecraft found a bright spot at the same location during Titan encounters in 2005 and 2004. Seen near Titan's upper edge in this false-color image from the VIMS instrument, the spot is almost 500 kilometers wide, and is brightest at infrared wavelengths. In addition to suggesting the uniquely colored spot is a persistent cloud possibly controlled by surface features, researchers also entertain the idea that the spot is caused by unusual surface material or extremely tall mountains. They also note the bright infrared spot could be hot. Further clues to the odd spot's nature will come during a planned encounter in July 2006 when Cassini's cameras will look at the spot during Titan's night. If it glows at night, it's hot.
Credit: VIMS Team, U. Arizona, ESA, NASA -
Amateur telescope making
There is a strong tradition of amateur telescope making within the amateur astronomy community.
The classic amateur telescope is the Newtonian reflector with a dobsonian type mount, also known as a dosonian telescope. Amateur telescope makers typically make the most critical parts of a Newtonian-reflector, which are the primary mirror and mount.
Making a telescope is a both fun and technically challenging. For a modest cost, a first class instrument can be constructed. Another reason to grind and figure the primary mirror of a telescope is that it is possible to produce a hand made mirror that is far superior to commercially made mirrors. It is well within the range of any reasonably competent person to produce a primary telescope mirror that is diffraction-limited.
The Newtonian reflector has two reflecting surfaces: the primary mirror (usually parabolic), and a small flat secondary mirror. The primary mirror reflects and focuses incoming parallel light rays back through the tube of the telescope until they are intercepted by a flat secondary mirror set at a 45 degree angle. This flat secondary mirror reflects the light sideways to an eyepiece mounted on the side of the telescope, where it converges at the focal plane.
Mirror making
The mirror is usually ground and polished to a shallow spherical section, and then carefully "figured" to a paraboloid using a special polishing lap and a rotating W shaped stroking motion. (The depth of the mirror's curve will define the focal length of the mirror and hence the f-stop of the telescope. If the focal length of the mirror is long enough, such as f/12, a spherical curve's performance will be nominally equivalent to a parabloid, and the more difficult task of achieving a parabolic shape becomes unnecessary. Also, the longer the focal length, the greater the resulting magnifying power of the primary mirror when used with a given eyepiece, although the field of view will be smaller.)
The shape of the mirror surface is periodically checked with a Foucault tester, this will be described in the mirror testing section.
Mirrors are usually ground from a "mirror-blank" of low-expansion borosilicate glass (Pyrex™ is the brand name). Alternatively, a special ceramic called Cer-vit is used. Cer-vit costs more, but produces mirrors that deform less as the temperature changes.
The mirror-blank is ground against a "tool," which is another piece of glass, either a thick piece of window glass or another mirror blank. An abrasive such as silicon carbide mixed with water is used between the mirror and tool. The tool is usually placed in a frame on a barrel in order to provide access from all sides.
To grind the hollow in the mirror, the mirror maker strokes the mirror-blank back and forth across the tool. Each stroke grinds the abrasive against the blank and tool. The mirror maker takes a step around the barrel every 10 or 30 seconds or so to average out surface errors. This ensures that the shape of the mirror grinds to a perfect concave spherical surface. Fresh abrasives and water are added as required.
During grinding, the piece of glass on top becomes concave as the piece of glass on the bottom becomes convex. The mirror is rough-ground using coarse abrasive until the curve begins to approach the desired depth (or radius).
The depth of focus is checked by wetting the mirror's surface, and seeing where a light's image focuses against a cardboard card
The same basic step is repeated, using successively finer abrasives. Silicon carbide is typically used from 60 down to about 500 grit, after which Aluminum oxide is used. Fine grinding to a 3 micrometre size abrasive will greatly speed up the polishing step.
It is important to clean the system carefully when reducing grit sizes to prevent scratching from the previous size abrasive. It is also important to periodically check the focal length of the mirror during the grinding process. The curve of the mirror will continue to deepen as long as the mirror is on top. If the curve becomes too deep, the system is flipped over, and grinding continues with the tool on top. This will cause the curve to become shallower.
After fine grinding is done, a polishing or "pitch" lap is made from the tool. A pitch compound is heated in a double boiler until it becomes liquid. This compound is poured over the mirror, and the tool is pressed on top of the tool and pitch so that the lap will take on the exact shape of the mirror. It is important to coat the mirror with rouge before this step to prevent the lap from sticking to the mirror. After the lap and mirror are separated, the lap has channels cut in it to let water and abrasives run off. Alternatively a rubber mold can be used when the lap is poured, to make the channels.
Then, using the lap, one begins to polish the mirror using rouge or cerium(IV) oxide. Polishing is very similar to grinding, except that the resistace between the mirror and the lap is much higher. Cleats must be fastened to the work surface to keep the lap and mirror from sliding. The scratches of the rouge are smaller than a wavelength of light, and the mirror thus becomes a specular (mirror-like) reflector.
Mirror testing
Foucault test
The basic trick to making a good mirror is to measure it, and then fix its problems by altering the lap to polish the problems away. The foucault test is the traditional mirror test for amateur mirror makers. It is also the least subjective of tests available to amateurs.
Although it is possible to test the unfinished mirror by putting it in a telescope assembly and making a star test, most mirror makers construct a simple device known as a Foucault tester.
Foucault test setup to measure a mirror
The Foucault tester consists of a pinhole light source and a vertical knife edge which is mounted on micrometers or home-made screw adjusters. The knife-edge is mounted at the focus. The light illuminates the mirror, and the user looks at the mirror past the knife-edge.
Foucault test appearance depending on knife-edge position
After the mirror is polished out, the mirror is placed vertically in a stand. The Foucault tester is set up at a distance close to the mirror focal point The tester is adjusted so that the returning beam from the pinhole light source is interrupted by the knife edge.
Viewing the mirror from behind the knife edge will show a pattern on the mirror surface. If the mirror surface is a perfect sphere, the mirror will appear evenly lit across the entire surface. If the surface is parabolic, the mirror will look like a donut or lozenge. Imperfections as small as a half-wavelength high (a quarter of a micrometer) show as flat spots or bumps.
It is possible to calculate how closely the mirror surface resembles a perfect paraboloid by placing a special mask over the mirror and taking a series of measurement with the tester. This data is then reduced and graphed against an ideal parabolic curve.
Issue #95 -
Astronomy Picture of the Day
2005 May 28
Himalayan Horizon From Space
Explanation: This stunning aerial view shows the rugged snow covered peaks of a Himalayan mountain range in Nepal. The seventh-highest peak on the planet, Dhaulagiri, is the high point on the horizon at the left while in the foreground lies the southern Tibetan Plateau of China. But, contrary to appearances, this picture wasn't taken from an airliner cruising at 30,000 feet. Instead it was taken with a 35 mm camera and telephoto lens by the Expedition 1 crew aboard the International Space Station -- orbiting 200 nautical miles above the Earth. The Himalayan mountains were created by crustal plate tectonics on planet Earth some 70 million years ago, as the Indian plate began a collision with the Eurasian plate. Himalayan uplift still continues today at a rate of a few millimeters per year.
Credit: Expedition 1, ISS, EOL NASA -
Amateur telescope making - Cont'd
Mirror testing - Cont'd
Matching Ronchi test
Ronchi testing is similar to the foucault test in set-up. A light source is emitted through a diffraction grating, reflected by the mirror, then passes through the refraction grating again and observed by the person doing the test.
The result is a pattern of interference that reveals the shape of the mirror. The interference is compared to a computer generated diagram of what the mirror should look like. Inputs to the program are line frequency of the diffraction grating, focal length and diameter of the mirror.
The ronchi test is much faster to set up than the foucault test but more subjective. It offers a quick glimpse at the mirrors shape and condition, and can quickly identify a 'turned edge' (rolled down outer diameter of the mirror).
Star test
Star testing tests the entire optical system of the telescope. It is a very subjective test that requires a high degree of skill to interpret, but is considered to be the best test available to amateurs.
A star is brought into field under high magnification and then observed inside and outside of focus. Diffraction rings appear around the star. The shape and brightness of the diffraction rings are analyzed to determine the quality of the mirror.
For primary mirror testing, all optical components are assumed to be perfect and the results are applied to the primary mirror only. In a cassigrainian telescope, for example, the secondary may be complete and the primary can be tested and adjusted to correct for errors in both the primary and the secondary.
Figuring the mirror
The true "art" of mirror making is in shaping the final curve or "figure" of the mirror. The figuring process begins after the mirror is fine ground and polished. The polishing lap continues to be used in this process.
The first goal in figuring a mirror is to obtain a perfectly spherical section. If the mirror is spherical, the surface of the mirror will appear to be evenly lit when inspected with the Foucault tester. The rotating "W" stroke described above will tend to bring the surface to a spherical shape.
It is important to allow the mirror to thermally stabilize or "cool down" after each figuring session. The friction of the lap against the mirror will cause the glass to expand slightly and will affect the figure.
The parabolic curve is obtained by slightly deepening the curve in the center of the spherical mirror. This is done by changing the "W" stroke so that more time is spent with the lap against the center of the mirror. It can take less than a minute of work to turn the spherical surface to a paraboloid, however the process is not exact and may need to be repeated numerous times. The shape of the curve can also be manipulated by flipping the mirror/lap over, or by using a small sub-diameter lap.
The Foucault tester and testing mask are used to obtain data used to calculate how closely the mirror matches a parabolic curve. Visually, the mirror surface will appear to have a slight circularly-symmetric donut appearance when viewed with the Foucault tester.
Aluminizing or "Silvering" the mirror
Although the finished mirror will work in the telescope without a reflective coating, the image will be very dim. So, a very thin coating of a highly reflective material is added to the front surface of the mirror.
Historically this coating was silver. Silvering was put on the mirror chemically. This was then polished. Silvering was typically done by the mirror maker.
Since the 1950s most mirror makers have the coating applied by a firm specializing in the work. Modern coatings usually contain Aluminum and other compounds.
The mirror is aluminized by placing it in a vacuum chamber with electrically-heated nichrome coils that can sublime aluminum. In a vacuum, the hot aluminum atoms travel in straight lines. When they hit the surface of the mirror, they cool and stick. Some mirror makers evaporate a layer of quartz on the mirror, others expose it to pure oxygen or air in an oven so that it will form a tough, clear layer of aluminum oxide.
Telescope design
The most common telescope design for the amateur telescope maker is the Dobsonian Telescope. The dobsonian very simple and easy to make. A Dobsonian telescope can be optimized for both planetary and deep sky observing. It is not suitable for astrophotography because it does not have the ability to automatically track the sky.
First, the amateur decides what size to construct. The difficulty of construction grows roughly as the square of the diameter of the mirror. A 4 inch (100 mm) mirror is a moderately easy science fair project. An 8 inch (200 mm) mirror is a good compromise between ease and constructing an instrument that would be expensive to purchase. A 12 inch (300 mm) mirror is difficult, and a telescope over 24 inch (600 mm) usually must be ground and lapped with mechanical assistance. Amateurs have constructed telescopes as large as 1 metre across (39 inches), but this is foolhardy for anyone other than the best-funded, experienced clubs.
Telescope Construction
Tube
Once the mirror is done, it is mounted in a mechanical tube. The idea is to maintain the optical alignment between the mirrors and eyepiece.
In a Newtonian telescope, the primary mirror is located at the bottom of the tube. The small secondary mirror is suspended in the middle of the tube at the top using a low-profile mount (called a spider). The eyepiece is aimed at the secondary mirror on the spider, from outside of the tube.
Alignment is achieved by adjustment. The secondary mirror's position and tilt must be adjusted. The primary mirror must be tilted to focus through the secondary on the eyepiece.
Circular disks called altitude bearings are attached to the side of the tube at its center of gravity so it can tilt on its mount. Usually a small finder telescope is attached to the tube to aid pointing.
Mount
In a Dobsonian mount, the altitude bearings rest on pads of teflon. This provides very little friction so that the telescope can be moved very small angles without jerking. These pads sit on the rocker box, which itself rotates in the azimuth on pads of teflon.
A dobsonian mount can be adjusted by computerized motors to track the stars, but the star field rotates, making the field of view useless for photography. It's theoretically possible to correct this with a rotating erection prism, but most people opt for a standard equatorial mount.
For astrophotography, an amateur equatorial mount is usually a "T" shape made of pipe, with roller bearings around the stem and top of the "T." The telescope tube is attached to one side of the T, with a counterweight on the other side to balance the weight of the telescope. The fixed axis (the stem of the "T," the one closest to the ground) is aimed at the pole-star, parallel to the axis of the Earth. In this way, moving the telescope to counter the rotation of the Earth requires movement only on one axis, the bearing wrapped around the stem of the "T."
Another form of equatorial mount is a two-tined fork. There are three bearings. One is wrapped around the "handle" of the fork. The handle of the fork is parallel with the axis of the Earth. The other two bearings are on the tines, and support the telescope tube from two sides. This mount is very popular for small professional telescopes because it weighs less (no counterweight) and since it gives better support, the tube and optics distort less. It's less popular with amateurs because it has three bearings, of two sizes, and it can be difficult to align the two secondary bearings.
Some amateurs construct setting circles on their mounts, or use motors that can move by very precise amounts. These let the amateur "dial in" astronomical objects by coordinate. A few amateurs have even constructed precision setting circles, and performed astrometry, measuring angles to nearby stars!
Issue #96 -
Astronomy Picture of the Day
2005 May 29
The Sagittarius Dwarf Tidal Stream
Explanation: Is our Milky Way Galaxy out to lunch? Recent wide field images and analyses now indicate that our home galaxy is actually still in the process of devouring one of its closer satellite neighbors. This unfortunate neighbor, the Sagittarius Dwarf galaxy, is now seen to be part of a larger Sagittarius Tidal Stream, a loose filament of stars, gas, and possibly dark matter that entangles the Milky Way. An artist's depiction of the stream is shown above. Speculation also holds that the Sagittarius Dwarf was once pulled through the Milky Way disk very close to our Sun's current location. An important resulting realization is that galaxies contain a jumble of clumps and filaments of both dim and dark matter.
Drawing Credit & Copyright: David Martinez-Delgado (MPIA) & Gabriel Perez (IAC) -
Aperture synthesis
Aperture synthesis is a type of interferometry that mixes signals from a collection of instruments to produce measurements having the same angular resolution as an instrument the size of the entire collection. At each separation and orientation, the lobe-pattern of the interferometer produces an output which is one component of the Fourier transform of the spatial distribution of the brightness of the observed object. In order to produce a high quality image, a large number of different separations between different telescopes are required (the projected separation between any two telescopes as seen from the radio source is called a baseline) - as many different baselines as possible are required in order to get good quality results.
Most aperture synthesis interferometers use the rotation of the Earth to increase the number of different baselines included in an observation (see diagram below). Taking data at different times provides measurements with different telescope separations and angles without the need for buying additional telescopes or moving the telescopes manually (the rotation of the Earth moves the telescopes for you!). Some instruments use artificial rotation of the interferometer array instead of Earth rotation, such as in Aperture Masking Interferometry.
Most aperture synthesis interferometers use the rotation of the Earth to increase the number of baseline orientations included in an observation. In this example with the Earth represented as a grey sphere, the baseline between telescope A and telescope B changes angle with time as viewed from the radio source as the Earth rotates. Taking data at different times thus provides measurements with different telescope separations.
This process is used in radio astronomy, where it was first developed by Martin Ryle and coworkers from the Cavendish Laboratory, Cambridge University. Martin Ryle and Tony Hewish jointly received a Nobel Prize for this and other contributions to the development of radio interferometry. The radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s. During the late 1960s and early 70s, as computers (such as the Titan) became available capable of handling the computationally-intensive Fourier Transform inversions required, they used aperture synthesis to synthesis first a 'One-Mile' and later a '5 km' effective aperture using the One-Mile and Ryle telescopes respectively. The technique was subsequently further developed in Very Long Baseline Interferometry to obtain baselines of thousands of km. Aperture synthesis is also used by a type of radar system known as synthetic aperture radar, and even in optical telescopes.
Issue #97 -
Astronomy Picture of the Day
2005 May 30
A Great White Spot on Rhea
Explanation: Why caused this great white spot on the surface of Saturn's moon Rhea? The spot was first noticed last year by the robot Cassini spacecraft now orbiting Saturn. Cassini's flyby of Rhea in April imaged in the spot in great detail. Astronomers hypothesize that the light-colored spot is the result of a relatively recent impact on the surface of the icy moon. The impact that likely created the crater also splashed light-colored material from the interior onto the darker surface. Rhea spans 1,500 kilometers across and is the second largest moon of Saturn after Titan. Rhea sports several other light colored surface features that are, as yet, not well understood.
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA -
History of telescopes
The credit for the invention of the telescope has been a subject of discussion. Thus, because Democritus announced that the Milky Way is composed of vast multitudes of stars, it has been maintained by some that he could only have been led to form such an opinion from actual examination with a telescope. Other passages from the Greek and Latin authors have similarly been cited to prove that the telescope was known to the ancients. But we are no more warranted in drawing such a conclusion without any evidence other than casual remarks, however sagacious, than we should be justified in stating that Seneca was in possession of the theories of Newton because he predicted that comets would one day be found to revolve in periodic orbits.
Refracting telescopes
There is an archeological finding of lenses is from Gotlandia in Sweden. These so-called Visby lenses can be dated to the second half of the 11th century. The one half is near a perfect ellipsoid and the other flat, making a perfect tool for handling light beams. Some of these lenses have a silver mounting and have been used as pendants. There are also unmounted lenses that may have been used as a loupe. They have been speculated to be components from an ancient telescope. However, if it would be, maybe it was something imported from the Middle East and it was anyway forgotten until about 1600.
The earliest documented telescope was that of Roger Bacon in the 13th century. It is quite certain that prior to 1600 the telescope was unknown, except to individuals who failed to see its practical importance, and who confined its usage to curious practices or to demonstrations of "natural magic."
The practical invention of the instrument was certainly made in the Netherlands about 1608, but the credit of the original invention has been claimed on behalf of three individuals, Hans Lippershey and Zacharias Jansen, spectacle-makers in Middelburg, and James Metius of Alkmaar.
The original Dutch telescopes were composed of a convex and a concave lens, and telescopes so constructed do not invert the image. Telescopes seem to have been made in the Netherlands in considerable numbers soon after the date of their invention, and rapidly found their way over Europe.
Galileo, happening to be in Venice in about the month of May 1609, heard that a Belgian had invented a perspective instrument by means of which distant objects appeared nearer and larger, and that he discovered its construction by considering the effects of refraction. Galileo states that he solved the problem of the construction of a telescope the first night after his return to Padua from Venice, and made his first telescope the next day by fitting a convex lens in one extremity of a leaden tube and a concave lens in the other one. A few days afterwards, having succeeded in making a better telescope than the first, he took it to Venice, where he communicated the details of his invention to the public, and presented the instrument itself to the doge Leonardo Donato, sitting in full council. The senate, in return, settled him for life in his lectureship at Padua and doubled his salary. Galileo may thus claim to have invented the telescope independently, but not till he had heard that others had done so.
Galileo devoted his time to improving and perfecting the telescope, and soon succeeded in producing telescopes of greatly increased power. His first telescope magnified three diameters; but he soon made instruments which magnified eight diameters, and finally one that magnified thirty-three diameters. With this last instrument he discovered in 1610 the satellites of Jupiter, and soon, afterwards the spots on the sun, the phases of Venus, and the hills and valleys on the Moon. He demonstrated the revolution of the satellites of Jupiter around the planet, and gave rough predictions of their configurations, proved the rotation of the Sun on its axis, established the general truth of the Copernican system as compared with that of Ptolemy, and fairly routed the fanciful dogmas of the philosophers. These brilliant achievements, together with the immense improvement of the instrument under the hands of Galileo, overshadowed in a great degree the credit due to the original inventor, and led to the universal adoption of the name of the Galilean telescope for the form of the instrument invented by Lippershey.
Johannes Kepler first explained the theory and some of the practical advantages of a telescope constructed of two convex lenses in his Catopirics (1611). The first person who actually constructed a telescope of this form was the Jesuit Christoph Scheiner, who gives a description of it in his Rosa Ursina (1630).
William Gascoigne was the first who practically appreciated the chief advantages of the form of telescope suggested by Kepler, viz., the visibility of the image of a distant object simultaneously with that of a small material object placed in the common focus of the two lenses. This led to his invention of the micrometer and his application of telescopic sights to astronomical instruments of precision. But it was not till about the middle of the 17th century that Kepler's telescope came into general use, and then, not so much because of the advantages pointed out by Gascoigne, but because its field of view was much larger than in the Galilean telescope.
The first powerful telescopes of this construction were made by Christiaan Huygens, after much labour, in which he was assisted by his brother. With one of these, of 12-ft. focal length, he discovered the brightest of Saturn's satellites (Titan) in 1655, and in 1659 he published his Systema Saturnium, in which was given for the first time a true explanation of Saturn's ring, founded on observations made with the same instrument. The sharpness of image in Kepler's telescope is very inferior to that of the Galilean instrument, so that when a high magnifying power is required it becomes essential to increase the focal length.
Giovanni Cassini discovered Saturn's fifth satellite (Rhea) In 1672 with a telescope of 35 ft., and the third and fourth satellites in 1684 with telescopes made by Campani of 100- and 136-foot focal length. Christian Huygens states that he and his brother made object-glasses of 170 and 210 ft. focal length, and he presented one of 123 feet to the Royal Society of London. Adrien Auzout (died in 1691) and others are said to have made telescopes of from 300 to 600 ft. locus, but it does not appear that they were ever able to use them in practical observations. James Bradley, on December 27, 1722, actually measured the diameter of Venus with a telescope whose object glass had a focal length of 212 ft. In these very long telescopes no tube was employed, and they were consequently termed aerial telescopes. Huygens contrived some ingenious arrangements for directing such telescopes towards any object visible in the heavens-the focal adjustment and centring of the eyepiece being preserved by a braced rod connecting the object glass and eyepiece. Other contrivances for the same purpose are described by Philippe de la Hire (Mém. de l'Acad., 1715) and by Nicolaus Hartsoeker (Miscel. Berol., 1710, vol. i. p. 261). Telescopes of such great length were naturally difficult to use, and must have taxed to the utmost the skill and patience of the observers. One cannot but pay a passing tribute of admiration to the men who, with such troublesome tools, achieved such results.
Issue #98 -
Astronomy Picture of the Day
2005 May 31
The Trifid Nebula from CFHT
Explanation: Unspeakable beauty and unimaginable bedlam can be found together in the Trifid Nebula. Also known as M20, this photogenic nebula is visible with good binoculars towards the constellation of Sagittarius. The energetic processes of star formation create not only the colors but the chaos. The red-glowing gas results from high-energy starlight striking interstellar hydrogen gas. The dark dust filaments that lace M20 were created in the atmospheres of cool giant stars and in the debris from supernovae explosions. Which bright young stars light up the blue reflection nebula is still being investigated. The light from M20 we see today left perhaps 3,000 years ago, although the exact distance remains unknown. Light takes about 50 years to cross M20.
Credit & Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT -
History of telescopes - Cont'd
Reflecting telescopes
Until Newton's discovery of the different refrangibility of light of different colours, it was generally supposed that object-glasses of telescopes were subject to no other errors than those which arose from the spherical figure of their surfaces, and the efforts of opticians were chiefly directed to the construction of lenses of other forms of curvature. James Gregory, in his Optica Promota (1663), discusses the forms of images and objects produced by lenses and mirrors, and shows that when the surfaces of the lenses or mirrors are portions of spheres the images are curves concave towards the objective, but if the curves of the surfaces are conic sections, the spherical aberration is corrected. He was well aware of the failures of all attempts to perfect telescopes by employing lenses of various forms of curvature, and accordingly proposed the form of reflecting telescope which bears his name: the Gregorian telescope. But Gregory, according to his own confession, had no practical skill; he could find no optician capable of realizing his ideas, and after some fruitless attempts was obliged to abandon all hope of bringing his telescope into practical use. Newton was the first to construct a reflecting telescope.
When in 1666 he made his discovery of the different refrangibility of light of different colours, he soon perceived that the faults of the refracting telescope were due much more to this cause than to the spherical figure of the lenses. He overhastily concluded from some rough experiments (Optics, bk. i. pt. ii. prop. 3) that all refracting substances diverged the prismatic colours in a constant proportion to their mean refraction; and he drew the natural conclusion that refraction could not be produced without colour, and therefore that no improvement could he expected from the refracting telescope (Treatise on Optics, p. 112). But, having ascertained by experiment that for all colours of light the angle of incidence is equal to the angle of reflexion, he turned his attention to the construction of reflecting telescopes. After much experiment he selected an alloy of tin and copper as the most suitable material for his specula, and he devised means for griding and polishing them. He did not attempt the formation of a parabolic figure on account of the probable mechanical difficulties, and he had besides satisfied himself that the chromatic and not the spherical aberration formed the chief faults of previous telescopes. Newton's first telescope so far realized his expectations that he could see with its aid the satellites of Jupiter and the horns of Venus. Encouraged by this success, he made a second telescope of 6k-in. focal length, with a magnifying power of 38 diameters, which he presented to the Royal Society of London in December 1671. A third form of reflecting telescope was devised in 1672 by Cassegrain (Journal des Savants, 1672). No further practical advance appears to have been made in the design or construction of the instrument till the year 1723, when John Hadley (best known as the inventor of the sextant) presented to the Royal Society a reflecting telescope of the Newtonian construction, with a metallic speculum of 6-in. aperture and 623/4-in, focal length, having eyepieces magnifying up to 230 diameters. The instrument was examined by Pound and Bradley, the former of whom reported upon it in Phil, Trans., 1723, No. 378, p. 382. After remarking that Newton's telescope had lain neglected these fifty years, they stated that Hadley had sufficiently shown that this noble invention does not consist in bare theory. They compared its performance with that of the object-glass of 123-ft. focal length presented to the Royal Society by Huygens, and found that Hadley's reflector will bear such a charge as to make it magnify the object as many times as the latter with its due charge, and that it represents objects as distinct, though not altogether so clear and bright.
Bradley and Molyneux, having been instructed by Hadley in his methods of polishing specula, succeeded in producing some telescopes of considerable power, one of which had a focal length of 8 ft.; and, Molyneux having communicated these methods to Scarlet and Hearn, two London opticians, the manufacture of telescopes as a matter of business was commenced by them (Smith's Opticks, bk, iii. ch. I). But it was reserved for James Short of Edinburgh to give practical effect to Gregory's original idea. Born at Edinburgh in 1710 and originally educated for the church, Short attracted the attention of Maclaurin, professor of mathematics at the university, who permitted him about 1732 to make use of his rooms in the college buildings for experiments in the construction of telescopes. In Short's first telescopes the specula were of glass, as suggested by Gregory, but he afterwards used metallic specula only, and succeeded in giving to them true parabolic and elliptic figures. Short then adopted telescope-making as his profession, which he practised first in Edinburgh and afterwards in London. All Short's telescopes were of the Gregorian form, and some of them retain even to the present day their original high polish and sharp definition. Short died in London in 1768, having realized a considerable fortune by the exercise of his profession.
Issue #99 -
Astronomy Picture of the Day
2005 June 1
White Dwarf Star Spiral
Explanation: About 1,600 light-years away, in a binary star system fondly known as J0806, two dense white dwarf stars orbit each other once every 321 seconds. Interpreting x-ray data from the Chandra Observatory astronomers argue that the stars' already impressively short orbital period is steadily getting shorter as the stars spiral closer together. Even though they are separated by about 80,000 kilometers (the Earth-Moon distance is 400,000 kilometers) the two stars are therefore destined to merge. Depicted in this artist's vision, the death spiral of the remarkable J0806 system is a consequence of Einstein's theory of General Relativity that predicts the white dwarf stars will lose their orbital energy by generating gravity waves. In fact, J0806 could be one of the brightest sources of gravitational waves in our galaxy, directly detectable by future space-based gravity wave instruments.
Credit: Tod Strohmayer (GSFC), CXC, NASA - Illustration: D. Berry (GSFC) -
History of telescopes - Cont'd
Achromatic Telescope
The historical sequence of events now brings us to the discovery of the achromatic telescope. The first person who succeeded in making achromatic refracting telescopes seems to have been Chester Moor Hall, a gentleman of Essex.
He argued that the different humours of the human eye so refract rays of light as to produce an image on the retina which is free from colour, and he reasonably argued that it might be possible to produce a like result by combining lenses composed of different refracting media. After devoting some time to the inquiry he found that by combining lenses formed of different kinds of glass the effect of the unequal refrangibility of light was corrected, and in 1733 he succeeded in constructing telescopes which exhibited objects free from colour. One of these instruments of only 20-in. focal length had an aperture of 21/2 in, Hall was a man of independent means, and seems to have been careless of fame; at least he took no trouble to communicate his invention to the world. At a trial in Westminster Hall about the patent rights granted to John Dollond (Watkin v. Dollond), 1 Hall was admitted to be the first inventor of the achromatic telescope; but it was ruled by Lord Mansfield that it was not the person who locked his invention in his scrutoire that ought to profit for such invention, but he who brought it forth for the benefit of mankind. 3 In 1747 Leonhard Euler communicated to the Berlin Academy of Sciences a memoir in which he endeavoured to prove the possibility of correcting both the chromatic and the spherical aberration of an object-glass. Like Gregory and Hall, he argued that, since the various humours of the human eye were so combined as to produce a perfect image, it should be possible by suitable combinations of lenses of different refracting media to construct a perfect object-glass. Adopting a hypothetical law of the dispersion of differently coloured rays of light, he proved analytically the possibility of constructing an achromatic object-glass composed of lenses of glass and water. But all his efforts to produce an actual objectglass of this construction were fruitless-a failure which he attributed solely to the difficulty of procuring lenses worked precisely to the requisite curves (Mem. Acad. Berlin, 1753). Dollond admitted the accuracy of Euler's analysis, but disputed his hypothesis on the grounds that it was purely a theoretical assumption, that the theory was opposed to the results of Newton's experiments on the refrangibility of light, and that it was impossible to determine a physical law from analytical reasoning alone (Phil. Trans., 1753, p. 289). In 1754 Euler communicated to the Berlin Academy a further memoir, in which, starting from the hypothesis that light consists of vibrations excited in an elastic fluid by luminous bodies, and that the difference of colour of light is due to the greater or less frequency of these vibrations in a given time, he deduced his previous results. He did not doubt the accuracy of Newton's experiments quoted by Dollond, because he asserted that the difference between the law deduced by Newton and that which he assumed would not be rendered sensible by such an experiment,
4 Dollond did not reply to this memoir, but soon afterwards he received an abstract of a memoir by Samuel Klingenstierna, the Swedish mathematician and astronomer, which led him to doubt the accuracy of the results deduced by Newton on the dispersion of refracted light. Klingenstierna showed from purely geometrical considerations, fully appreciated by Dollond, that the results of Newton's experiments could not be brought into harmony with other universally accepted facts of refraction. Like a practical man, Dollond at once put his doubts to the test of experiment, confirmed the conclusions of Klingenstierna, discovered a difference far beyond his hopes in the refractive qualities of different kinds of glass with respect to their divergency of colours, and was thus rapidly led to the construction of object-glasses in which first the chromatic and afterwards the spherical aberration were corrected (Phil. Trans., 1758, p. 733).
We have thus followed somewhat minutely the history of the gradual process by which Dollond arrived independently at his invention of the refracting telescope, because it has been asserted that he borrowed the idea from others. Montucla, given for his invention, was dead, and his son brought an action for infringing the patent against Chainpness. There is no report of the case, but the facts are referred to in the reports of subsequent cases, It appears that workmen who had been employed by Mr Moor Hall were examined, and proved that they had made achromatic object-glasses as early as 1733. Dollond's patent was not set aside, though the evidence with regard to the prior manufacture was accepted by Lord Mansfield, who tried the case, as having been satisfactorily proved.
It is clearly established that Hall was the first inventor of the achromatic telescope; but Dollond did not borrow the invention from Hall without acknowledgment, in the manner suggested by Lalande. His discovery was beyond question an independent one. The whole history of his researches proves how fully he was aware of the conditions necessary for the attainment of achromatism in refracting telescopes, and he may be well excused if he so long placed implicit reliance on the accuracy of experiments made by so illustrious a philosopher as Newton. His writings sufficiently show that but for this confidence he would have arrived sooner at a discovery for which his mind was fully prepared. It is, besides, impossible to read Dollond's memoir (Phil. Trans., 1758, p. 733) without being impressed with the fact that it is a truthful account, not only of the successive steps by which he independently arrived at his discovery, but also of the logical processes by which these steps were successively suggested to his mind.
The triple object-glass, consisting of a combination of two convex lenses of crown glass with a concave flint lens between them, was introduced in 1765 by Peter, son of, John Dollond, and many excellent telescopes of this kind were made by him.
The limits of this article do not permit a further detailed historical statement of the various steps by which the powers of the telescope were developed. Indeed, in its practical form the principle of the instrument has remained unchanged from the time of the Dollonds to the present day; and the history of its development may be summed up as consisting not in new optical discoveries but in utilizing new appliances for figuring and polishing, improved material for specula and lenses, more refined means of testing, and more perfect and convenient methods of mounting.
About the year 1774 William Herschel, then a teacher of music in Bath, began to occupy his leisure hours with the construction of specula, and finally devoted himself entirely to their construction and use. In 1778 he had selected the chef-d'oeuvre of some 400 specula which he made for the celebrated instrument of 7-ft. focal length with which his early brilliant astronomical discoveries were made. In 1783 he completed his reflector of 184 in. aperture and 20-ft. focus, and in 1789 his great reflector of 4-ft. aperture and 40-ft. focal length. The fame of these instruments was rapidly spread by the brilliant discoveries which their maker's genius and perseverance accomplished by their aid. The reflecting telescope became the only available tool of the astronomer when great light grasp was requisite, as the difficulty of procuring disks of glass (especially of flint glass) of suitable purity and homogeneity limited the dimensions of the achromatic telescope. It was in vain that the French Academy of Sciences offered prizes for perfect disks of optical flint glass. Some of the best chemists and most enterprising glass-manufacturers exerted their utmost efforts without succeeding in producing perfect disks of more than 31/2 in. in diameter. All the large disks were crossed by striae, or were otherwise deficient in the necessary homogeneity and purity.
Issue #100 -
Astronomy Picture of the Day
2005 June 2
Sculpting the South Pillar
Explanation: Eta Carinae, one of the most massive and unstable stars in the Milky Way Galaxy, has a profound affect on its environment. Found in the the South Pillar region of the Carina Nebula, these fantastic pillars of glowing dust and gas with embedded newborn stars were sculpted by the intense wind and radiation from Eta Carinae and other massive stars. Glowing brightly in planet Earth's southern sky, the expansive Eta Carinae Nebula is a mere 10,000 light-years distant. Still, this remarkable cosmic vista is largely obscured by nebular dust and only revealed here in penetrating infrared light by the Spitzer Space Telescope. Eta Carinae itself is off the top left of the false-color image, with the bright-tipped dust pillars pointing suggestively toward the massive star's position. The Spitzer image spans almost 200 light-years at the distance of Eta Carinae.
Credit: Nathan Smith (Univ. of Colorado), et al., SSC, JPL, Caltech, NASA -
Canberra Deep Space Communications Complex
The Canberra Deep Space Communications Complex (CDSCC) is located in Australia at Tidbinbilla in a valley of the Murrumbidgee River, about half an hours drive out of Canberra in the Australian Capital Territory, accessible from either the northern or southern suburbs from the Paddy's River Road. See map below: the Tidbinbilla Nature Park is marked by the star, with the station in the valley mid-way towards the Weston Creek (district) suburban area.
Tidbinbilla Nature Reserve Locality Map
It is part of the worldwide Deep Space Network run by NASA's Jet Propulsion Laboratory (JPL). The complex is commonly referred to as the Tidbinbilla Deep Space Tracking Station and was officially opened on the 19th of March 1965 by the then Prime Minister of Australia Sir Robert Menzies.
As at March 2005 there are four antennas in use, one 26-metre, two 34-metre and one 70-metre, which is the largest steerable parabolic antenna in the Southern Hemisphere. The CDSCC also uses the Parkes radio telescope in central New South Wales at busy times to receive data from spacecraft.
70 m telescope at the Canberra Deep Space Communications Complex
The station is separated from Canberra by the Murrumbidgee River, but most notably by the Coolamon Ridge and Urambi Hills that help shield the city's radio frequency (RF) noise from the dishes. Located nearby is the Tidbinbilla Nature Reserve.
The CSIRO manages most of NASA's activities in Australia. Since March 2003, Raytheon Australia has managed the CDSCC on behalf of the CSIRO and NASA.
The station's collimation tower is some 3 km to the north-west, on Black Hill.
Issue #101 -
Astronomy Picture of the Day
2005 June 3
M27: The Dumbbell Nebula
Explanation: The first hint of what will become of our Sun was discovered inadvertently in 1764. At that time, Charles Messier was compiling a list of diffuse objects not to be confused with comets. The 27th object on Messier's list, now known as M27 or the Dumbbell Nebula, is a planetary nebula, the type of nebula our Sun will produce when nuclear fusion stops in its core. M27 is one of the brightest planetary nebulae on the sky, and can be seen in the constellation Vulpecula with binoculars. It takes light about 1000 years to reach us from M27, shown above, digitally sharpened, in three standard colors. Understanding the physics and significance of M27 was well beyond 18th century science. Even today, many things remain mysterious about bipolar planetary nebula like M27, including the physical mechanism that expels a low-mass star's gaseous outer-envelope, leaving an X-ray hot white dwarf.
Credit & Copyright: Joe & Gail Metcalf, Adam Block, NOAO, AURA, NSF -
Effelsberg 100-m radio telescope
Since its inauguration in 1972, the Effelsberg 100-m radio telescope is still, to the present day, one of the world's largest fully steerable telescopes. It operates at wavelengths from about 7 mm to 90 cm. The telescope is operated by the Max Planck Institut für Radioastronomie in Bonn, Germany. The Effelsberg 100-m telescope was involved in several such surveys, including the one, to name just one example, at 408 MHz (73 cm) by Haslam et al. (1981, 1982; Astron. and Astrophys.).
Effelsberg 100-metre radio telescope, Germany
General Information
Organization: Max Planck Institute for Radio Astronomy in Bonn, Max-Planck-Gesellschaft
Location: Effelsberg, North Rhine-Westphalia
Wavelength regime: radio 408MHz-86GHz
Completion date: 1971
Webpage
Physical characteristics
Telescope style: parabolic reflector
Diameter: 100 m
Collecting area: 31,000 m²
Mounting: alt-azimuth fully steerable primary
Dome: none
Issue #102 -
Astronomy Picture of the Day
2005 June 4
First U.S. Spacewalk
Explanation: In 1965, forty years ago on June 3rd, astronaut Edward White made the first U.S. spacewalk. Tethered to his Gemini IV capsule, White is pictured above holding a compressed gas "zip gun" for maneuvers in his right hand. His spacewalk began over the Pacific Ocean near Hawaii and ended 23 minutes later above the Gulf of Mexico. Of course, the term spacewalk is a bit deceiving as White was falling freely in low earth orbit alongside his capsule manned by fellow astronaut James McDivitt. In free-fall, White was able to control his motions by firing bursts from his gun until its supply of compressed gas ran out. He ultimately returned, exhausted, to the two-man Gemini capsule.
Credit: Gemini 4 Mission, NASA -
Five College Radio Astronomy Observatory
The Five College Radio Astronomical Observatory (FCRAO) was founded in 1969 by the Five College Astronomy Department (University of Massachusetts, Amherst College, Hampshire College, Mount Holyoke College and Smith College). From its inception, the observatory has emphasized research, the development of technology and the training of students—both graduate and undergraduate.
FCRAO Radome-enclosed 14-m Telescope
The initial FCRAO telescope was a customized low-frequency antenna to search for pulsars in the galaxy. The development of instrumentation within the FCRAO labs contributed to the discovery of the binary pulsar system PSR 1913+16 by Joseph Hooton Taylor Jr. and Russel Hulse and for which they received the 1993 Nobel Prize in Physics.
Issue #103 -
Astronomy Picture of the Day
2005 June 5
A Milky Way Band
Explanation: Most bright stars in our Milky Way Galaxy reside in a disk. Since our Sun also resides in this disk, these stars appear to us as a diffuse band that circles the sky. The above panorama of a northern band of the Milky Way's disk covers 90 degrees and is a digitally created mosaic of several independent exposures. Scrolling right will display the rest of this spectacular picture. Visible are many bright stars, dark dust lanes, red emission nebulae, blue reflection nebulae, and clusters of stars. In addition to all this matter that we can see, astronomers suspect there exists even more dark matter that we cannot see.
Credit & Copyright: John P. Gleason, Celestial Images -
Giant Meterwave Radio Telescope
The Giant Meterwave Radio Telescope (GMRT), located near Pune in India, is the world's largest radio telescope at metre wavelengths. It is operated by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research, Bombay. An office of NCRA is located in the Pune University campus right next to IUCAA. There are fourteen telescopes randomly arranged in the central square, with a further sixteen arranged in three arms of a "Y"-shaped array (similar to the VLA) giving an interferometric baseline of about 25 km. The GMRT is an interferometer which uses a technique known as aperture synthesis to make images of radio sources.
Giant Meterwave Radio Telescope (GMRT), India
Each antenna is 45 metres in diameter and, instead of a solid surface like many radio telescopes, the reflector is made of wire rope stretched between metal struts in a parabolic configuration. This works because of the long wavelengths (21 cm and longer) at which the telescope operates. Each antenna has four different receivers mounted at the focus. Each individual receiver assembly can rotate so that the user can select the frequency at which to observe.
The maximum baseline in the array gives the telescope an angular resolution (the smallest angular scale that can be distinguished) of about 1 arcsecond at the frequency of neutral hydrogen (1,420 MHz).
Astronomers from all over the world regularly use this telescope to observe many different astronomical objects such as galaxies, pulsars and supernovae.
Each year on National Science Day the observatory invites the public and pupils from schools and colleges in the surrounding area to visit the site where they can listen to explanations of radio astronomy, receiver technology and astronomy from the engineers and astronomers who work there.
General Information
Organization: NCRA
Location: 10 km east of Narayangaon, India
Wavelength regime: radio 50 to 1500 MHz
Completion date: first light 1995
Webpage
Physical characteristics
Telescope style: array of 30 parabolic reflectors
Diameter: 45 m
Collecting area: 60,750 m²
Mounting: alt-azimuth fully steerable primary
Dome: none
Issue #104 -
Astronomy Picture of the Day
2005 June 6
Saturn: Dirty Rings and a Clean Moon
Explanation: Eating surface ice from Enceladus might be healthier than eating ice from Saturn's rings -- it certainly appears cleaner. From their apparent densities and reflectance properties, both the rings of Saturn and its shiniest moon, Enceladus, are thought to be composed predominantly of water ice. For reasons that are not yet understood, however, many of Saturn's ring particles have become partly coated with some sort of relatively dark dust, while the surface of Enceladus appears comparatively bright and clean. The contrast between the two can be seen in the above image taken last month by the robot Cassini spacecraft now in orbit around Saturn. Bright Enceladus shines in the background in contrast to the darker foreground rings. The reason why Enceladus is so bright is currently unknown but might involve bringing fresh water to its surface with water volcanoes.
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA -
Green Bank Telescope
The Robert C. Byrd Green Bank Telescope (GBT) is the world's largest fully mobile radio telescope. It is part of the National Radio Astronomy Observatory (NRAO) at Green Bank, West Virginia (USA). The telescope honors the name of Senator Robert C. Byrd.
Green Bank Telescope: the largest fully steerable single dish in the world, 100 x 110 m.
The telescope sits at the heart of the United States national radio quiet zone, a large area where all radio transmissions are either limited or banned outright, to help the telescope function properly.
Description
The surface area of the GBT is 100 by 110 meters with a separate actuator for each of the 2,004 surface panels. The panels are made from aluminum to a surface accuracy of less than 0.003 inches RMS. Infrared lasers are used to measure the position of each plate so that if one is to move out of place the actuator can move it back into it proper position, resulting in a surface that can be kept in an optimal state throughout an observation with the telescope. A laser alignment system also allows for precise knowledge of the positioning of the dish by utilizing reflectors in the mountains and around the base of the telescope itself.
The telescope is unusual in that the mirror is not a symmetrical dish, but is a section of a much larger parabolic figure with the receiver where the prime focus would be of the entire mirror. As a result, the support for the receiver does not in any way obscure the mirrors view of the sky.
Issue #105 -
Astronomy Picture of the Day
2005 June 7
Galaxies in View
Explanation: Galaxies abound in this cosmic scene, a well chosen telescopic view toward the northern constellation of Ursa Major. Most noticeable are the striking pair of spiral galaxies - NGC 3718 (above, right) and NGC 3729 (below center) - a mere 52 million light-years distant. In particular, NGC 3718 has dramatic dust lanes sweeping through its bright central region and extensive but faint spiral arms. Seen about 150 thousand light-years apart, these two galaxies are likely interacting gravitationally, accounting for the warped and peculiar appearance of NGC 3718. While a careful study of the deep image reveals a number of fainter and more distant background galaxies, another remarkable galaxy grouping known as Hickson Group 56A can be found just to the right of NGC 3718. Hickson Group 56A contains five interacting galaxies and lies over 400 million light-years away.
Credit & Copyright: Johannes Schedler (Panther Observatory)
