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Edmund Scientific's Popular Science Library 1
Looking at the Moon
Reading Star Charts
What is a Barlow Lens?
Binocular Collimation
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(from Edmund Mag 5 Star Atlas (30091-18))
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COMPARATIVELY near at 240,000 miles, the moon is a fascinating object showing amazing detail in even the smallest telescope. Technically the resolution of moon objects is about two times Dawes Limit or R=9/D. Since linear distances on the moon are approximately 1 mile per second, it is easy to convert angular measure to linear. For example. a 3-in. refractor is rated 913 or 3 seconds of arc, which means you can resolve a crater 3 miles in diameter or two similar craters side by side. Much smaller objects in the form of a line can be seen but are not resolved. A typical example is the Straight Wall which is only about 1/8 mile wide and so about 118 second in angular width. This is seen "plain as day" but without detail with a 3-in. telescope.
PHASES. One orbit of the moon around the earth is a lunation. It taices 29 days, beginning with the New Moon or no-moon when thernoon is in line with the sun and so gets no light on its earth-facing side. But in a day or two the moon moves a little east of the sun, so that a narrow crescent catches the sunlight. The crescent gets fatter every day and in 7-1/2 days the moon is half-illuminated, which phase is called First Quarter because 7-1/2 days is one-fourth of a lunation. After the first quarter, the sunny side continues to get fatter and fatter, finally becoming the fully illuminated Full Mooon. After the full moon the lighted area reduces in reverse fashion with the light on the opposite side. Finally only a thin crescent remains and you are back to New Moon again.
LOOKING AT THE MOON. Most moon objects are seen best when on or near the terminator since then the object has a bright side and also a dark shade. However, the full moon is interesting and shows well the ray system surrounding Tycho, and to a lesser degree, Copernicus and Kepler. Tycho is a beautiful example of a rater, 54 miles in diameter and 3 miles in depth, Copernicus is the same diameter but shallower with multiple waves which classify it as a Ringed Plain. The full moon does not obscure crater Aristarchus at the edge of the 2nd Quadrant. This is the brightest spot on the moon. The darkest spot is Grimaldi in the 3rd Quadrant, close to the edge. This is an oval crater with a mean diameter of about 120 miles. The terminator occurs in all parts of the moon. It sweeps across the moon and back every month, giving you a choice of left or right lighting. From new moon to full, the terminator is the sunrise line; from full to new, it is the sunset line. Either sunrise or sunset light is equally good for most objects, but there are some exceptions, notably the Straight Wall which is a cliff some 800 ft. high sloping toward the east. The wall is seen as a bright line with sunset light, indicating the phase should be about a day past Last Quarter. Lighting from the sunrise side shows no light at all but only a wide, dark shadow.
One of the prettiest scenic areas is the Sinus Iridum or Bay of Rainbows in which Cape Heraclides may be seen (with some imagination) as a beautiful moon maiden when the moon is about 11 days old. Good examples of rills or clefts are Ariadaeus and Hyginus, both slightly below center in the map, both seen best around First Quarter.
DIRECTIONS ON THE MOON. If you look south at the moon, it is natural to label the moon with the same directions of earth, that is North is at the top of the moon and East is on the left. Even when you turn a map upside down, the directions retain their identity. The upside down view is the conventional moon map and it shows the moon as it is seen with an astronomical telescope. CLICK HERE TO SEE ILLUSTRATION
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(From The Edmund Sky Guide (30095-35))
READING STAR CHARTS. A brief look at a star chart or a list of stars will usually turn up a plethora of star designations. Some have almost lyrical sounding names like Aldebaran or Alpheratz, while others have Greek letter designations or simply a number. Many stars have two names and a few have three, although almost always one is preferred over the others.
Most of the 60 or so brightest stars in the sky (visible from mid-northern latitudes) are known by their descriptive Arabic, Greek or Roman names. These names, although meaningless in English, usually translate into a logical word picture. Betelgeuse, for example, is believed to be ancient Arabic for "armpit of the mighty one." Dubhe is "the back of the great bear," and Spica is "the ear of corn" in Latin. However, the majority of stars are not known by these colorful names and are simply designated by a number or letter. The first of these systems was developed by the German Johann Bayer in his star map published in 1603.
Bayer generally listed the stars in order of brightness by letters in the Greek alphabet, alpha for the brightest, beta for second brightest, gamma for third, and so on. What sometimes confuses beginners is that the constellations are not written in their normal form when used in the Bayer system but rather their genetive form. Thus the brightest star in Taurus,
commonly known as Aldebaran, becomes Alpha Tauri, Elnath is Beta Tauri, and Gamma Tauri, being a fainter star, doesn't have a popular name.
Many of the big constellations contain more bright stars than the 24 letters in the Greek alphabet can accommodate. Seeking to remedy this problem the British astronomer John Flamsteed, about a century after Bayer, designated each star in a constellation with a number, thus eliminating the limiting factor in the Greek alphabet. The numbers were applied from the western side of the constellation toward the east. In some constellations Flamsteed named more than a hundred stars as he included all he believed were within the limit of naked eye visibility.
The stars Flamsteed missed are designated with catalog numbers
taken from extensive lists published by observatories that specialize in star positions. Several examples are indicated on the maps. Another naming system, common in southern hemisphere constellations, utilizes English letters which at times is confusing because some English letters look identical to some Greek letters. Modern astronomers ignore most of the English lettering system except for the use of capital letters from R to Z and double caps in all combinations to designate variable stars within a constellation. Wide double stars or strings of stars are sometimes given superscripts in the form of a small number to the upper right of the letter designations. In some of these instances the stars appear so close together that they both go by the same designation but are individually distinguished by the superscript.
Learning star designations is not difficult but it does take time to sort out all of these differences. It's like learning how to read a map for the first time.
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(From Edmund Scientifics All About Telescopes (30090-94))
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A BARLOW lens is a negative lens used inside the focal plane of a telescope objective. Its normal diverging action reduces the convergence of the light cone, forming a larger image at a slightly greater distance. All Barlow lenses are designed for a certain magnification factor--usually 2x--but work well over a moderate range of powers.
TYPICAL BARLOW SYSTEM. A drawing of a Barlow system begins with the usual light rays from objective to image, except knowing the Barlow will enlarge the primary image, you make it just that much smaller, Fig. 1. A and B spacing distances are then calculated for the desired magnification, using Case 5 equations. The linear field of eyepiece is set off at the final image plane, and the light ray intercepts are extended from the Barlow lens to edge of final image, Fig. 2. If you want to locate the objective stop, it can be done graphically by extending the light rays backwards, as shown in Fig. 2. As can be seen, the objective stop is a virtual image; if calculated, you use Case 3 equations. The position of objective stop must be known if you want to calculate (Case 1) the exit pupil position. In most cases, only the A and B spacing distances are needed. Glare stops can be fitted anywhere along the light cone.
FOCUSING MOVEMENT. Normally a Barlow setup requires "out" movement of the focusing tube. A goodly amount of "out" movement is supplied by the Barlow tube itself, Fig. 5. The net result is that "in" movement of the focusing tube is needed for the popular 2x setup, Fig. 6. Fig. 7 illustrates in reverse fashion--with eyepiece alone.
SPECIAL SETUPS. The parfocal Barlow (Fig. 9), has the same focus for either eyepiece alone or with Barlow--the focusing tube does not move. For this kind of setup, you simply space A and B distances from the primary image plane, which is made coincident with the focal plane of the eyepiece used alone. A Barlow system is sometimes built-in as a permanent part of a telescope. Fig. 10 is an example. Because the diagonal is close to the primary image, which itself is reduced in size, it is possible to field the full light cone of an f/8 mirror with a 3/4 inch diagonal. For the arrangement shown, spacing distance B is fixed at about 8 in. You select some suitable magnification and then calculate F as shown. A good -quality simple plano-concave lens of the specified focal length will usually perform quite well.
You have to focus "out" about 1/2 inch, indicating that the Barlow setup itself must focus "in." Increased M. pushes the image back and requires more "out" movement. The maximum case for the equipment specified is shown in Fig. 8. This takes actual "out" movement of the focusing tube. To determine the focusing tube movement on a drawing like this, you measure from primary image plane to the position the focal plane of the eyepiece would normally occupy if used alone.
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(From Edmund Scientifics Collimators and Collimation (30090-72))
COLLIMATION of a binocular means simply that the two telescopes are to be made parallel. This applies to the optical axes. If this adjustment is considerably at fault, you will see a double image through your binocular. A smaller error in collimation will permit you to see a single image but only at the expense of more or less eye-strain. The double imagery of a poorly-collimated binocular can be seen by closing one eye while looking at a distant object through the binocular, and then opening the eye quickly. For an instant or two you will see the double image, but your eyes will work automatically and quickly fuse even widely separated images. CLICK HERE TO SEE ILLUSTRATION.
In the diagram, a simple cross is shown as the target, but you can look at a chimney, telephone pole or other distant, well-defined object. The image separation may be normal right-left, matching your eyes (Fig. 2), or the images may be crossed (Fig. 3) CLICK HERE TO SEE DIAGRAM.
The correction is made by moving one or both objectives laterally. This movement is always opposite to the direction you want image to shift; if you want the image to move up, you move the objective down, etc. The mechanical method of moving the objectives laterally is usually an eccen- tric mount and eccentric ring (Fig. 4). The movement is not large, rarely more than 11/32 inch from center, equal to about 11.2 degree in angular measure. Some imported glasses still feature the older style of screw adjustment (Fig. 5).
Many inexpensive binoculars have no collimation adjustment at all. Figs. 6 and 7 show how the light path direction is changed by shifting one or both binocular objectives laterally. CLICK HERE TO SEE DIAGRAM. Fig. 6 shows the light path for a center-of-field object. The mechanical axis coincides with the optical axis. The light enters parallel with the optical axis and emerges the same way.
What happens when the objective is moved laterally is shown in Fig. 7. The former center-of -field object point is no longer the actual center of field--it has become an off-axis object. Since the target is now off-axis, the light bundle emerges at an angle. These diagrams show a simple astro telescope; with any erecting telescope, the emergent beam gets an additional flip and skews off in a direction opposite to the example shown. The lateral movement of objective in an eccentric mount can be made in any of the three ways shown in Fig 8. CLICK HERE TO SEE ILLUSTRATION Best control is obtained with the straightline movement, C and D, Fig 8. This simple circular movement can be used for collimating by eye. Click here to continue this article...
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