Universe Today
How can you fairly compare one telescope to another? It’s all in the (angular) resolution.
The entire point of introducing telescopes into astronomy is to make distant things more visible. And to do that, you need high resolution.
We’ve all taken our phone cameras and started pinching and zooming, and very quickly, the results become rather lackluster. Images start to get all blurry and pixelated because when you zoom all the way in, you reach the resolution limit of your camera. If you had a more powerful camera, you could keep pinching and zooming and still get a clear, not blurry, not pixelated image.
Same thing for a TV. If you have an older TV with low resolution and you go up close to it, you can see the individual pixels, and then the image doesn't look as great. But if you have a newer TV, then you can still see a sharp crystal-clear picture even nice and up close.
Normally, when non-astronomers think of resolution, we think of our phone or computer screen or TV. We think of how many pixels are jammed together on the screen. The more pixels, the higher resolution. The sharper, the crisper, the better your display. This is something called linear resolution.
This is the density of pixels, down a line or squeezed into an area. But in astronomy, this is less than useful because different objects are at different distances. If you point a telescope at the moon, you can see some features like mountains or even individual boulders. You have a certain linear resolution when looking at the moon. But when you point your telescope at, say, Jupiter, all you can make out are gigantic cloud bands that are tens of thousands of kilometers across.
Your linear resolution looking at Jupiter is much, much worse because Jupiter is so far away. So to standardize everything, we measure a different kind of resolution, something we call angular resolution. This makes the resolution a known property of the telescope, something we can calculate, something we can write down. And this is independent of what you're trying to look at so we can judge and compare one telescope against the other in a fair way.
In astronomy, we usually measure angular resolution in terms of things called arcminutes and arcseconds. This is a very old, even ancient, method of dividing up the sky. And for once, we have the case where an old ancient tradition in astronomy still makes sense and describes exactly what it's trying to accomplish.
I want you to imagine that you're standing out on a big empty plane in the middle of nowhere, and you can see the horizon all around you. I want you to imagine that horizon is a giant circle surrounding you, and you will divide this circle into 360 degrees. So, 360 little chunks on this circle going all around you. Now you divide each of those individual degrees into 60 sections of its own.
So you take that one little degree, and now you chop that up into 60 little sections. Those little sections are called arcminutes or minutes of arc. And then you can take each one of those individual arcminutes, and you can subdivide each arcminute into sixty arcseconds. And if you need even finer divisions than that, we switch to decimals because we quickly get tired of all that divide by 60 nonsense.
And if you're wondering why 360 degrees, why 60 arcminutes in a degree, why 60 arcseconds in a minute, you can thank the ancient Babylonians who also gave us 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute. Coincidence? Absolutely not.
And it's angular resolution that we use to judge telescopes because the angular resolution is independent of what you're looking at. It's just something you know, how finely you can divide a circle surrounding you. And then from here, you can look at objects at different distances with the same angular resolution and you end up with different linear resolution. So if you have very, very fine angular resolution and you look at something up close, you can see tiny, tiny little details. And then if you look at something far away, you can't see those tiny details, but the angular resolution tells you how well you're going to do when looking at all these different objects.
