It took me a while to understand the concept of a field of view (FOV) when working with my telescope.
I had always thought that FOV was something you could measure simply with your eyes, but it turns out that it’s a term used to describe how much area of the sky can be seen at once.
In this post, I will explain what field of view really is, and I’ll also find out how you can easily calculate FOV.
What’s A Field Of View In Astronomy?
The field of view (FOV) is the area that an observer can see when they look at a part of the sky.
It depends on how far away something is from us. For example, if I were to look up at the night sky with my naked eye, I would be able to see only a few stars in the sky.
If I were to use binoculars, I could see more stars. And if I use a telescope, I’ll be able to see a lot more stars and star constellations.
The field of view gives me a larger image of what I can see with your telescope.
However, the further away something is, the less detail I will be able to see.
This is because light takes time to travel through space, and so distant objects appear smaller than nearby ones.
That’s why I need a powerful telescope that’s much more sensitive to light to see details of a distant galaxy.
When astronomers talk about the FOV of telescopes, they are talking about the size of the image produced by the telescope as seen by someone looking at the object.
For example, imagine that you have a telescope that has a 2-inch mirror.
If you put this telescope next to a person standing 10 feet away, then the person will be able to see the entire 2-inch diameter circle of the telescope.
But if you move the telescope 20 feet away, the person will only be able to see half of the 2-inch diameter circle.
Now, let’s say you want to know the exact distance between two galaxies. You could measure their separation using a ruler or measuring tape.
But what if you wanted to do it without having to go out there and measure them yourself?
Well, you could just take a picture of one galaxy and compare its size to another galaxy in the same photo.
This is called “astrometry” and it’s used all the time by professional astronomers to determine distances between things like stars and planets.
Astronomers also use astrometry to find exoplanets.
So, when you’re trying to figure out where a planet might be located, you can use astrometry and field of view to help you pinpoint the location.
Why Do I Need To Know My Field Of View?
There are many good reasons why knowing the field of view is important for my backyard astronomy.
It Helps Me Understand My Image Size
As I mentioned, the field of view is the size of the image of the sky I will be able to see with my telescope.
This means, the better my field of view, the more stars I’ll be able to observe in the sky. It will give me a wider field and greater magnification options.
It Helps Me Choose Which Telescope To Buy
Choosing the right telescope is very important. A telescope with a wide field of view allows me to see more stars and star clusters than a telescope with a narrow field of view.
A wider field of view will also make it easier to spot faint objects such as nebulae and galaxies.
It Can Help Me Find More Objects In The Sky
A wider field of view allows more objects to fit into the frame of my telescope. This means that I can capture more images of celestial objects.
It Helps Me Decide Which Eyepiece To Get
If I plan on getting an eyepiece with a larger aperture (the opening in the front of the telescope), I will need a bigger field of view.
It Helps Me Understand What Magnification To Aim For
Magnification refers to how detailed the image of the object appears through the eyepiece. The higher the magnification, the smaller the image becomes.
You may not realize it, but the field of view plays a role here too.
If I choose a magnifying power that is too high, I won’t be able to see much detail because the small field of view will limit the amount of light hitting my eye.
I always aim for a magnification level that allows me to see details in the objects I am observing.
For example, if I am looking at Jupiter, I don’t want to try to get a close-up look at the moons orbiting around Jupiter. Instead, I want to focus on Jupiter itself.
How To Calculate Field Of View
The way how to calculate your telescope’s field of view depends on your eyepiece and the different specifications the manufacturer of your telescope and eyepiece provided.
Most telescope eyepieces will indicate focal length, lens configuration, and AFOV (apparent field of view).
Types Of Field Of View
There are two types of field of view that you can calculate: apparent field of view and true field of view.
Let’s take a look at the different image scales of these two types to find out how to calculate them.
Apparent Field Of View
An apparent field of view is the angle of the field of view in degrees that I can see with my telescope.
If I have a focal length of 25 mm, then I will have a 50-degree apparent field of view.
However, this is just the field of view when I look through your eyepiece without attaching it to my telescope.
So when an eyepiece manufacturer says that an eyepiece has a 50 degrees AFOV, then this means that I will be able to see 50 degrees of the sky through this particular eyepiece – without the use of my telescope.
When I then attach the eyepiece to my telescope, the field of view will be reduced to the true field of view.
True Field Of View
The true field of view is the actual angle of the field of vision that I can see through my telescope. It includes any obstructions between the eyepiece and the object being observed.
To calculate the true field of view, also called TFOV, I simply take your apparent field of view and divide it with the magnification of my telescope.
Some telescopes tell you the exact magnification, but you can also easily calculate this. Just take the telescope’s focal length and divide it by the focal length of the eyepiece.
Instead of the apparent field of view, some manufacturers also just provide the diameter of the eyepiece’s field stop.
You can also calculate the true field of view with the field stop. Just multiply 57.3 times the field stop diameter. Then divide this by the telescope’s focal length.
Both the focal length and the field stop diameter must be in mm to get the correct result.
Calculating Field Of View With The Drift Method
Another method to calculate the field of view is using the drift method. This method uses the focal length of the eyepiece or the magnification.
The drift method is based on the concept that stars seem to cross the sky westwards. This happens at a speed of 86,100 seconds (23 hours and 56 minutes).
What You’ll Need For The Drift Method
There are a couple of things you will need when calculating your field of view with the drift method.
Your telescope must use an equatorial mount or any mount which shows the correct set declination.
It’s also a good idea to have a star atlas or a star app on your phone. This will help you check the different declinations of celestial objects and stars.
You will also need a scientific calculator, as well as a stopwatch.
Using The Drift Method To Calculate Field Of View
This is a hands-on way to calculate the field of view, but the first thing is to pick a bright enough object in the sky.
It’s important that the star is easily visible and it should also be near the celestial equator. Some good stars are Virgo Porrima, Taurus Aldebaran, or Orion Mintaka.
These are close to the celestial equator and they are easy to see.
Then I look up the bright star’s declination in my star atlas, or you can also use star software for this.
Now, I need to set up my eyepiece and the telescope. I check again that I can see my chosen object well enough.
My telescope doesn’t, but if your telescope has a motoring drive, you should turn this off now.
Next, I look through the eyepiece and ensure my star moves westward from the center of my eyepiece.
The chosen object has to be in the center of the eyepiece or else the measurements for the diameter of the field of view will be off.
I usually adjust my telescope slightly, so the star is going through the middle of my view.
Now that I am set up properly, I start taking measurements. I adjust the right ascension of my telescope so my object is slightly outside my view on the Eastern edge.
Then I pick up my stopwatch and see how long it takes for the object to show in the view of my eyepiece, then the star drifting across and disappearing from my field of view.
I note this down in seconds. Just to be sure that my measurements are correct, I take a couple more readings.
Typically, I take three readings and then average them.
Time to get the scientific calculator out. The true field of view can now be calculated using this formula: (Drift Time * (cos(star’s declination)) * 360°) / 86,160
Does Field Of View Impact The Brightness Of The Image?
The field of view does not impact the brightness of the image. However, it can affect the sharpness of the image.
If I have a large FOV, there may be some blurring in the edges of the image. To avoid this, I try to keep the size of the FOV small.
The smaller the FOV, the sharper the images will be.
Apps And Software That Help To Calculate Field Of View
If you don’t want to use a traditional star atlas, then apps and other software can be very handy to calculate the field of view.
Here are some of the apps I found useful.
Stellarium is one of the most-used apps for hobby astronomers. It’s ideal when you want to calculate the field of view for observing or even astrophotography.
It allows you to enter the lens specifications and sensor data of a DSLR camera. However, you will need the Oculars plugin for this.
Although FOViewer Delux is only available for android phones, it’s very handy to find a telescope’s magnification and the right eyepiece combination.
A big benefit is that you can superimpose a field of view on some popular stars and celestial objects.
This field of view calculator does what it says on the tin. Just put in all the information, and it automatically calculates the field of view of your telescope or astro-camera.
Field Of View In Astronomy Binoculars
Field of view is used in a range of different lens devices. From telescopes and cameras to astronomy binoculars, it’s an important measurement that impacts what an observer can see.
While you will need to calculate the FOV with a telescope, a pair of binoculars should show the FOV on the side of one of the eyepieces.
There are typically a few numbers on the eyepiece, but the FOV can usually be found under the magnification and objective F size.
For example, if the numbers on your eyepiece say “9.2/400 ft/120 m” then the 9.2 is the field of view.
The other numbers indicate that if you use your binoculars at a distance of 120 m, then you will be able to see 400 feet in width and height.
Unfortunately, not all binocular brands indicate the field of view so easily on their eyepieces. One of my favorite binoculars didn’t have the FOV written on.
This meant I had to calculate it. Taking the above example with “400 ft/120 m”, take the first number (400 ft) and divide this by 52.4.
The result will be your field of view in degrees.
Some manufacturers only give you the first specification in meters. If that’s the case, you will need to divide this first number by 16 instead.
This will show the correct field of view for your specific pair of binoculars.
Frequently Asked Questions
How Do Telescopes Increase Field Of View?
The main reason why telescopes increase the FOV is that they magnify the image. The more powerful the telescope, the larger the image becomes.
However, the size of the image depends on other factors as well, such as magnification.
What Is a Wide Field Of View?
Wide field of view (WFOV) is simply an alternative term for the apparent field of view wider than 65˚.
Some astro cameras can capture images even with a 90° field of view. This captures more stars and celestial objects in an image.
In conclusion, knowing how to calculate FOV is important for many reasons. Whenever I am looking to get a new eyepiece for a telescope, finding out about the size of the field of view is one of my priorities.
I like observing as many objects in the sky as possible. For astrophotographers, FOV is essential because it impacts the size of the image that can be captured. If you want to make the most of your backyard astronomy, knowing the different methods of calculating the field of view is a great skill to have.