True Field of View

True Field of View

An astronomer not only has to be familiar with the night sky but he also must know his telescope inside and out before he can explore the unknown (at least to him) regions of outer space. One critical characteristic of the telescope that must be determined is its true field of view (FOV), the area of the sky that you see when looking through the eyepiece.

Once you know how big of piece of the sky you’re seeing through an eyepiece, you can plot your course through the stars using a star chart.

To measure the FOV of your eyepiece, find a star located near the celestial equator. You can usually display this line on a star chart program, or locate it on a printed star chart. Use it to help you pick a star. We will take advantage of the fact that a star located on the celestial equator drifts about 1 degree every 4 minutes as the night goes on.

Center the star in the eyepiece and turn off the clock drive. Record the time it takes the star to drift to the edge. If the star takes 2 minutes to drift to the edge, the distance from the center of the eyepiece to the edge is ½ degree or the distance across the whole field of view is 1 degree. Simple, eh?

For my 6-inch f/8 newtonian, the star took 2 minutes 52 seconds to drift from the center to the edge of my 1 inch eyepiece. That’s almost 3 minutes or ¾ of a degree. So the true field of view is 1 ½ degrees. Using the same method I found that my finder-scope has a FOV of 5.25 degrees. I am ready to start plotting star hops on my star charts

Now I can set up my star chart program to create finder circles to match my finders FOV and my eyepieces FOV. Above is a star hopping chart to find M13 in Hercules. I use the freeware program Cartes du Ciel to make my star charts. I have one for each Messier object.