Exploring Telescopes' Visual Perspective: A Crash Course on Key Concepts
Straight Up Astronomy:
Eyes Wide Open, y'all! We're diving into the nitty-gritty of the field of view in astronomy. To break it down, the field of view is like how much sky you're peeping through your space-goggles, measured in degrees.
Take, for instance, if we had eyes in the front and the back of our heads. Then we'd be rocking a 360-degree field of view, 'cause we'd be able to see everything around us. But in reality, we're always looking forward, and a typical human eye without any help has an approximate field of view of about 210 degrees.
Now, in astronomy, the field of view commonly refers to how much sky you can check out through the eyepiece of a telescope. If one scope has a larger field of view than another, you'll be able to see more of the surrounding sky with that one.
Measuring the Field of View With Your Telescope
Telescopes don't function like binoculars because you can switch out the eyepieces. Changing the eyepiece means changing the magnification and, consequently, the field of view.
Calculating the field of view ain't as easy as you'd think. There are a few methods, but we're gonna cover the easiest one here. Let's break it down into two steps.
- Step 1: Magnification
Figuring out the magnification is the piece of cake part. Most amateur astronomers already know this trick, but I'll run through it for the newbies out there. The formula is:
Telescope magnification = telescope focal length / eyepiece focal length
Every telescope has a focal length, which specifies how far light travels from the objective lens (or primary mirror) to the eyepiece where the image is formed. You'll find this figure somewhere on a sticker on the optical tube of your telescope, or you might need to look up the specs online.
All eyepieces also have a focal length, which can usually be found on the barrel of the eyepiece or on top. You divide the telescope's focal length by the eyepiece's focal length to get the magnification.
For example, let's say my scopes' focal length is 650mm, and I'm using an eyepiece with a focal length of 25mm. That means I'm seeing things nearly 26 times larger. If, instead, I use a 3mm eyepiece, I get a whopping 217x magnification.
- Step 2: Calculating the True Field of View
This is where things get a little tricky. Your eyepiece has an apparent field of view (AFOV) value that's unique to the eyepiece itself. This is the number of degrees of sky that the eyepiece would show you if you held it directly up to your eye without any telescope. (Sadly, that wouldn't be very useful; we need a telescope to magnify and focus the view!)
Not all eyepieces have the apparent field of view specified, so you might need to refer to the eyepiece's or telescope's original packaging or search it up online. If I can't find the specs, I usually take the apparent field of view as 50 degrees, especially if the eyepiece isn't top-notch.
The true field of view is the number of degrees your eyepiece shows you when you use it with your telescope. To calculate it, you divide the eyepiece's apparent field of view by the magnification it provides when used with the telescope.
For example, I've got a zoom eyepiece that can go from a 24mm focal length to an 8mm focal length, with an inherent apparent field of view of 60 degrees. With my 650mm scope, it gives me a magnification of about 27x when used at 24mm. So, 60 divided by 27, and I get about 2.2 degrees, which is roughly the width of four full Moons.
On the other hand, when it's set to an 8mm focal length, it provides about 81x magnification, an inherent apparent field of view of 40 degrees, dividing 40 by 81, and I get approximately 0.49 degrees, which is slightly less than the full moon's width.
Why Worry About the Field of View?
There are three major reasons to consider the field of view.
- Navigating with Star Charts: If you're not using a computerized GoTo telescope, you're probably star-hopping. That means you're using a star chart to find your targets. Awareness of the field of view is crucial because otherwise, you won't know which stars will be visible through your eyepiece, and you could get lost trying to find the next one.
- Focusing: As a general rule, the higher the magnification you're using, the smaller your true field of view will be. Small objects, like planets or planetary nebulae, often require a smaller field of view for easy observation, but for focused views of star clusters and nebulae, a wider field of view is best. Keep in mind that if your view isn't properly focused, adjusting the focus on nearby stars can help ensure your target is sharp.
- Rich Observing Experience: High magnification can make your target appear larger, but it can also make the experience less visually appealing, as you'll have a smaller view of the surrounding stars. A larger field of view can help preserve the context and depth of the objects you're observing, making the overall observing experience richer and more meaningful.
Is the Best Field of View Out There?
Generally, there's no "best" field of view, as the ideal field of view depends on what you're observing and the telescope you're using. Small objects, like the Moon, planets, or small planetary nebulae, can often be observed with a smaller field of view, but larger objects like star clusters and nebulae typically require a wider field of view. However, keeping an open mind and enjoying the view is always the name of the game!
- In astronomy, the field of view is comparable to the amount of sky observed through a telescope's eyepiece, often measured in degrees.
- Changing the eyepiece in a telescope alters both the magnification and the field of view, as the formula for magnification is telescope focal length divided by eyepiece focal length.
- The true field of view is calculated by dividing the eyepiece's apparent field of view by the magnification it provides when used with the telescope.
- The true field of view plays a significant role in navigating with star charts, focusing, and creating a richer observing experience in astronomy, especially when dealing with small objects like planets or planetary nebulae.
- When comparing telescopes, refractors often have a smaller field of view compared to reflectors, making them less suitable for observing large objects like star clusters and nebulae.
- Understanding the relationship between telescopes, eyepieces, the field of view, and the objects observed in space-and-astronomy contributes to the advancement of science and the appreciation of technology in our exploration of the universe.
