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What Is The Light-gathering Power Of An 8-inch Telescope Compared To A 4-inch Telescope?

When it comes to choosing a new telescope, one of the most crucial aspects of its performance is how much light it can gather. With more light, the more (and fainter, distant) objects you will be able to see. But how much more powerful is an 8″ telescope over a 4″ telescope. Let’s find out!

So, what is the light-gathering power of an 8-inch telescope compared to a 4-inch telescope? The light-gathering power of an 8-inch telescope is 4x times greater than a 4-inch telescope.

You’re probably wondering how I got to those figures.

So let us now break down how I got to these numbers and what it all means for using a telescope.

How Do You Calculate The Light Gathering Power Of A Telescope

You calculate the light-gathering power of a telescope with the following equation:

π ×(diameter of objective)2/4.

That symbol is Pi by the way, if you are not familiar with it.

In mathematics, its commonly accepted that Pi = 3.14159

So with this in mind, we just need to do a little bit of converting and then plug the numbers in.

So, for an 8-inch telescope, the diameter of the objective is 200 Millimetres (or mm).

Now doing the formula again we get:

π ×(200)2/4 = 10,000

Now over to a 4-inch telescope.

Being have the aperture and having half the objective lens diameter length, we know it is 100 Millimetres (or mm).

Plugin the numbers in we get:

π ×(100)2/4 = 2500

And as you can see, 10,000 is 4x as much as 2,500.

I used this Pi calculator, by the way.

Another means of expressing this is by comparing the ratio of the respective diameters squared, in cm:

So, an 8 Inch objective (20cm) vs a 4″ (10cm) objective:

([20× 20] ÷ [10× 10] = 4).

What Is Light Gathering Power Of A Telescope?

The light-gathering power of a telescope is the ability of a telescope to collect light compared to the human eye. It is undoubtedly a telescope’s most important feature.

It is dependent on the function of the diameter of the objective lenses; also known as the aperture.

Therefore, comparing different telescopes can be done via the size of their apertures, which will, in turn, give us their respective light-gathering abilities.

The advantage of collecting more light with a larger aperture is that it will enable the user to observe fainter stars, nebulae, and very distant galaxies.


4x the power.

And as light-gathering is so essential to performance, an 8″ telescope is will unveil considerably more of the night sky than its ‘smaller’ equivalent.

Because light gathering is the most important purpose of an astronomical