t this point you are probably thinking of presents for the family astronomer. Alternatively, if you are the family astronomer, you will be leaving a trail of “helpful hints” among friends and family about what you would like to see under the tree. In this first article of Christmas suggestions we look at telescopes, the astronomical “big ticket” items.
You want the telescope you buy to mark the beginning of a lifetime of discovery, not to end up collecting dust in a closet. The fundamental commandment here is: if you are not an expert, deal with an expert. Go to your local science store. Discuss what you want, try things out before buying them. Don’t deal with stores where the salespeople know less about telescopes than you do. Last year a local optician was selling a kit for a small telescope. However, since the display model was assembled completely wrongly, one could not expect to get much useful advice from that quarter.
Telescopes comprise two main components. Firstly there is the “objective” which collects the light and forms an image of the piece of sky seen by the telescope. This is viewed through a magnifying lens called the “eyepiece.” The objective of a telescope may be a lens or a mirror. If the former, the telescope is known as a refracting telescope, or refractor; if the latter, it is known as a reflecting telescope, or reflector. A refracting telescope should have an objective lens at least 75mm in diameter. These telescopes come in two main types: achromats, where the problem of false colour fringes around bright objects has been pretty well corrected, and apochromats, where it is fully corrected. Apochromats are the Cadillacs among telescopes. Prices start at a few hundred dollars for a good quality achromat, and perhaps three times that for a similarly-sized apochromat.
Size for size, reflecting telescopes are cheaper, but since reflectors require a secondary mirror five or more centimetres in diameter in front of the objective to send the light to where it can be fed to an eyepiece, don’t get a reflecting telescope with an objective mirror smaller than about 15 cm. Once again, expect to spend at least a few hundred dollars on a telescope that is worth owning.
When visiting department or “big box” stores, we often see telescopes for sale. These often bear claims like ìincredible 400 times magnification,, and look impressively hi-tech. These instruments are usually useless. Warning sign number one is the magnification. The rule of thumb for the maximum usable magnification for a telescope is that it is roughly the diameter of the objective in millimetres. Thus, for 400 times magnification, the objective should be at least 40 cm in diameter. In most cases these telescopes have objectives in the five to eight cm range, which is totally inadequate. In addition the lenses are often plastic, and the accessories are usually non-standard. Do not buy one of these.
However, the extra effort will be worthwhile. The smile on the face of the family astronomer when he or she gets that first view of the Moon, Jupiter or Saturn will be something you won’t forget.
Jupiter rises at 3 p.m., Mars at midnight and Saturn at 4 a.m. The Moon will reach First Quarter on the 1st.
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.