Observing Projects I Putting the Eyes to Work

Observing Project 1A - Pressing the Dawes Limit

Some of the most enjoyable sights in astronomy are those you can see without a telescope. There are many objects and phenomena in the sky that are both beautiful to see and test the ability of the observer to push his or her eyes to their limits. In this next section, we'll take a look at some of these and find out just how good you are.

An oncoming car has two headlights. If the car were to back away from you, those headlights would appear to move closer together with increasing distance. Eventually the lights would become so close together that it would become impossible for you to separate them from each other and they would merge into a single point of light. The minimum angular distance between two light sources at which you can still distinguish two lights is called the Dawes limit. The ancient American Indians used to test the visual acuity of their children by having them look at a star and see if they could tell if there was only one or maybe more.

A favorite target used for this test is the star at the bend in the handle of the Big Dipper. The star is named Mizar and at first glance appears to be a rather ordinary star, shining bluish-white at magnitude +2.2. If your eyes are good, you may detect that Mizar is not alone,but has a companion nearby. Shining about 12 arc minutes6 to Mizar's northwest is a fainter star that is nearly lost in Mizar's glow. The star is called Alcor and shines at magnitude +4.0. Splitting Mizar and Alcor is a very tough challenge, especially if your sky is light polluted, which may make seeing Alcor difficult under any circumstances. Though these two stars appear to be a pair, in fact Alcor lies more than three light years farther in the background than Mizar does. Mizar is about 78 light years distant while Alcor is just over 81 light years distant. The two stars only appear to be close together, but in fact this system is not a double star at all. Each one is independent. Mizar in actuality is a double star by itself. Its partner is easily visible in any telescope shining about 14 arc seconds to Mizar's southwest at magnitude 3.9. If your eyes are sharp enough to split Mizar

6 One arc minute equals 1/60th of one degree. A degree is divided into 60 minutes. Each minute in turn is divided into 60 seconds, just as it is on a clock.

and Alcor, you're ready to try and take on more difficult challenges. Twelve minutes is tough under any circumstances. Can you split a pair of stars right near the Dawes limit? If you can manage Mizar and Alcor, then move southward to Libra and try to split Zubenelgenubi. The brightest point of light in the constellation Libra glimmers at magnitude 2.7 and is also a pair of stars. Zubenelgenubi is much tougher to split than is the Mizar and Alcor pair. The stars are much closer together, only about 230 arc seconds apart, not far above the Dawes limit for the unaided eye. The secondary star is also much fainter than is Alcor, shining at only magnitude +5.1. The brighter component shines so much brighter that it can drown out the fainter star in its glare. Unless your sky is fairly dark, spotting the companion will be very difficult. Splitting close double stars near the limit of the resolution of your observing instrument requires time and patience. It does not matter whether you are using the human eye or the mighty Keck reflectors, when you are working at the limits of your equipment, you need to wait for just that right moment when the eye is relaxed and the air is still for just that one precious second.

Observing Project IB - Deep Sky Visual Acuity (Stellar Objects)

Riding high in the winter sky is the beautiful open star cluster called the Pleiades, listed in the Messier catalog as M45. The cluster, also known as the "seven sisters," is so named for the seven fairly bright stars that can be discerned within the hazy patch of light. The cluster itself shines with an overall brightness of about magnitude +1.5. These are relatively newborn stars, just recently born from the stellar womb (though the gas that surrounds them is not part of the cloud from which the stars formed). The stars are surrounded by a bluish glow that consists of interstellar dust and gas interacting with the stars. The stars and gas glow together to form one of the deep sky's most beautiful jewels and by far the brightest object in Charles Messier's legendary catalog. It also makes yet another ideal testing ground for your night visual acuity. How many of the seven sisters can you actually see? The brightest member of the cluster is Alcyone, a young blue-white star shining at

Figure 1.2. M45 at the upper right of the totally eclipsed Moon. Twelve members are visible. 35 mm SLR piggybacked on a Celestron Super C8 Plus.

Figure 1.2. M45 at the upper right of the totally eclipsed Moon. Twelve members are visible. 35 mm SLR piggybacked on a Celestron Super C8 Plus.

magnitude +2.8. Atlas is the star farthest to the east shining at magnitude +3.6. Working to the west, Merope is magnitude +4.1, Maia shines at magnitude +3.8 and Electra is magnitude +3.6. These five are the only ones that I have been able to see from my northern New Jersey home. There are four other stars, Taygeta at magnitude +4.3, Pleione at magnitude +5.0, Celaento at magnitude +5.5 and Asterope at magnitude +5.8 that are visible to the unaided eye in dark conditions, but I have never been able to detect them without binoculars. Through binoculars, the Pleiades will number more than a dozen stars. A telescope will reveal yet even more within the hazy bluish white patch of light. How many can you find with the unaided eye? If you can find more than four members, then your eyesight is most keen and your sky fairly transparent. If you can bag seven, then you are seeing to magnitude 5.5 and your sky is extremely dark. If you can see Asterope, then you should consider having astronomy club meetings where you live, for your eyes and sky are good for seeing clear down to the naked-eye limit.

Observing Project 1C - The Light Pollution Census

A great test of the darkness of your sky that we alluded to earlier in this chapter is to do a census of the Great Square of Pegasus. How many stars can you see within the Square? If you can count only two, you are seeing to only magnitude +4.4. Picking up four stars takes you to magnitude +5.0. There are a total of eight stars brighter than magnitude +5.5 and if you can count as many as 16, you are seeing to magnitude +6.0. If you have extremely good night vision and a very dark sky, you might be able to see to magnitude +6.5 in which case you may count as many as 35 stars within the square. The Great Square makes a great test because it is most prominent in the sky during late autumn and winter. During this time of year, the atmosphere is normally very stable because the ground cools very rapidly at night and chills the air close to the ground. This has the effect of greatly diminishing the

Figure 1.3. Great Square of Pegasus mag 6.5. Graphic created by author with Redshift 4.

temperature lapse rate and lends great stability to the lower atmosphere. With stable conditions prevailing on many nights, use the Great Square of Pegasus to judge the darkness of your autumn sky and the sharpness of your eyes. Count carefully, be discriminating and above all, be honest with yourself. Remember you are doing a serious scientific project and you must take great care not to let your desire to prove how clear your sky is interfere with an honest and objective result. How many stars can you see in the square?

Observing Project ID - Deep Sky Visual Acuity (Non-Stellar Objects)

Stars are fairly easy to make out because all of their luminosity is concentrated into a nearly immeasurable point of light. Try to imagine how much more difficult it would be to see if the total light were to be spread out over several square degrees. Such is the case with deep sky objects such as nebulas, star clusters and galaxies. That is what we will test with this next drill. How many objects in Charles Messier's famous catalog can you find with the unaided eye? Messier was a comet hunter in eighteenth century France. To aid his fellow comet hunters and him, Messier set out to catalog all the objects in the sky, which could easily be mistaken for comets. By the time he was finished, he had logged and listed over one-hundred of the deep sky's grandest wonders. In this project, we will attempt to see how many of these we can find without optical aid. Most are well below the threshold required for naked-eye visibility, but some can be seen easily from any location. The open star cluster M45, which we explored earlier, is the brightest single object in the catalog at total magnitude +1.5. This light is scattered across more than 6 square degrees of sky, making the overall brightness seem much fainter than the number would suggest. Magnitude estimates of an object's brightness in astronomy are measures of total light. The more the total light is spread out, the lower the overall surface brightness becomes. To simulate this effect, pour some sugar out on a table and concentrate the grains as tightly as you can. Now see the difference in apparent brightness when the sugar grains are spread out over a wide area. There is the same amount of sugar crystals just as a 1.5 magnitude star produces the same number of photons as the Pleiades cluster. The light of the star is much more concentrated, therefore it appears much brighter because it is concentrated into one single point.

Check out the Messier catalog list printed at the back of this book in Appendix A. How many of the objects brighter than magnitude six can you see? To the south of M45 in the winter sky is the bright emission nebula ("bright" is both an adjective and a technical classification of this object) M42. The Great Orion Nebula hangs from the belt of mighty Orion the hunter. The glowing gas cloud shines at magnitude +2.5 and cannot be mistaken for any other object in the sky. This showpiece of the heavens is located nearly on the celestial equator and thus visible to astronomers all over the world. Over to the northwest, try for the Andromeda Galaxy (M31). Andromeda shines at only magnitude +4.6 and that light is spread over a wide area of sky. Andromeda measures about 1.8 degrees long by 29 minutes wide. This is about the area covered by three full moons. Under moderate light pollution conditions, you might not be able to see it at all without binoculars. Finding

Andromeda is deeply satisfying for many amateurs for it represents the farthest place you can possibly see with the unaided eye. Remember that averted vision!

Star clusters make inviting targets in the summer sky. Here are three globular clusters brighter than magnitude 6. In Hercules, try M13 at magnitude +5.8. This cluster will require a dark sky and a good eye, but it is distinguishable covering a span of about 17 arc minutes. Through a telescope, M13 is the grandest globular cluster in the northern sky, but is a great test for the unaided eye. To the south is the globular cluster M4, near Antares in Scorpio. Though it lists as slightly brighter than M13 at magnitude +5.6, its light is spread out over a wider area making it more difficult to see. In addition, M4 never gets very high above the horizon for northern hemisphere viewers, unlike M13, which sails nearly overhead during summer nights. Riding a little higher in the sky to the northwest of M4 in the constellation Serpens is the globular cluster M5. Also listed at magnitude +5.6, it is somewhat more compact than M4 making viewing a little easier. Still both of these clusters represent challenges even in a dark sky. In the southern hemisphere are two of the skies grandest jewels, the globular clusters 47 Tucanae and Omega Cen-tauri. Both of these monster globulars are easily visible to the unaided eye and outshine any cluster in the north. Because they are only visible from the southern hemisphere, Messier could never add them to his catalog.

Open star clusters can be even more difficult because they do not have the bright cores that characterize globulars. Still there are some that you can hunt down without any optical aid. Next to M45, the sky's next brightest open cluster is called Praesepe in Cancer. Also known as the "Beehive Cluster" it is listed as M44 in the Messier catalog. The cluster lists at magnitude +3.5, but the light is scattered over more than a full degree of sky. A tighter grouping lies near the tail of Scorpius called M6. This open cluster near the Scorpion's tail is only 21 minutes across, about the size of many globulars and its total light measures magnitude +4.6. M6 has a nearby partner called M7. This open cluster lists more than a full magnitude brighter at +3.3 but it covers more than four times as much sky so the two clusters appear to have a very similar surface brightness.

The Messier catalog offers a treasure trove of deep-sky wonders. How many can you find on your own? If you can find ten, you've got a great sky and keen eyes.

Observing Project 1E - The Sixth Naked-Eye Planet: Uranus

In the late eighteenth century, William Herschel was scanning the skies from his observatory looking for comets when he chanced upon a sixth-magnitude star that his charts did not show. After observing the object for several nights it appeared to move across the sky in a slow but steady eastward motion. Herschel though he had discovered a new comet. But further observations yielded even more startling results. The object was moving in an orbit that was not very comet-like. The object's orbit appeared to be circular, like that of a planet and not the highly elongated path followed by a comet. The object also seemed to exhibit more of a planet-like appearance, without the fuzzy coma characteristic of comets and it had no tail to be seen at all. After several weeks of careful research, Herschel stunned the world

Figure 1.4. A speck in the darkness: Uranus. Celestron Super C8 Plus and Meade Pictor 216 XT at f/10. Photograph by author.

by announcing the discovery of his new planet. Astronomers around the world raced to their telescopes to see the new wonder of the solar system. Many modern names were floated for the new world, including "Herschel" for its discoverer and "Georgian Star"for Britain's King George III,but the name that stuck was "Uranus," the mythical father of Saturn and grandfather of Jupiter.

Uranus' discovery was historic, for the solar system had its first ever new member. The new world measured in at 29,000 miles in diameter, about the size of three and one-half Earth's. Uranus for most of its history was considered to be a gas giant much like Jupiter and Saturn, consisting mainly of gaseous hydrogen and helium. The Voyager 2 flyby in 1986 changed all that. It showed the planet to be substantially different from the first two Jovian worlds in that it was made up of large quantities of ices, thus earning the new moniker "ice giant." Neptune also turned out to be more like Uranus after Voyager 2's 1989 flyby of that planet.

Though Herschel needed a telescope and a lot of good luck to find Uranus, all you need is that dark sky, good eyes and a star chart to find the seventh planet from the sun. The planet generally shines at magnitude +5.8, placing it just above the threshold for naked-eye visibility. Finding it among the other 6,000 points of light in the sky magnitude six or brighter is an intimidating challenge. Popular astronomy magazines such as Sky & Telescope or Astronomy will print charts annually, showing the location and path of Uranus through the evening sky for the year. To find such a faint nondescript object, work to become intimately familiar with the sky in that area. That way, anything that appears unusual will immediately jump out at you. As you carefully scan that area of sky on a nightly basis, you will see one of the faintest objects in the area move slightly. The motion is normally towards the east, but as Earth overtakes Uranus in its orbit each year, our point of view causes the illusion called retrograde motion. The planet appears to stop in its path and then begin to track backwards towards the west for several months before resuming normal motion again. In the darkest of skies, the planet may also betray its true green color, but for most of us it will just appear to be its normal shade of gray.

Figure 1.4. A speck in the darkness: Uranus. Celestron Super C8 Plus and Meade Pictor 216 XT at f/10. Photograph by author.

Observing Project 1F - The Very Young Moon

Each month as the Moon swings around Earth in its orbit it passes between Earth and the Sun (most of the time slightly above or below it). As the Moon passes this position it reemerges into the evening sky as a crescent, which grows fatter each night, as the Moon appears to move farther away from the Sun in the sky. In this project we will attempt to determine how soon after new moon can you first detect the crescent.

There are several factors that improve or degrade your prospects for success in this project. The most important variable is the angle that the ecliptic makes against the sky. The ecliptic is the imaginary projection of the plane of Earth's orbit against the celestial sphere. All the planets and our Moon orbit in a path around the Sun that always remains on or close to the ecliptic. Since the Moon never strays far from this line, the angle the ecliptic makes with the horizon is vital to your observing prospects. An object can be very close to the Sun and yet be easily visible if the ecliptic makes a steep angle with the horizon. At other times, the object can be twice as far from the Sun and yet be nearly invisible if the ecliptic makes a shallow angle with the Sun. The best time to try and bag a very young moon is during the months of February, March and April. During these months, the ecliptic makes its steepest angle with respect to the horizon because Earth's north pole is tipped so that it is pointing behind the planet with respect to its orbital path. Solar system objects close to the Sun appear to stand almost directly above it at sunset. During August, September and October the reverse situation occurs when Earth's north pole leads in our orbit and the ecliptic appears to lie almost flat with respect to the western horizon at sunset. The reverse of this relationship occurs if you are trying to locate objects at sunrise. The most favorable months are August, September and October while the late winter and early spring months present poor viewing geometry at dawn. These relationships are also inverted if you are reading in the southern hemisphere.

Any good astronomy publication will tell you not just the day when new moon occurs but in fact the precise moment of new moon, usually in Universal Time. If the moon is new precisely at the time of your local sunset, you will have the opportunity to view a twenty-four hour old moon at sunset the next night. On nights where the viewing geometry is favorable, go out and find a spot with a clear view of the western horizon. Note the spot where the Sun sets and about thirty minutes later look about one fist-width (an average person's fist at arm's length subtends an angle of about 10 degrees) above the sunset point or slightly to the south. A twenty-four hour old moon should be fairly easy to spot, showing a hairline crescent hanging in an almost ghostly fashion just above the western horizon. Most people will not notice the moon until the next night. The extremely thin crescent, just 7% illuminated and only 11 degrees from the Sun, can be very difficult if any haze exists and makes for a good observing challenge.

Once you've succeeded at this, try an even younger moon. At twenty hours, the moon is barely 6% illuminated. The youngest crescent that anyone has reportedly sighted with the unaided eye (that can be verified) was only fourteen hours old. At that age the Moon is only about 6 degrees east of the Sun and is only 4% illuminated. Such a feat requires a very clear sky, perfect eyesight and viewing technique and a good shot of good luck. Can you do it? Check your ephemeris for next month's new moon and see if you can give it a try.

Observing Project 1G - Elusive Mercury

We all learned in grade school that there are five classic planets visible to the unaided eye. Venus and Jupiter flare brilliantly in the night sky and are unmistakable. Saturn also shines brightly and prominently all night long when favorably placed. Mars is unique with its orange-red glow. All of these planets shine high in a dark sky when favorably placed and require no effort to find. Poor Mercury is left to suffer in anonymity, spinning around the Sun at breakneck speed completing a circuit every eighty-eight days. As it passes between Earth and Sun (inferior conjunction) it enters the morning sky for several weeks, then passes behind the Sun (superior conjunction) and enters the evening sky. The period of visibility when the planet is in the morning or evening sky is referred to as an apparition. The planet will remain in the morning or evening sky for seven to nine weeks at a time depending on its distance from the Sun. The planet however will only be visible for a fraction of the time, for two to three weeks with the unaided eye while it is farthest from the Sun and reasonably bright.

Observers wishing to sight Mercury must learn to deal with two important contradictions. First is the assumption that Mercury must be easiest to see when it ranges farthest from the Sun. Mercury orbits the Sun in a path that has a greater eccentricity (deviation from circular) than any other planet besides Pluto. This causes the planet at some apparitions to roam much farther from the Sun than at others. As a result when Mercury reaches its greatest elongation (farthest angular distance from the Sun for a particular apparition) at the same time it is closest to the Sun (perihelion), it never strays more than 18 degrees from the Sun. If Mercury reaches greatest elongation at the time of its aphelion (farthest from the Sun) it can appear as far as 28 degrees from the Sun. Northern hemisphere observers are at an unfortunate disadvantage. The problem is that when Mercury reaches greatest elongation at aphelion, it is always at a time when viewing geometry is at its most unfavorable. Greatest elongations when viewing geometry is most favorable occur only with Mercury near perihelion. As a result, when Mercury wanders farthest from the Sun, the ecliptic lies flat with respect to the horizon and the planet is buried in the glare of twilight. When Mercury sticks close to the Sun, the viewing angle is at its best. This is the best time to look for the elusive innermost planet even though it is much closer to the Sun than it would otherwise be.

The second contradiction is that it is best to look for Mercury between the time of greatest elongation and inferior conjunction (in an evening apparition, in a morning apparition inferior conjunction comes first). This is when the planet is closest to Earth and therefore should be at its brightest. In fact, nothing could possibly be farther from the truth. Mercury is a unique object in all the cosmos in that it is the only celestial body that grows fainter as it draws closer! No other body in the universe does this. No other body changes brightness through such a range as Mercury either. Mercury is actually at its brightest (magnitude -2.0, brighter than Sirius) when it is near superior conjunction, when it is farthest from Earth. As it draws nearer to Earth, it fades to magnitude zero by greatest elongation. Over the next two weeks as it nears its closest point to Earth, it completely fades out of sight, growing as faint as magnitude +4.6 just before inferior conjunction. We'll discuss this remarkable aspect of Mercury's behavior a little later on when we look at it through the telescope.

Finding the planet is a simple matter of knowing exactly where to look and then being there at the right time. Trying to find the planet too soon after sunset will cause frustration because the sky is still too bright. Waiting too long will cause Mercury to sink too low to the horizon. The best time to look is during the time period starting about seven to ten days before greatest elongation in the evening sky or during the same period after greatest elongation in the morning sky. Begin looking about thirty minutes after sunset. During times of favorable viewing geometry, look just above and a little to the south of the sunset or sunrise point. As the pink sunset sky fades to deep blue, pinkish Mercury will appear. During this time, the planet will be shining at brighter than magnitude zero and will stand out nicely. During an evening apparition the planet will quickly retreat to the horizon and is lost within the next half-hour. If you are observing in the morning sky, see how long you can keep Mercury in sight as dawn approaches. The planet is at magnitude zero at greatest elongation but about two weeks later, you should be able to see it as bright as magnitude -1.0. This should be bright enough to allow any observer to track the planet until sunrise.

When you succeed, you will join a rather exclusive club. Everyone on Earth has seen the four other bright planets, but not one person in a thousand has consciously looked at Mercury and realized what it is. Now with our eyes well trained and exercised, let's begin the work of assembling the equipment needed to complete the integrated observing system.

CHAPTER TWO

The Integrated - Observing System.

Part II: Your Equipment

After spending my childhood and teen years observing under bright city or suburban lights with a small-aperture department store refractor telescope, I entered my college years with the dream of a larger instrument that would bring the deep sky into view for me. The two-inch Tasco refractor served its purpose well for showing the visible planets, Moon, and Sun. Seeing the deep sky beyond the solar system, the outer planets, faint comets, or the asteroid belt's largest denizens was hopelessly beyond my department store telescope's limited reach. As I began to search among the various manufacturers, the vast number of choices available rapidly overwhelmed me. Should I consider another refractor or a reflector? If I were to choose a reflector, what type should it be? What I decided to do was make a list of qualities that I needed to have in a telescope and then set about finding the one that would come closest to meeting my unique needs. Here are the items that made up my list:

(1) Aperture: It had to be big enough to permit viewing deep sky objects and stars to a reasonably faint magnitude. Getting to magnitude 14 (Pluto) would require at least eight inches.

(2) Portable: It had to be small enough to permit me to move it up or down stairs, transport easily by car and light enough to carry over moderate distances.

(3) Optical Quality: I was going to pay what amounted to a lot of money for a college student so it had to be a serious instrument that would provide the best, brightest, crispest images possible for my investment.

(4) Versatile: The telescope had to be capable of a full range of operations from wide-field viewing to providing high-power detail both as a visual and photographic instrument.

(5) Durability: This was going to be my telescope for many, many years. It would have to be solidly built and be able to withstand the rigors of decades of use.

(6) Upgradeable: As my needs and skills in amateur astronomy grew through the years the telescope had to be able to grow with me.

(7) Serviceable: In the unlikely event that there was ever a mechanical problem with the scope or a part needed to be replaced or if I just needed advice on how to work something, the manufacturer of the telescope had to be able to provide it.

When you create this list, make certain that you organize the items in the order in which they are important to you in case you need to or are willing to compromise. In my case, all seven of these items were absolutely non-negotiable. For example, without aperture there is absolutely no reason for me to be buying a telescope. I already owned one and wanted to replace it because the one I had was not big enough. It would have made no sense to replace one small scope with another. Portability was number two because I had only a limited amount of space in which to store a telescope and needed to be able to move it as necessary to be able to get it to the observing site. All my equipment had to fit into a college student's car (1981 K-Car, don't laugh at me). It then had to be hauled from the car to the place where the telescope would be set up, often a distance of a hundred feet from where I would park. A huge telescope weighing more than a hundred pounds just would not get the job done. The quality of the optics was another issue. My plan was to spend about $2,000 on my new scope. For that money, it had to have the best optics I could afford. This required that I put in a fair amount of time learning about the strengths and weaknesses of various optical systems that were available. I also needed a scope that was versatile. It had to be of the right focal length and size so that it could make photographic images of the deep sky with reasonable speed yet also provide crisp images at high power when called for visually. It also had to be durable, capable of withstanding not only the test of time, but also the rigors of repeated transportation, bumping and jostling without not only remaining intact, but without requiring constant readjustment each time I put it in the car. It had to be upgradeable, meaning capable of accepting add-on accessories such as drive motors, eyepieces and camera adaptors. As my skill grew and I wanted to probe deeper into the heavens, the telescope had to be able to grow with me and be capable of using advanced equipment beyond its own equipment package. Finally, I demanded a telescope that would be easily serviceable in the rare event that something went wrong with it. I set my sights on a scope with a good warranty and a company with a good chain of dealers where a telescope could be taken for repair or at least for shipment back to the factory for maintenance.

Remember that not everything on my list may appear on yours and you may in turn have needs that I did not. If you're mounting the scope on a permanent pier, then its not likely that portability is important to you, especially if that pier is to be surrounded by a dome. You may desire the capability for full computer control of your telescope or Global Positioning System capability to eliminate any need for manual navigation of the heavens. I have always found these tasks among the most pleasurable in astronomy and I take great pride in finding my way around the sky. If you would prefer just to hit the GO TO button, there's nothing wrong with that if it increases your pleasure in astronomy or at least lowers your frustration level. I would just prefer to find it myself. Anyway, when I bought my scope in 1986, there was no such technology available. You may have an absolute price cap to live with or other restrictions. Make out your own list and set your priorities accordingly.

There are some things you should always remember about buying a telescope. If you are about to lay out money for a serious telescope, be aware that it is an investment that will hold good value. Telescopes are not cars. Optical systems do not degrade in quality from being used so plan to take very good care of it because its going to be with you for many years. Mine is now nineteen years old, still looks like its fresh out of the box even though we've been all over the universe together. I expect we will travel the cosmos together for many more years to come. Though I'm planning to add a second telescope to my collection, it is going to be a smaller scope for quick low-power viewing of the skies on short notice. Secondly, the scope is only as good as the quality of its optics and mount. A telescope with poor optics will produce images that are distorted either in shape, color or both. A telescope also can have the world's best optics but if you put it on a shaky mount, an image magnified 100 times will only show you a vibration magnified 100 times.

By now, you probably realize that I've spent some time taking backhanded swipes at "500x" department store telescopes. They may serve their purpose reasonably well as entry-level instruments, but if you buy one realistically believing it will produce 500x, forget it. My third key thing to remember is that the primary function of a telescope is not to magnify. Its primary function is to collect large amounts of light and bring that light to a clear, sharp focus. Magnification is a secondary consideration and in fact is not performed by the telescope at all, but rather by the eyepiece. A telescope's ability to clearly present magnified images is directly related to how much light it can collect. If you try to magnify an object 500 times using a 2-inch telescope (that's 250x per inch of aperture), all you will wind up with is a blur magnified 500 times. A telescope of this size cannot magnify with that much power and bring the image to focus. Even medium-size telescopes of 8 inches aperture (that's 62.5x per inch) cannot withstand the use of that much power. In reality, a telescope with good optics should be able to focus an image at a maximum of 50x per inch of aperture. Any more than that results in a loss of image brightness, clarity and contrast that grows progressively worse as more power is used. If you try to push that two-inch department store refractor to more than 100x, you are going to be sorely disappointed. Never buy a telescope that is marketed on the basis of its magnifying power. If you look at ads for Celestron or Meade telescopes, you will never ever see their instruments advertised on the basis of magnifying power. With an eyepiece of the proper focal length, any telescope can be made to magnify an object 500 times. Aggravating the situation further is the possibility that the lenses of that department store telescope may not be made of the best material available. Top-quality refractors are made with optical quality glass and top-end scopes may use a lens element made of calcium fluorite. Cheap scopes may have lenses made of plate glass, Pyrex or even worse, plastic! This will create imperfect or even badly distorted images even at low powers.

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment