Calculating The True Field Of View Of A Telescope Eyepiece
One of the things that originally puzzled me when I first bought my telescope was the business of apparent field of view and true (or actual) field of view.
When you look through your telescope, the area of the sky you can see is known as the true (or actual) field of view. The field of view (FoV) of your telescope is determined by the design of the eyepiece of the telescope.
To calculate the FoV you need to first find out the value of the apparent FoV. Every eyepiece has its own value of the apparent FoV, which is supplied by the eyepiece manufacturer.
Next, work out the value of magnification. To do this, divide the value of the focal length of your telescope by the focal length of the eyepiece. For example, if your telescope has a focal length of 650mm (which is the case for a Sky-Watcher Explorer 130P), and your eyepiece has a focal length of 10mm, the magnification for your telescope will be 650 / 10 = 65.
Finally, divide the apparent FoV by the magnification to get the true/actual FoV. For example, if the eyepiece has an apparent FoV of 50 degrees and a magnification of 65X, then the actual FoV is equal to 50 degrees / 65X = 0.77 degrees.
If the eyepiece has an apparent FoV of 55 degrees and a magnification of 65X, then the actual FoV is equal to 55 degrees / 65X = 0.85 degrees.
So, the greater the apparent FoV, the greater the actual/true FoV. Also note that lower magnifications will give you a greater FoV than higher magnifications.
If you do not know the apparent FoV of the eyepiece, you can get a rough figure for the actual/true FoV by using the following formula:
42/Magnification
So, for the above example, we would get 42/65 = 0.65 degrees.
This is only an approximate figure, but it is good enough to give you an idea of how much of the sky you will be able to see when you use a particular eyepiece in your telescope.
About The Author:About the Author: John Dixon documents his astronomy exploits in his blog www.MyAstronomyBlog.com. He also maintains a bookkeeping software application called MyBookkeepingManager.
Recent Articles On This Topic
Planet Observing - An Astronomy Beginners Primer
Here are a few tips on viewing our neighbors in the Solar System.
Astronomy Using Binoculars
Although they are not as appealing as a telescope, a good pair of binoculars are an excellent alternative, and will enable you to establish whether or not astronomy is for you without needing to go through the expense of buying a telescope. Binoculars are easy to use, they are light, and they are versatile.
Astronomy Eyepieces
The purpose of an eyepiece is to magnify the focused image, and different eyepieces can be used to give different magnifications and fields of view. You will need to experiment with different ones to find the best one for a particular application.
What To Look For In A Telescope Mount
Although when anyone ever thinks about astronomy they automatically think about the telescope, it is also important to have a good mount and tripod.
Is The Sky-Watcher Explorer The Right Telescope For Me?
The Sky-Watcher Explorer 130P is aimed squarely at the beginners market. It is a relatively cheap (about £200) telescope that will provide a good introduction to telescope astronomy.
What Is The Significance Of A Telescope's Focal Length?
All things otherwise being equal, slower focal ratios produce less aberrations, but in many designs, a faster focal ratio produces a more compact telescope, meaning there is always some sort of compromise.
Calculating The True Field Of View Of A Telescope Eyepiece
One of the things that originally puzzled me when I first bought my telescope was the business of apparent field of view and true (or actual) field of view.
Three Types Of Telescopes - Astronomy 101 For Beginners
Of the many types of telescopes, a good starting price for a beginner telescope will be around $400.00.
New Telescope Eyepieces Will Improve Even The Finest Telescopes
Should I Change My Eyepiece? Now that you have bought or perhaps have been given a new telescope, you're thinking about making it even better. One the first things new telescope owners usually replace are the telescope eyepieces.
Refractor Telescopes Vs Reflector Telescopes
If you are interested in purchasing a telescope, you have probably heard about refractor and reflector telescopes. However, you might be unaware as to the differences between the two and which one you should choose. The following will discuss both of these telescopes in more detail.
Navigation
