How to Calculate the Field of View of Your Telescope
Last Updated: December 12, 2022
As you gaze up at the starry night sky, have you ever wondered how much of it you’re actually able to see through your telescope? The answer lies in the concept of Field of View, aka the amount of sky that can be observed at any given time through your telescope eyepiece.
A wider FOV means you can take in more of the sky in a single glance, while a narrower FOV gives you a more focused and detailed view of a particular object or area. Mastering this concept will allow you to better utilize your telescope and make the most of your stargazing experiences.
Field of View Formula
To calculate the field of view of a telescope, you will need to know the focal length of the telescope and the eyepiece used.
The formula for calculating the field of view is:
For example, if you have a telescope with a focal length of 1000mm and you are using an eyepiece with a focal length of 25mm, the field of view would be:
FOV = 25 / 1000 * 57.3 = 1.43 degrees
The value of 1.43 degrees in this formula is the result of the calculation for the field of view of the telescope. The field of view is the angular size of the area that can be seen through the telescope at a given time. In this case, the field of view is 1.43 degrees, which means that the area visible through the telescope will be 1.43 degrees wide.
A degree is a unit of angle measure, with 360 degrees in a full circle. One degree is equal to 60 arc minutes, which is why the value 57.3 is used in the formula (this is the number of arc minutes in a degree).
It’s also important to note that the field of view can vary depending on the size of the eyepiece and the distance of the object being viewed. A wider eyepiece will generally have a wider field of view, while a narrower eyepiece will have a narrower field of view. Similarly, an object that is closer to the telescope will appear larger in the field of view than an object that is farther away.
Thanks to the power of HTML and CSS, I have been able to create the below telescope Field of View calculator that will hopefully make your life a little easier.
Telescope FOV Calculator
Enter the telescope's focal length and the eyepiece's focal length below to calculate the field of view (FOV).
The field of view is degrees.
A word on “angular size”
The angular size is a measure of the apparent size of an object as seen from a particular perspective. It is typically used by deep sky object observers and astrophotographers and it is measured in units of angular measures, such as degrees, arc minutes, or arc seconds. The angular size of an object can be thought of as the angle formed by two lines that extend from the observer’s eye to the opposite edges of the object.
Objects that are closer to the observer will appear larger, even if they are physically smaller than objects that are farther away. This is why the Moon, which is actually much smaller than the Sun, appears to be about the same size as the Sun when viewed from Earth – it is much closer to us and that is reflected in its angular size.
Arc Minutes | Arc Seconds | Degrees |
1 | 60 | 0.01667 |
5 | 300 | 0.08333 |
10 | 600 | 0.16667 |
15 | 900 | 0.25000 |
20 | 1200 | 0.33333 |
25 | 1500 | 0.41667 |
30 | 1800 | 0.50000 |
35 | 2100 | 0.58333 |
40 | 2400 | 0.66667 |
45 | 2700 | 0.75000 |
50 | 3000 | 0.83333 |
In general, to convert from arc minutes to arc seconds, you can multiply the number of arc minutes by 60. To convert from arc minutes to degrees, you can divide the number of arc minutes by 60. To convert from degrees to arc seconds, you can multiply the number of degrees by 3600.
The full Moon has an angular size of about 31 arcmin or 0.5 degrees.
Alternative method: Drift Timing
The Drift Method takes advantage of the fact that the Earth rotates 360 degrees in 24 hours, which means that a star on the celestial equator takes 24 hours to return to the same position. This means that the Earth turns through one minute of arc every four seconds.
To use the drift method, the observer first points the telescope at a star and uses the focusing knob to make the star appear as a bright point of light. The observer then times how long it takes for the star to drift out of the field of view of the telescope, which can be used to calculate the FOV of the telescope.
Here is an example of how to use this method:
- Point the telescope at a bright star and use the focusing knob to make it appear as a sharp point of light.
- Observe the star for a minute or so and note how long it takes for the star to drift out of the FOV of the telescope.
- Divide the time it took for the star to drift out of the FOV by four seconds per minute of arc (the time it takes for the Earth to rotate one minute of arc). For example, if it takes 60 seconds for the star to drift out of the FOV, the FOV is 15 minutes of arc (60 seconds / 4 seconds per minute of arc = 15 minutes of arc).
- To convert the FOV from minutes of arc to degrees, divide the number of minutes of arc by 60 (the number of minutes in a degree). In this example, the FOV is 0.25 degrees (15 minutes of arc / 60 minutes per degree = 0.25 degrees).
This technique was mostly used by astronomers centuries ago. Nowadays we have access to a plethora of apps and calculators that make this calculation much easier and quicker.
The role of aperture and focal length in determining the field of view
Aperture and focal length are the two main factors that determine the FOV of a telescope. The aperture is the diameter of the objective lens or mirror, and it’s crucial for gathering light and giving you a clear and detailed view of celestial objects.
The focal length determines the magnification and how much of the sky you can see at any given time. Together, these factors determine the angular FOV of a telescope and the amount of sky you can observe in a single glance.
The larger the aperture, the more light is gathered inside the optical tube assembly (OTA).
Understanding the relationship between field of view and magnification
FOV and telescope magnification are related, but they’re not the same thing. FOV refers to the amount of sky you can see at any given time, while magnification determines the size and clarity of the image of a particular object.
As a general rule, increasing the magnification of a telescope will narrow the FOV, while decreasing the magnification will widen the FOV. So, it’s important to find the right balance between these two factors to optimize your observations.
Practical examples for determining the optimal field of view
- For observing deep sky objects such as galaxies and nebulae, a wider field of view is generally better. These objects are rather large, even though we can not see them with our naked eyes. For example, the Andromeda galaxy is as large as six moons next to each other. A telescope with a field of view of 1-2 degrees would be a good choice for this type of observation.
- For observing planets and other small objects, a narrower field of view is generally better. This allows you to see more detail in the object, as it fills a larger portion of the eyepiece. A telescope with a field of view of 0.5-1 degree would be a good choice for this type of observation.
- For observing the moon and other bright objects, a medium field of view is generally best. This allows you to see a good amount of detail, without the object appearing too small in the eyepiece. A telescope with a field of view of 0.75-1.5 degrees would be a good choice for this type of observation.
Ultimately, the best field of view for your observations will depend on your personal preferences and the specific objects you want to observe. Experimenting with different telescopes and eyepieces can help you to find the best combination for your needs.
Using angular sizes to plan your stargazing adventures
If you are tired of squinting at tiny objects in the sky or trying to fit a massive constellation into your telescope’s tiny field of view? Well, never fear – the answer to your viewing woes is right at your fingertips (or rather, right in front of your eyes).
By knowing the angular size of an object you want to observe in advance, you can easily prepare your telescope and plan the best eyepiece to make the most of your stargazing experience.
To give you a little headstart, I have prepared a list of 10 deep sky objects below. If you are looking for the full list, you can find it in my article on the Messier Catalog.
- The Orion Nebula (M42) has an angular size of 1.42×1.0 degrees (85×60 arcminutes)
- The Andromeda Galaxy (M31) has an angular size of 2.96×1.05 degrees (178×63 arcminutes)
- The Hercules Cluster (M13) has an angular size of 0.277 degrees (16.6 arcminutes)
- The Dumbbell Nebula (M27) has an angular size of 0.133×0.095 degrees (8.0×5.7 arcminutes)
- The Pleiades (M45) has an angular size of 1.83 degrees (110 arcminutes)
- The Whirlpool Galaxy (M51) has an angular size of 0.183×0.117 degrees (11×7 arcminutes)
- The Sunflower Galaxy (M63) has an angular size of 0.167×0.1 degrees (10×6 arcminutes)
- The Crab Nebula (M1) has an angular size of 0.1×0.067 degrees (6.0×4.0 arcminutes)
- The Ring Nebula (M57) has an angular size of 0.023×0.017 degrees (1.4×1 arcminutes)
- The Pinwheel Galaxy (M101) has an angular size of 0.367 degrees (22 arcminutes).
Now go out there, use the new formula you’ve learned about a little earlier, and enjoy the stars. And remember, the sky’s the limit (well, technically it’s infinite, but you get the idea).
Telescopes are complex optical systems and it can take a little time to learn how to use them optimally. I hope to make this learning process a little easier by providing some of my tips and my experience on how to get the most out of your telescope.
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