Embarking on a celestial journey with a telescope opens a window to the wonders of the night sky, revealing intricate details of distant objects.
Central to this experience is the concept of telescope resolution, the telescope’s ability to unravel fine features and distinctions within celestial realms.
In this exploration, we will delve into the fundamental aspects of telescope resolution, understanding how it influences our observations and shapes our perception of cosmic phenomena. From the theoretical limits to practical considerations, join us on a journey to comprehend the intricacies of telescope resolution and enhance your astronomical pursuits.
What is telescope resolution?
Telescope resolution refers to the ability of a telescope to distinguish fine details in celestial objects.
It is measured in terms of the smallest angular separation between two points that can be observed as separate entities. Telescope resolution is crucial for revealing intricate features such as craters on the Moon, rings of Saturn, or multiple stars in a binary system.
But before delving too far into the intricate world of resolution, it’s essential to grasp the fundamental units of measurement used by astronomers—arc seconds, arc minutes, and degrees. These units play a crucial role in quantifying the size of celestial objects in the vast expanse of the night sky.
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Degrees, minutes, and arc seconds
Degrees
The word arc refers to the arc of the sky and if you look from the horizon to zenith, which is the highest point in the sky (directly overhead), you can break down that entire arc into 90 degrees.
There are several quick and easy ways you can measure the degrees in the sky.
You can use apps and software to see exactly where the degree markers exist but you can also use a more crude method involving your hands to help you measure these distances.
If you make a fist and put it out at arm’s length, that fist makes up roughly 10°. If you spread out your pinky and thumb (think Hawaiian surfer), that’s approximately 25°.
You can also use some constellations to help you.
One popular point of reference for observers in the Northern Hemisphere is the Big Dipper. The distance from Megrez to Dubhe (which make up the top part of the bowl of the ladle) is about 10 degrees. And the distance from Dubhe to Polaris (the north star) makes up just under 30°.
Other times, your instruments can help you instantly gauge how large a section of the sky is. If you’re looking through your finder scope, you’re probably seeing close to 6° of the sky.
Minutes
You can break down each degree into 60 minutes of arc which is usually displayed as 60’. For some reference here, the full moon is about 30’ across. That’s also how wide the sun is (which is why we can have a total eclipse).
You might be surprised to find out how large some deep sky objects can be when measured in degrees and minutes. For example, the Andromeda Galaxy is 3° across which is six times the diameter of the full moon!
Now that you have a grasp on degrees and minutes, we can move on to arc seconds.
Arc Seconds
Each arc minute can be broken down into 60 arc seconds (60”). Arc seconds can be used as measurement units for the planets to give you some reference.
For example, when I observed Mars at opposition in December 2022 it had a disk diameter of 17.1 arc sec. But at its next opposition in September 2035 it will be close to 25 arc sec. That’s an increase of almost 50% in size!
To give you some perspective, Jupiter at its smallest is 30 arc-seconds in diameter. And at opposition? The planet can appear almost 50 arc-seconds across!
Out of all of the different measurement units, arc seconds are most relevant to resolution in a telescope.
Now let’s talk about resolution
Resolution is the measure of how much detail your telescope can make out.
This concept is easily illustrated if you have ever flown into a city at night.
From a higher altitude, a lot of the light inside the city looks like it flows together. But as you start to get closer, you can start to distinguish between street lights, vehicles, buildings, etc. The same idea applies to objects inside your eyepiece especially for things like double stars.
Double stars often appear like a single star when viewed with the naked eye but as soon as you whip out the binoculars or your telescope, you can “split” them and you may see an additional star (or even multiple stars)
It’s an especially satisfying feeling whenever the stars are different colors!
Resolution on a telescope is measured based on the smallest distance in “arc seconds” between two points that can be seen as individual objects in the telescope.
How to find the resolution of your telescope
You can estimate the resolution of your telescope by using a formula: 120/aperture (mm) or 4.5/ aperture (inches).
Let’s apply this to my Dobsonian with 200mm in aperture.
120/200mm = .6.
This means that my theoretical resolution limit should be slightly over half an arc second. That’s just slightly smaller than the Cassini Division on Saturn’s rings when Saturn is at its closest, which explains why I often see it pretty good in my 8 inch reflector telescope.
But just to clarify, if my telescope has a resolution limit of, let’s say, 0.6 arc seconds, and there are two stars positioned only 0.5 arc seconds apart, they will appear as a single star when observed through my telescope.
Just be aware that this upper limit on resolution is not a “guaranteed” limit because of the unsteadiness of the sky. At times, I could struggle to do better than one arc second in resolution!
Some telescopes will advertise the resolution limits but other times you will have to work out the formula to find out the limit for yourself.
Size chart of common night sky objects
| Celestial Object | Size in Arc Seconds |
|---|---|
| Moon | Approximately 1,800 arc seconds (30 arc minutes) in diameter. |
| Jupiter | Varies from about 30 to 49 arc seconds in diameter during opposition. |
| Saturn | Varies from about 15 to 20 arc seconds in diameter. |
| Mars | Ranges from about 3.5 to 25.1 arc seconds during opposition. |
| Venus | Varies from about 10 to 63 arc seconds in diameter. |
| Andromeda Galaxy (M31) | 3 degrees and 6 arc minutes, equivalent to 13,600 arc seconds. |
| Orion Nebula (M42) | Approximately 85 arc seconds in diameter. |
| Pleiades Star Cluster (M45) | Around 110 arc minutes, equivalent to 6,600 arc seconds. |
| Ring Nebula (M57) | Roughly 1.5 arc minutes, equivalent to 90 arc seconds. |
| Hercules Cluster (M13) | Approximately 20 arc minutes, equivalent to 1,200 arc seconds. |
| Sombrero Galaxy (M104) | About 5 arc minutes, equivalent to 300 arc seconds. |
| Whirlpool Galaxy (M51) | Approximately 11 arc minutes, equivalent to 660 arc seconds. |
| Beehive Cluster (M44) | Around 95 arc minutes, equivalent to 5,700 arc seconds. |
| Lagoon Nebula (M8) | Approximately 120 arc seconds in diameter. |
| Crab Nebula (M1) | About 6 arc minutes, equivalent to 360 arc seconds. |
Final word
Telescope resolution serves as the key that unlocks the mysteries of the cosmos, allowing astronomers to peer into the intricate details of celestial objects. From discerning the delicate features of distant planets to unveiling the subtle beauty of double stars, the resolution of a telescope shapes the richness of our observational experiences.
As we navigate the celestial world, understanding and appreciating the factors influencing telescope resolution enhances our ability to explore the vast expanse of the night sky and revel in the wonders that lie beyond our earthly bounds. May your future astronomical endeavors be guided by the clarity and precision that telescope resolution affords. Happy stargazing!
Want to get started in astronomy?
Our free telescope cheat sheet breaks down the key factors to choosing a telescope and shows you how to get stunning views of planets, nebula, and galaxies!
