Crazy Astronomy helps amateur astronomers to get info about the live sky and also to learn many things about astronomy.
Friday, December 31, 2010
Mission planned to probe Uranus
Thursday, December 30, 2010
How to make your own telescope at home
You can build this first simple instrument without difficulty while allowing you to learn the structure and operation of telescopes in general. Although it is simple, it can reveal the craters of the Moon and the satellites of Jupiter. It is also very useful for demonstrating lens aberrations. You really need to build this telescope as a necessary step towards understanding the solutions employed in the second and improved
telescope model. In the section "From Lenses to Optical Instruments", you saw how a telescope works; here I simply remind you that the objective lens produces an image of the object observed, and this image is magnified by the eyepiece.
Figures 2 and 3 show our first telescope, which is made using easy-to-find materials. The components of this instruments are:
1 - ring to secure the eyepiece lens from behind
2 - ring for centering the eyepiece lens
3 - eyepiece: lens with focal length of 20-50 mm. You can buy o
ne in an optical or photographic shop, or you can get one free by using the lens of a disposable camera.
4 - ring to secure the eyepiece lens from the front
5 - a cardboard tube for the eyepiece. You can use the tube from a roll of plastic food wrap or paper towels. You can also use short sections of this tube to make rings 1, 2, and 4 which you need to hold the eyepiece lens in place
6 - coupling between the eyepiece tube and the main t
ube. This is a hollow cylinder with an outer diameter that fits snugly into the end
of the main tube and an inner diameter that provides a snug but movable fit to the outside of the eyepiece tube. You can make the coupling using several plywood disks glued together or using a polystyrene cylinder with a hole bored through it. If you use polystyrene, you will need to add an opaque covering at each end.
7 - main tube. Use a cardboard or plastic tube about as long as the focal length of the objective lens and with an outside diameter of 50-60 mm about. Suitable sources include map mailing tubes and core tubes for carpets, drawing paper, or wrapping paper.
8 - objective lens. You can use a common eyeglass len
s with a focal length of 500-1000 mm. You can buy it in a optical shop. Ask the optician to reduce the lens diameter in order to fit it precisely into the tube cap
9 - diaphragm. Cut it from a black card, then open a hole of about 15 mm in diameter in the center of the disk10 - Cap of the tube. If you buy a tube for drawing sheets, you should have a cap which will be useful for retaining the objective and the diaphragm. Otherwise, you can made it with a disk of cardboard. Make a series of radial cuts around the edge of the disk to make a set of tabs, Moisten the tabs; then place the tube cap on one end of the principal tube and bend the tabs around the outside of the tube. Glue the tabs together where they overlap, but be careful not to glue the cap to the principal tube yet. When the glue is dry, slip the cap off and cut in the cap a hole a few mm less in diameter than the outside diameter of principal tube.
Figure 3 - The components of the first telescope |
The distance between objective and eyepiece lenses must be equal to the sum of their focal lengths. The eyepiece tube must stick out a few centimeters so you can move it to focus the telescope. Make the length of the principal tube short enough to allow you to grip the protruding part of the eyepiece tube with your fingers as you adjust the focus.
The eyepiece tube must slide smoothly in its channel, but it should not be loose enough to fall out if you hold the telescope vertically. Paint the inside of the tubes with black opaque paint (matte finish) or India ink . Secure the cap of the main tube to keep it from pulling away from the tube.
HOW TO USE THE FIRST TELESCOPE
Do not use the diaphragm at first, but leave the objective at the greatest aperture. Point the instrument towards a distant object. Move forward and backward the eyepiece tube until the image is as distinct as possible. You will soon realize that the image is of poor quality and is never distinct. This simple objective lens has many defects that produce the poor-quality image. You can reduce some aberrations by decreasing the lens aperture. This is why we use a diaphragm on the objective. It is a round disk of black stiff paper with a 15-mm-diameter hole in the center. This diaphragm, placed in front the objective, reduces both the effects of the lens defects and the brightness of the image. As a consequence, you can only observe objects brightly illuminated by the Sun. To minimize the chromatic aberrations, you will need to replace the spectacle lens you used as the objective for your first telescope with an achromatic lens, as we shall see below. |
As soon as you use your telescope, you will see that you cannot hold it steadily enough in your hands to maintain a stable image. You will need to build a support (fig. 2) to help you to point your instrument and keep it steady. You can mount this support on a photographic tripod by means of a 1/4 W threaded hole.
The first telescope will give you a good feel for lens aberrations (fig. 3). In this simple instrument, chromatic aberration is the most conspicuous. The aberrations can be greatly reduced by means of careful lens design. As it is not possible to limit all kinds of aberrations using only a single lens, objectives and eyepieces are created using multiple lenses. By selecting different types of glass for the various lenses and using appropriate surface curvatures and distances between lenses, it is possible to control in a satisfactory manner the aberration of the system. In general, the success of an objective or an eyepiece in correcting aberrations depends on the number of lenses used to make it.
For the second telescope, shown in figure 10, we use an achromatic objective, made up of two lenses of different shapes, one converging and the other diverging. Sometimes they are glued together by means of Canada Balsam or a synthetic resin (cemented doublet), other times they are kept separated (air-spaced doublet). These two lenses have different indices of refraction, one high (Flint glass), and the other low (Crown glass). Hence, the chromatic aberrations of the two lenses act in opposite senses, and tend to cancel each other out, thus producing a much more distinct image than a single lens could achieve.
Usually, these objectives are constructed to reduce other types of aberration as well. Obviously, achromatic objectives vary in quality. In some of them, it is still possible to perceive a residual chromatic aberration, or the images they produce are well focused in the center only, or they produce a pincushion or barrel distortion. Figure 3 describes the main optical aberrations.
In our first telescope, we used a simple magnifying glass as the eyepiece. Also eyepieces made up of a single lens are affected by several aberrations, particularly chromatic, and with a single lens it is not possible to eliminate them. In the early 1700s, Huygens showed that he could eliminate chromatic aberration in an eyepiece using a system of two lenses. Since then, many eyepiece models have been designed to obtain better and better corrections, a wider field corner, etc. However, eyepieces always retain the same basic function of magnifying the real image formed by the objective. The main parameters that characterize an eyepiece are the following:
Parameter | Defines |
---|---|
MODEL | Aberration corrections |
FOCAL | Focal lengths combine to determine the magnification power of the telescope |
FIELD | Determines how wide the image appears to the eye. A wider field makes the telescope more comfortable to use |
EYE RELIEF or EYE DISTANCE | Indicates the proper distance from the eye to the eyepiece lens |
DIAMETER | Indicates the outside diameter of the eyepiece tube. Most eyepieces have diameters of either ~24 mm or ~32 mm |
In addition to those shown in figure 4, other types of eyepieces can be made by using more lenses. Such fancy lenses are made for special purposes, and they are usually expensive.
With the first telescope you built, images were inverted, and in the section "From Lenses to Optical Instruments" I explained why. But astronomers don't really care whether they see star images "straight up" or "upside down." In fact, with the exception of the Sun, all stars are so distant that not even with the most powerful telescopes has anyone ever seen their disks. They appear to us always as points of light, and to see a point of light upright or overturned does not make any difference. However, many people would like to use their telescopes for terrestrial observations, in which case "right side up" does make a difference. |
Several different methods allows you to erecting images without significantly degrading their quality. Figures 5, 6, 7, 8, 9 show the main erecting systems. These optical devices are sold with a case and tubes for connecting them with the eyepieces and the focusing systems.
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During the construction of this second telescope (fig. 10), we will use improved technology and manufacturing methods to achieve better performance than we could get from our simple first telescope. To build this instrument, you will need: - an achromatic objective with a diameter between 40 and 100 mm, and with a focal length between 600 and 1200 mm
- an eyepiece with a focal length between 20 and 40 mm. Any model show in figure 4 is good, with the exception of the Ramsden eyepiece
- a rack and pinion focusing system. It is made up of two tubes sliding one into the other. The inner one is moved by a rack and pinion couple
- an image erecting system (see figures 5-9)
- the main tube in aluminum #1. Buy it with a length equal to the objective focal length. Its inside diameter must be greater than the diameter of the objective mounting bracket
- adapter ring in black plastic or aluminum
- coupling ring in black plastic or aluminum
- light shade tube.
You can buy objective, eyepiece, erecting and focusing systems from suppliers who advertise in astronomy magazines, or you can ask an amateur astronomy club for advice. In any case, make sure to choose diameters for your components such that they will fit each other; otherwise, you will need to fabricate fitting rings. You will have to make the principal mechanical parts with a lathe. If you do not have one, you can go to a machine shop. Since the parts are all quite simple, you shouldn't need to spend a lot. In any case, ask for a cost estimate. Students of high schools, technical colleges and universities can often get access to their school laboratories. If you want to get your own machine tools, you can find commercial Chinese-made lathes that are available for less than a thousand dollars. For the same price, you can buy also a small used lathe.
RESOLVING POWER AND MAGNIFICATION POWER
The magnification of the telescope (M) is given by the ratio between the objective and eyepiece focal lengths: M = Fob/Fep. You cannot simply magnify at will, seeing more and more details. The maximum magnification you can reach with a telescope is limited by the diameter of the objective. The larger the diameter of the objective, the closer are the points it is able to distinguish as separated.
The resolving power (RP) of a corrected objective, expressed in seconds of arc, is given by RP" = 120/D where D is the diameter of the objective in millimeters. The human eye has an RP of about 60". Hence, the maximum magnification you can obtain from an objective (MM) is given by the ratio between the RP of the eye and that of the objective: MM = RPeye / RPob.
For instance, an achromatic objective with a diameter of 80 mm has an RP of 120 / 80 = 1.5". Hence, the right magnification using this objective should be 60 / 1.5 = 40X. In practice, you can double this value, but it is better avoiding to go further, because the amount of visible detail will not increase. In the end, follow this simple rule: the magnification power of a telescope has not to exceed the diameter of its objective, expressed in mm. Check the real RP of your instrument by means of double stars whose angular distances are tabulated in astronomical books.
The most spectacular heavenly body to observe with a telescope is without a doubt the Moon. The best time to observe the Moon with your telescope is at the first quarter, when it appears only half illuminated. Under these conditions, lunar mountains and craters project long shadows, making them better visible from the Earth.
Make your first observations with the simple telescope, the one with the eyeglass lens as objective. At the beginning, keep the objective at the maximum aperture. At the edge of the objects, you can see the blue color at one side, and the orange color at the other side. These colors are produced by chromatic aberration. The image will appear quite confusing. Now place the diaphragm on the objective. It will greatly reduce the aberrations, you see the difference! But on the other hand, the brightness of the image will be dramatically decreased as will the resolving power. Using an achromatic telescope, instead, these defects are by comparison nearly imperceptible even without a diaphragm. In fact, with this type of instrument, the diaphragm is not needed.
Other objects to observe are the nearest planets. Jupiter shows four satellites aligned along the equatorial plane, appearing as a model of the solar system. For observing the Rings of Saturn, you will need of an instrument of good quality and high magnification power. The comparison between the apparent sizes of Jupiter and Saturn give you an idea of great distances in astronomy. You can also see Venus, which shows phases as the Moon, and you can even see star clusters and double stars
Wednesday, December 29, 2010
Hubble Discovers a Strange Collection of White Dwarf… Dwarfs
Read the rest of Hubble Discovers a Strange Collection of White Dwarf… Dwarfs
Tuesday, December 28, 2010
Brightest Radio Supernova
Bridge Between the Stars - NGC 602: Hubble Visualization
It’s been awhile hasn’t it? Time may have passed, but absence makes the heart grow fonder. For those of you who have missed our very special dimensional looks into the Cosmos, then it’s high time we let our minds and eyes relax and we take a 200 thousand light-year distant journey towards the edge of the Small Magellanic Cloud for a look at a bright, young open cluster of stars known as NGC 602… (...)Read the rest of Bridge Between the Stars - NGC 602: Hubble Visualization by Jukka Metsavainio
When Galaxies turn Pink!
inwheel_Galaxy">Pinwheel Galaxy (M101), are pretty unremarkable, and are just a dusty whitish-blue color. But if we take other, similar galaxies, like NGC 3184, we find something interesting. Specifically, we find a few areas in these spiral arms that are pink
When The Black Hole Was Born
The new research is based on observations with some of the largest ground-based telescopes in the ...
Monday, December 27, 2010
Quadrantid Meteors: Fire over Ice
Welcome to viewing conditions for the Quadrantid meteor shower!
Despite the cold, many meteor-watching veterans will tell you that the Quadrantids put on one of the very best displays of shooting stars all year. And this year moonlight will not a factor, because the the Moon is new on the night of January 3–4, when the shower peaks.
The shower is rich but brief, with 60 to 120 or more meteors visible per hour under ideal conditions for just 2 to 4 hours. The peak this year is predicted to come around 1h Universal Time on the 4th, well timed for Europe and Central Asia but a little early for North America (8 p.m. Eastern Standard Time, 5 p.m. Pacific, on the evening of the 3rd).
However, outbursts of Quads have been seen several hours early or late. Also, the meteors that arrive late in this shower tend to be brighter than the early ones. Minor activity has been reported as much as a week before and after the peak date.
The shower’s radiant (between the end of the Big Dipper’s handle and the head of Draco) is descending toward the horizon at nightfall if you live anywhere north of 37°, and it's circumpolar if you live above latitude 41° north. Very few meteors appear in the sky when a shower’s radiant is this low. But those that do are spectacular “earthgrazers” that skim along the upper atmosphere far across the sky. Just one of these can make your night. So keep an eye out on the evening of the 3rd. Any Quadrantid earthgrazers will be flying from the north-northwest after dusk, from due north around 8 or 9 p.m., and from north-northeast later in the evening.
Not until 1 a.m. does the radiant start climbing high enough in the northeast for the shower to really get cooking. Early risers note: the radiant is highest before the first light of dawn, which is 5 to 6 a.m for most of you.
The shower’s name comes from the defunct constellation Quadrans Muralis, the "mural quadrant," which Joseph-Jérôme de Lalande added to the sky in 1795. He and his nephew used one of these wall-mounted sighting devices for measuring positions of celestial objects.
The shower itself was one of the first discovered. Adolphe Quetelet of Brussels Observatory identified it in the 1830s, shortly after the American discovery of the Perseid shower captured astronomers’ attention and set meteor science on its way.
Sunday, December 26, 2010
Making Sense of Saturn's Rings
Sure, Galileo Galilei observed them in 1610 (though he didn't know what he was seeing), and Christiaan Huygens deduced the rings' true nature in 1655. But it wasn't until NASA's Voyagers and, especially, Cassini got "up close and personal" with the rings that we started to appreciate just how complicated they are.
In fact, despite four centuries of study, astronomers still don't know how or when these iconic bands came to exist.
Most any textbook will offer two possible causes: Either an early satellite strayed too close to Saturn and was ripped to shreds by tidal forces, or those same tides kept a primordial disk of matter from assembling into moons. A really goodtextbook might include a third option, proposed about 20 years ago, that a massive comet (perhaps a Centaur object) ventured too close to the just-forming Saturn and shed enough icy material to create a ring system.
None of these ideas have wowed planetary theorists, and for good reason. Six years of close scrutiny by Cassini have confirmed what ground-based infrared observers suspected as long ago as 1990: the rings are almost entirely water ice. In fact, after allowing for the meteoritic contamination they've picked up over time, they likely werepure ice when they formed — and there's the rub. Any sacrificial satellite or primordial building blocks would likely have been a roughly 50:50 mix of ice and rock. That's the composition of most satellites not only circling Saturn but throughout the outer solar system as well.
So how can you pulverize a moon without adding lots of rocky rubble to the rings? The answer, says Robin Canup (Southwest Research Institute), is to take it apart very, very carefully. In a paper appearing in the December 16th Nature , Canup puts forth a new scenario that she's been perfecting over the past year. The key, she says, is understanding what happened to Saturn's moons after they formed — and realizing that (unlike Jupiter) Saturn has only one large satellite, Titan.
Just as the giant planets must have been dragged toward the Sun by interactions with the solar nebula, Canup posits that any big moons created in the primordial disk of matter around Saturn would have spiraled inward and been gobbled up. The last of these, roughly Titan's size, would have gotten so hot from tidal stresses that its water ice melted, causing any rocky matter to sink to the core. As it continued to migrate inward, the doomed moon crossed the threshold (known as the Roche limit) inside of which the planet's tidal forces began tearing it to pieces.
Here's the genius of Canup's model: the Roche limit for disrupting a rocky body is much closer to a planet that the one for ice (because rock is much denser). So while the moon's icy exterior was literally falling apart, the rocky core remained intact and eventually fell into Saturn. All the chips-off-the-block left behind, orbiting close to the planet, would have been nearly pure ice.
Moreover, there would have been a lot of them — totaling perhaps 10 billion billion tons, a thousand times more mass than estimates for what the ring system holds now. "It would have been a vastly more massive initial ring," she admits. But that's not a deal-breaker: since rings like to spread out, over time lots of this stuff would have slid inward and into Saturn, and lots of it would have migrated outward — beyond the Roche limit — where it could clump together into stable satellites built like enormous ice cubes. The inner moon Tethys, for example, is 660 miles (1,066 km) across, yet it has a bulk density of only 0.97 g/cm3.
Other planetary scientists like what they see. As French ring specialists Aurélien Crida and Sébastien Charnoz conclude in an accompanying commentary, "Canup's model offers, for the first time, a convincing starting point for a consistent theory of the origin of Saturn's rings and satellites."
If she's right, the Saturnian ring system has been around for nearly 4½ billion years. Yet the ongoing gravitational tugs of war between the rings and little moons like Atlas argue that the rings formed much more recently, something like 500 million years ago. Moreover, meteorites should have dirtied up the rings' once-pure ice in as little as 100 million years — unless the system is far more massive than has been assumed.
With more time and a little luck, Cassini should be able both to estimate how much meteoritic grit is soiling the rings and to determine the mass of the ring system to within a few percent — two important results that I suspect Canup is eager to see
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Amateur telescope making
Grinding a mirror using an abrasive and a smaller tool over 300 mm mirror |
8" Newtonian reflector telescope |