# Most important feature of a telescope

I was taking an astronomy test, the following question came up:

The most important function of the astronomical telescope is:

1. Resolving power
2. Light gathering power
3. magnifying power
4. focal length

The focal length obviously means nothing here, as well as the magnification (that can always be done later). As far as resolving power and light gathering power go though, they both seem to be "equally" important, in that without the first, there is no detail, and without the second, there is no image.

It does seem that exposure time can compensate for a lack of light gathering power (what matters is the product between the two), but nothing can compensate for resolving power.

My professor seems to think the answer is resolving power. Why is this? Is this some well known idea in astronomy?

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I think resolving power is important, as it's the derivative of the others. You won't see any dimmer objects with huge magnification and too little light. You'll get many blurred dots if you gather lots of light but don't make it useful. Only combining the two you achieve a good resolving power - a decent image of distant objects. – SF. Nov 6 '13 at 15:48
The most important feature of a telescope is the astronomer using it. – Keith Thompson Nov 10 '13 at 23:34
That test is naive. There is no such thing as the most important feature. All are important, in different ways. One may become more important in a certain context. – Florin Andrei Jan 14 '15 at 20:29

$$\theta = 1.22 \frac{\lambda}{D}$$
where $\lambda$ is the wavelength of light and $D$ is the diameter of the lens.