SKY WATCH December 1995

by Harry Adams


The Holiday Season is upon us. No doubt many children and adults will receive telescopes and binoculars this Christmas. Unfortunately there are many more disappointing astronomical instruments to be purchased than there are satisfactory ones. The typical department store or camera shop usually can be counted on to carry one or two of the variety best described as toys. These are usually proclaimed as having 450 times magnification and have sleek black tubes with fancy looking accessories. After one or two frustrating sessions, these instruments usually languish in a closet until they are unloaded at a garage sale. Another budding astronomer meets disappointment at the hands of advertising hype.

How can you tell what telescope is best for you? First of all, try to buy from someone who knows telescopes. Clerks in camera shops and department stores will not usually fall into this category. Find a dealer in telescopes or scientific instruments instead. These stores will carry a variety of telescopes and accessories and can help you to decide what instrument best suits your needs. They will also provide after sales service and can tell you how to make certain adjustments by yourself.

Before you buy, look at the ads in Sky and Telescope or Astronomy magazines to get an idea of the types and sizes of telescopes that fall into your price range. Don't forget to convert the prices to Canadian dollars. ( I always multiply the US$ price by 1.5.) Keep in mind that a beginning astronomer doesn't need the biggest instrument on the planet. There are hundreds of things to be seen in telescopes under 100 mm (4 inches) in aperture and it will probably take several years for a truly dedicated observer to get around to all of them.

What do you look for when evaluating a telescope? Beginners usually store their telescopes indoors between observing sessions, so portability is an issue. This places a upper limit of about 200 mm (8 inches) on the aperture, since anything bigger is just too heavy to be easily carried in and out. (Although I do know people who lug around 14 inch Schmidt-Cassegrains on a regular basis.) Best to stay under 100 mm (4 inches) for a beginner.

What about the quality of the optics? You will have to take the advice of the dealer on this. Until you can look at some stars and learn to interpret their diffraction patterns, this is all you can do. Even the most reputable telescope manufacturers produce the odd lemon. A good dealer will help you return the telescope if its optics are defective. Some points are worth remembering. Most telescope problems are a result of poorly aligned optics. Newtonian reflecting telescopes tend to need alignment every few weeks. This is not a difficult procedure but if you are not mechanically inclined, stick to a refractor. A refractor costs more but comes aligned and stays aligned. Schmidt-Cassegrains are trickier to align but only require it once or twice a year.

What kind of eyepieces should you buy? The kind of eyepiece you buy is determined by the f-ratio of your telescope. The f-ratio is the ratio of the focal length of the main lens (objective) to the diameter (aperture) of the objective. A 100 mm aperture telescope with a 1000mm focal length has an f-ratio of 1000/100 = 10, sometimes written as f/10. If the f-ratio is 10 or greater, then buy Kellner type eyepieces. They are reasonably priced and perform well on high f-ratio telescopes. If the f-ratio is 8, then Kellner will do for low magnifications, but you should consider higher quality Plossl eyepieces for higher magnifications. If the f- ratio is less than 8, then only buy Plossls or better quality eyepieces. Kellners are not up to the critical requirements of low f-ratio telescopes.

Also never buy eyepieces made in the 0.96 inch diameter format common to department store telescopes. It is nearly impossible to find good quality lenses in this format. Stick to the standard 1.25 inch format. These lenses can be made to much higher standards simply because it is easier to work with the bigger pieces of glass used in their manufacture.

How many eyepieces should you buy? Eyepieces magnify the image produced by the main lens or mirror of the telescope. The shorter the focal length of the eyepiece the higher the magnification it will produce. For example, if the focal length of the telescope's main lens is 1000 mm and the eyepiece is 10 mm, then the magnification is 1000/10 = 100 times. The highest power eyepiece should produce a magnification no greater than twice the telescopes aperture in millimetres. So for a 100 mm aperture telescope the highest magnification would be 2 x 100 = 200 times. (Contrast this with the 450 times claimed for the 60mm camera shop telescope!) The lowest power eyepiece should give a magnification close to but not less than the aperture in millimetres divided by 5. So for the same 100 mm aperture telescope this would be 100 / 5 = 20 times. One or two more eyepieces with intermediate magnifications will round things out perfectly. Buy the low power and one intermediate power eyepiece first and add to collection at birthdays. An eyepiece case helps to preserve their quality.

Finder scopes on all but top of the line telescopes are usually a joke. Not much you can do but ask your dealer if you can trade it in if you don't like it. Try to get one that has alignment rings front and back. The single ring finder mounts don't work very well. The 6 x 30 mm finder is the minimum size I would recommend.

Finally, the mounting is what makes or breaks introductory level telescopes. A good firm tripod with a soundly built telescope pointing system is a must. The telescope should be easy to point and should stay where it is pointed. Slow motion controls on both axes of motion are required. Vibrations should disappear quickly if the telescope is tapped.

Not everything is available in moderate sized telescopes, so shop around, ask a lot of questions and beware of deals that sound too good. They usually are.

December's star chart is for 21h EST. The chart shows the sky for the middle of the month, looking to the South, from the Horizon (curved line near the bottom) to the Zenith (the point directly overhead, labelled Z on the chart). The directions southeast (SE), south (S) and southwest (SW) are shown on the Horizon line. Brighter stars are represented by larger dots, fainter stars by smaller dots. All times are given in Eastern Standard Time this month.

The Sun slows in its southerly trek this month rising at 7:45 midmonth and at 7:52 by month's end. Sunset occurs at 16:24 on the 15th and eight minutes later on the 30th.

Mercury is 4 degrees above the southwest horizon at sunset on the 15th and 11 degrees high on the 30th. This is not a favourable apparition.

Venus, at magnitude -4, is prominent after sunset, riding between 14 and 19 degrees above the southwest horizon in the latter part of December.

Mars shines at magnitude 1.3 and hangs 8 degrees above the southwest horizon at sunset, below Venus.

Jupiter is too close to the Sun to observe for most of the month and moves into the morning sky after conjunction with the Sun on the 18th. Look for it 5 degrees above the southeast horizon just before sunrise on the 30th.

Saturn shining at magnitude 1.1 is 34 degrees above the southern horizon after sunset and sets around midnight. The rings are still nearly edge on and present an unusual view in the telescope.

Moon FM - 6th, LQ - 15th, NM - 21st, FQ - 28th

Geminid Meteor Shower occurs from the 4th to the 16th with the maximum on the 13th and 14th. Expect 90 meteors per hour under ideal conditions, or about 30 per hour in town. This shower produces many bright, slow meteors.

Binocular observers can hunt for the galaxy M31 in Andromeda. This enormous galaxy is almost in our backyard, lying only 2 million lightyears away. Sharp eyed observers can pick it out with the naked eye.

References: Handbook for Visual Meteor Observations, Paul Roggemans, Ed., Sky Publishing Corp. Burnham's Celestial Handbook, R.Burnham, Jr., General Publishing Co., Toronto. The Observer's Handbook of The Royal Astronomical Society of Canada, edited by Roy L. Bishop, University of Toronto Press Incorporated.


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