2008 Writing
D e s e r t E x p o s u r e
September
2008
Constellation of the Month: Telescopium, the Telescope
Down on our southern horizon during September evenings is the faint constellation Telescopium, the Telescope. Since this modern instrument was invented in 1608, there is no mythology related to it. This obscure group of stars received its name from Frenchman Nicolas Louis de Lacaille, after visiting the Cape of Good Hope in 1751-52 on an expedition to map the southern sky. He observed 10,000 southern stars. Returning to France, he took some unassigned star groups and made 14 constellations.
 |
Click the image for a larger map.
Poking over the southern horizon during September evenings, the constellation Telescopium becomes visible for just a few hours. Telescopium occupies only 252 square degrees, making it 57th of the 88 official constellations. The northern part of the original constellation was cut off and given to Scorpius, Sagittarius and Ophiuchus. The star Peacock is actually the brightest star in Pavo, the next constellation south. |
There are two basic types of telescopes, refracting and reflecting. Telescopium represents the refracting type, which has a convex or magnifying lens in the front of a long tube. The lens gathers light from the sky and focuses it down the tube to the focal plane, which is usually less than an inch across. Another magnifying lens — the eyepiece — below the focal plane enlarges the view. Each eyepiece has a focal length stamped on it. If you take the focal length of the lens at the top of the telescope and divide it by the focal length of the eyepiece, you have the magnification that you will get when you look through the telescope.
But astronomers are not primarily interested in magnification; they are interested in the ability of the telescope to gather light. It makes objects visible that are too faint to be seen by the eye alone. The amount of light a telescope can gather is related to the area of the objective (front) lens. If you increase the diameter of the lens times 2, you increase the amount of light by a factor of 4 (2 squared), allowing you to see stars 1.5 magnitudes fainter. To see ever fainter objects in the sky, astronomers need larger and larger lenses. At some point, the lens gets so large and heavy that it is hard to support at the top of a telescope tube.
The objective lens in a real telescope is actually made of two lenses, designed to allow each to correct the distortions caused by the other. Each lens has two surfaces that must be precisely figured, for a total of four surfaces. The allowable error of each surface is tiny fractions of an inch, making a telescope lens very difficult and expensive to make.
Telescope makers came up with a solution to allow them to make larger telescopes while only having to precisely figure one surface: the reflecting telescope. Instead of a large lens at the front of the telescope, a large magnifying mirror is placed at the bottom of the tube. Light strikes the mirror and is reflected back up the tube to a small flat mirror that reflects the focused light out the side of the telescope tube where the eyepiece is located. This form was invented by Sir Isaac Newton, and so is called a Newtonian Reflector. There are many other configurations of the reflecting telescope, but in every case, it is much cheaper to manufacture and mount a reflector than a comparable sized refractor. The largest reflecting telescopes currently in operation are over 32 feet across, while the largest refractor is only 40 inches across.
The Planets for September 2008
September starts with three planets, Mars, Venus and Mercury, clustered just above the west-southwestern horizon as it starts to get dark. They will be hard to see in the evening twilight; get out your binoculars. These three planets will be close together during early September, with the Moon joining them on the 1st. The three planets will join the first magnitude star Spica during the third week of month. As the month ends, Mercury, Mars and Spica will sink into the glare of the Sun, while Venus climbs higher in the sky.
| Watch the Skies (all times MDT)
|
Of the three planets, Venus will be the easiest to be seen, and is the only one that still visible at month's end. Venus spends most of the month in Virgo, but slips into Libra as September ends. At mid-month, Venus is magnitude -3.9, while its disc is 11.4 seconds-of-arc across and it is 89 percent illuminated. The planet is becoming less full as it swings around the Sun and comes nearer the Earth. Its approach makes Venus appear larger and brighter.
Mercury appeared in our evening sky late last month and is still there. It reaches its farthest point from the Sun on Sept. 11, before turning around and heading back toward the Sun. Mercury will also be approaching the Earth, going from 69 percent sunlit on Sept. 1 down to 36 percent on Sept. 20. It will grow in size from 6.2 seconds-of-arc across to 8.4 while it fades from magnitude 0.1 to 0.6, making it even harder to see.
Mars has been in our evening sky for months, and this month it makes its last appearance. Still in Virgo, it shines at magnitude 1.7. Mars is a tiny 3.8 seconds-of arc across, and is just barely visible as September begins. By month's end, it will have disappeared into the Sun's glare.
High in the southeastern sky as twilight ends, the only other naked-eye planet currently visible, Jupiter, is moving slowly eastward among the stars of Sagittarius. At magnitude -2.5, it is the brightest object in this portion of the sky. On the 15th, Jupiter is 41.6 seconds-of-arc across, slowly shrinking as the Earth pulls away from it. With a telescope, you will easily be able to make out the four Galilean moons, tiny 6th magnitude diamonds in a crooked line parallel to Jupiter's equator. Jupiter sets around 1:15 a.m.
The Sun has been heading south for almost three months, and finally reaches the celestial equator on Sept. 22. Here in the northern hemisphere, autumn begins as we leave our hot summer months behind. So start getting those cold-weather clothes out for the cool fall nights and "keep watching the sky"!
An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.