How to Calculate the Field of View of the Sky-Watcher SK909AZ3 Refractor Telescope

The area that you can see through your telescope is known as the true or actual field of view. The field of view of your telescope is determined by the design of the eyepiece of the telescope. Calculating the field of view of a telescope can be done by following a few steps.

Difficulty: Easy

Instructions

1. Find out the value of the apparent field of view. Every eyepiece has its own value of the apparent field of view. This value of your eyepiece is supplied from the manufacturer.

2. Find the value of magnification. The value for magnification can be found out by dividing the value of the focal length of your telescope by the focal length of the eyepiece. Assuming your telescope has a focal length of 750 mm and the focal length of your eyepiece is 10 mm, the magnification for your telescope is equal to 750 / 10 = 75. It is usually denoted as 75X. The focal length for this model is 900 mm.

3. Divide the value of the apparent field of view by the value of magnification to get the value of the field of view. For example, your eyepiece has apparent value of field of view at 50 degrees and magnification of 75X, then the value of the actual field of view is equal to 50 degrees / 75X = 0.66 degrees.

Tips & Warnings

• Take care that the magnification is not too high and the field of view does not have a very small value. Under such cases, finding the objects you want to watch through your telescope may become a little difficult.
• Make sure that your telescope does not have too large of a magnification. This would magnify the turbulence between the telescope and the object you are looking at. Ideally, the magnification should be 2X per each mm of aperture for good viewing.
• Start with a lower magnification to spot the object you want to observe. Once you have spotted the object, use a larger magnification telescope for a bigger image. Using a large magnification in the beginning can be often confusing.