Selecting a Christmas telescope

Christmas is coming! If you are considering getting a telescope for the family astronomer, you are probably about to go shopping, and worrying about making the right choice. You want to open new horizons, not acquire another cupboard dust gatherer. There are some traps out there. I hope these notes help a little in avoiding problems.

The two main bits in a telescope are the objective, which is a large lens or mirror that has the task of collecting light and making an image, and the eyepiece, which magnifies that image. Most astronomical objects are faint, so the objective should be as big as possible or affordable, in order to collect the maximum amount of light. For astronomy, don’t bother with a telescope having an objective smaller than about 70 mm. The resolution, or ability to see fine detail, depends upon the size and quality of the objective. Our eyes cannot resolve the fine detail the telescope objective captures. That is where the eyepiece comes in. It magnifies the detail revealed by the telescope objective to a point where our eyes can discern it. That means, in general, the larger the telescope objective, the better its ability to discern fine detail, so the larger the magnification we can use. This gives us a rule of thumb that always applies; an average quality telescope will support a maximum magnification of about one times per millimetre of objective diameter. So a telescope with an objective 75 mm in diameter will be useful for magnifications up to about 75 times. A high quality telescope may stretch to about two times per millimetre of objective diameter, so our 75mm objective might be usable with a magnification of around 150 times. That is not a magnification to be sneezed at. It will reveal the rings of Saturn, the cloud belts of Jupiter, the largest satellites of those planets, double stars and star clusters. The Moon will look incredible. Instead of a disc in the sky it will become a mountainous, exotic landscape.

Unfortunately there are telescopes out there that really should not be sold. These usually turn up in department and discount stores, where the sales people know little about what they are selling. These offer magnifications of 300 times or even more. Something is wrong here; our rule of thumb suggests such a telescope would need an objective 30 cm in diameter, and a telescope tube at least that size. Is the box big enough? These instruments are usually useless.

Since 2009 is the International Year of Astronomy, some manufacturers have produced some really inexpensive telescopes – less than $100 in some cases. However you need to be sure, so I suggest really strongly you go to the local science store for buying telescopes. Deal with people who know telescopes and will stand behind what they sell. There are major Internet dealers, but in that case have a chat a member of the local astronomy club first. Get advice before reaching for the credit card. Good Luck!

Jupiter lies in the Southwest during the evening. Mars and Saturn rise around midnight and 5 a.m. espectively. Venus lies low in the predawn sky. The Moon will be Full on the 2nd and Last Quarter on the 6th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Tel (250) 493-2277, Fax (250) 493-7767

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