Rule

Rule #2 The maximum usable magnification of a telescope depends on the size of its objective. During times of very good weather, the maximum magnification used should be no more than 50 times per inch of aperture.

Given Rule Number Two, it’s no wonder the view in the 2.4 inch telescope is less spectacular. The normal maximum magnification of this telescope is 2.4 multiplied by 50 which amounts to 120x! We could have probably seen just as much detail at 120x as we did at 300x. In fact we might have seen more detail at the lower magnification since the brighter image would have made any color in the image appear richer. Richer colors often cause subtle details to be seen that might otherwise have been missed!

There are rare exceptions to Rule Number Two, if you happen to live in a place with very dry air (say New Mexico or Arizona) and the atmosphere happens to be extremely steady on the night you are observing, you might be able to use up to 100x per inch. But even in such an unusual climate, the air won’t be steady enough for that much magnification on most nights. I live in the southeastern U.S. where the air is normally very humid and prone to some turbulence. Maximum magnification for me is more like 30x per inch!

I wouldn’t let Rule Number Two bother me too much if I were you. High magnifications are really only useful when observing bodies which are within the solar system; i.e., the sun, moon, and planets. Other objects which are much farther away (such as galaxies, glowing interstellar clouds, and star clusters) are normally best seen with relatively low magnifications. That is, magnifications much lower than 50x per inch. Such extremely distant bodies are called deep sky objects. Deep sky objects are often so huge that they will fill your entire field of view even at low power. Also, these objects are typically not as bright as planets, so the brightening from using low power usually helps you see more detail not less. The brighter you make a deep sky object, the more likely you are to see any color it may have.

Besides, even if you were using your telescope in the best of conditions, the physical characteristics of light place a limit on what you can see regardless of magnification. This effect is known as Dawes’ Limit and is named after the scientist who first described it. Dawes’ Limit essentially says that for each particular aperture, there is a minimum size for any detail which can be seen. The bigger the objective, the smaller the detail. Thus, even under excellent viewing conditions, the smallest possible detail which may be seen in a six inch telescope is still much finer than the smallest possible detail visible through a 2.4 inch instrument. A telescope’s resolution can be thought of as the tiniest possible detail visible given the conditions (be they good or bad) under which you are observing. Dawes’ Limit represents the maximum resolution which may be achieved with a particular telescope under perfect observing conditions.

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.