There is another reason why you won

There is another reason why you won’t want to use high power with a small telescope. The true field of view is the amount of sky which you can see when viewing through your telescope. The higher the power you use, the smaller the true field of view. 2.4 inch telescopes are notorious for having a very small true field of view even at low magnifications. At high magnification their true field of view is ridiculously small. A small true field of view makes it very hard for you to find an object when looking through the telescope. When you finally do see it, you will only get a glimpse for a few seconds before the Earth’s rotation causes the object to leave your field of view! Add to this the relatively shaky mounts which come with many of the lowest priced 2.4 inch telescopes and . . . well you’ll need a lot of patience.

By-the-way, many manufacturers offer motorized drives for telescopes which may be used to counteract the effects of the Earth’s rotation. Such a drive will automatically keep an object in your field of view.

There are also reasons besides magnification which contribute to the smallness of the average 2.4 inch telescope’s true field of view. For instance, the relatively small diameter of its eyepiece or ocular is an important factor (throughout this book you may see the terms eyepiece and ocular used interchangeably). The eyepiece is the part of the telescope that you put your eye near to get a view. Eyepieces on many 2.4 inch scopes are less than an inch in diameter: 0.965 inches to be exact. Eyepieces on most other telescopes are 1¼ inches in diameter. The larger lenses in the larger oculars, in certain instances, allow the true field of view to be wider by increasing the apparent field of view. The apparent field of view of an eyepiece can be thought of as how big the view seems to be to your eye. The maximum width of the view that the typical human eye can take in at once is an angle of about 120 degrees. So if you look into an eyepiece and the image it shows you extends across one third of your eyesight, the apparent field of view of the eyepiece is 120 divided by 3, that is, 40 degrees.

One more thing you should know. Even some 4.5 inch or 5 inch reflector telescopes come with 0.965 inch oculars, so watch out!

Rule #3 A slight increase in an objective’s diameter may yield a very large increase in the amount of light collected!

This is one of the most interesting truths about telescopes. In illustration, let us compare a telescope containing a 3 inch objective with one possessing a 4 inch objective. The 4 inch objective is only 33% wider than the 3 inch, yet this slightly wider objective will collect 58% more light! Why is this true? Even though the diameter of the objective is only slightly bigger, the surface area of the objective increases by a much larger amount. The more surface area there is, the more light the objective can gather. For those of you with some math background, remember the area of a circle is pi times r squared (where r is the radius of the objective; that is, half of the diameter).