Finding Your Way Around The Night Sky - Part Two.
.. By Chris Stellis
WHAT IS STAR HOPPING
Star hopping is the technique of using stars as landmarks to blaze a path across the sky. Many amateur observers use star hopping to find deep-sky objects. You don't need a computerized drive, electricity or special gadgets of any kind. All you need is a dark sky and a good set of star charts. The best way to become good at star hopping is to do it. There are three rules when doing a star hop.
* Get a good finder scope
* Use good charts or Star Atlas
* Know how much sky you are seeing
Get a Good Finder Scope
The job of a finder scope is to allow you to accurately aim your telescope at a celestial object. A finder does this in one of two ways. It either shows the object or the star field where the object is located. The two best options in finder scopes are the traditional straight-through finder and the unit-power finders, commonly referred to as Telrads. The traditional straight-through finder uses aperture and magnification to show the object. And even if the object is too faint to be seen through the finder, it will show enough stars to identify the correct field. The big disadvantage of the traditional finder is that the image does not match your naked eye view of the sky. The view is usually inverted, which can be very disorienting. This is where the unit-power (1X) finder comes into play.
The Telrad and other examples of the 1X finder project a red circle or a red dot against a clear plastic screen. You look through the screen at the night sky and compare the view with that represented on a star chart. If you can identify a pattern of stars in the area of your target and center that area within the circle or against the dot, then your object should be visible in the telescope. Unit-power finders are very intuitive in their use. The view matches your view of the sky and the chart's representation. The disadvantage is that you rely on naked eye stars to aim the telescope. If you observe under light-polluted skies, a Telrad won't be of much assistance. The telescope may not be, either. Even if your aim is dead-on accurate, the object may not be visible through the sky glow.
Use Good Star Charts or Star Atlas
A good set of star charts will do two things. First, the charts will translate easily to the naked eye view. It's important for beginning observers to use charts showing big chunks of sky on each page. If the charts show only a portion of the constellation in which your target resides, it will be difficult to match the chart to the naked eye view. Second, the charts will plot enough stars to make star hopping practical. If the chart only shows stars to 5th or 6th magnitude, some deep-sky objects won't have any nearby reference stars.
There are a lot of good beginning star charts from which to choose. The Sky Atlas 2000.0 charts by Wil Tirion and Roger Sinnott are excellent. This can be bought for around £25 from Amazon. Each chart usually includes at least one constellation in its entirety, making comparison with your naked eye view a breeze. Stars to magnitude 8.5 are plotted which means you'll be able to use either your Telrad or traditional finder to aim the telescope. Areas of sky that overflow with bright deep-sky objects are given their own more detailed sections.
Know How Much Sky You Are Seeing
The key to matching naked eye, finder scope and telescope views to the star chart is knowing how much sky each is presenting. The naked eye view is pretty straightforward. The view through a 1X finder is fairly easy to match up too. However, if you use a Telrad-style reflex finder that projects a circle against the sky, it's important to know how much sky is confined to that circle. A Telrad's outer circle encloses a four-degree area. The innermost circle is half-a-degree across, about the size of the full Moon. You can make a simple tool that will allow you to easily match the Telrad view with star charts. Draw a circular pattern on a clear transparency sheet equal in diameter to a four-degree section of sky on your charts. This overlay makes comparison of the two views very easy. It just happens that a straight-through 8X50 finder also shows four-to-five degrees. Your overlay can be used for either device.
Finally, you will have to match the view through the eyepiece to the chart. Begin by determining how much magnification your eyepiece provides:
Magnification = Telescope focal length (in millimeters) / Eyepeice focal length (in millimeters)
For instance, a 32-mm eyepiece provides 38X magnification in a telescope with a 1,220 millimeter focal length. That's the same focal length as an 8-inch, f/6 Dobsonian. A 16-mm eyepiece produces 76X in that same scope and a 8-mm eyepiece produces a 152X view.
Next, determine the true field of view of that eyepiece. You can determine the approximate true field of view for any eyepiece using this formula:
True Field = Eyepiece Apparent Field (in degrees) / Magnification
For example, a typical Plossl eyepiece has an apparent field of view of 52-degrees. A 32-mm Plossl in an 8-inch, f/6 Dob magnifies 38X. The true field of view (52/38) works out to about 1.4-degree. This is a good, wide field of view to use when looking for objects. Once you find the object, experiment with other eyepieces and magnifications until you find the one that presents the best view.
If you've got good charts, you know the field of view of your finder scope and eyepieces, then you're set to explore the night sky.
What You Can See With A Telescope
Some of the types of celestial objects you can view are:
THE MOON--Prepare for an awesome spectacle. The moon's disk has a pastel-cream and grey background, streamers of material from impact craters stretch halfway across the lunar surface, river-like rilles wind for hundreds of miles, numerous mountain ranges and craters are available for inspection. At low or high power the moon is continually changing as it goes through its phases. Occasionally you will be treated to a lunar eclipse. Note this is a very bright object in a telescope and some sort of moon filter, preferably a variable type, will save your eyes.
THE PLANETS -- Observation of planets will keep you very busy. You can see Jupiter with its great red spot change hourly, study the cloud bands and watch its moons shuttle back and forth. Study Saturn and its splendid ring structure, watch Venus and Mercury as they go through their moon-like phases. Observe Mars and see its polar cap changes or watch the dust storms and deserts bloom with life. Uranus, Neptune and Pluto can be seen easily with 8" or larger telescopes.
STAR CLUSTERS -- There are two types of star clusters- (1) open star clusters (also called galactic clusters) which are loosely arranged groups of stars, occasionally not too distinctive from the background stars, and (2) globular star clusters which are tightly packed groups of many millions of stars.
NEBULAE -- These are glowing clouds of gas falling into two types- (1) planetary nebulae which are relatively small ball-shaped clouds of expanding gases and are believed to be the remnants of stellar explosions, and (2) diffuse nebulae which are vast, irregularly-shaped clouds of gas and dust.
GALAXIES-- These are vast, remote "island universes" each composed of many billions of stars. Galaxies exist in a variety of sizes with regular and irregular shapes.
COMETS -- Magnificent comets are routinely visible through telescopes.
DOUBLE (BINARY) STARS -- These are pairs of stars orbiting around a common center of gravity, often of different and contrasting colors.
What you can see is dependent on a lot of factors. The most important of these for astronomy is aperture. Other important factors are optical quality, steadiness of your tripod and mount, seeing conditions, your location (city or rural), brightness of the object and your experience. You won't be able to see the American flag on the surface of the moon or black holes. You won't see as much color as you see in astrophotographs (photos of celestial objects) because these utilize long exposure times which allow the light and color to build up on the film.
Most telescopes can be used to see things on the Earth. You can use them for long distance terrestrial viewing, nature study, sports action, surveillance or general land usage. You can also easily photograph terrestrial objects since a telescope can be used as a long telephoto lens by attaching the body only of a 35mm SLR camera. T-Ring and T-Adapter accessories are also required.
Astrophotography is also a rich and rewarding experience. With many telescopes it is relatively easy, but takes patience and experience to produce excellent results. Taking your own astrophotographs is a thrill as you can share the results with others.
CCD IMAGING -- The last few years have brought to the amateur astronomer a large assortment of CCD (Charge Coupled Device) cameras. Electronic imaging opens up a whole new vista for amateur astronomers who can obtain images quickly and from urban locations.
Star Map Sites
Custom star maps and satellite pass predictions.
Downloadable starmaps in PDF format.
Generate custom planisphere-style star maps.
Interactive star charts and sky info with Java applet.
Star Hopping Guidebooks
The Observer's Sky Atlas by Erich Karkoschka (Springer-Verlag; New York; 1998). An overlooked but excellent guide to finding a choice selection of targets.
Stars and Planets: A Viewer's Guide by Gunter Roth (Sterling Publishing; New York; 1998). A fine guide with charts to selected deep-sky objects.
Star Hopping: Your Visa to Viewing the Universe by Robert Garfinkle (Cambridge University Press; Cambridge; 1994). The author provides 14 star-hopping tours for the telescope owner.
Star Hopping for Backyard Astronomers by Alan M. MacRobert (Sky Publishing; Cambridge, MA; 1993). Good charts and illustrations take you on 14 star-hopping tours of selected regions of the sky.
Turn Left at Orion by Guy Consolmagno and Dan M. Davis (Cambridge University Press; Cambridge; 2000). A great star-hopping guide to the sky's best 100 objects, with finder charts and eyepiece sketches of object appearances.
ASTRONOMY - FREE SOFTWARE
Lists of most popular software.