The Lake County Astronomical Society

FOV Star-Hopping

There's a trick used by every star-hopper: hunting down objects by "stepping off" using the field of view (FOV) in the telescope. But you first have to determine the actual field of view for the telescope and eyepiece combination you're using. There are several methods, but the one that's most accurate also happens to be easy to use at your scope.

A star on or near the celestial equator moves westward at the rate of 15 degrees every hour, or 1 degree every 4 minutes. The following procedure uses that principle to find the field of view of a telescope with any given eyepiece. If you have an equatorial mount, set the scope up and align it with the pole just as you would for an evening's observing. Turn the scope to point at a right angle to the polar axis. It will now be pointed along the celestial equator. If you have an altazimuth mount, such as on a Dobsonian-mounted telescope, you will need to use a star chart to locate a star along the celestial equator. Find any convenient star near the equator and position it at the western edge of the field of view. Note the time. Let the star drift to the east side. Note the time. Divide the time in minutes that it took the star to drift across the field by 4 to get the number of degrees in the angular field of view. For instance, if it takes 6 minutes for a star to drift across the field, the angular field of view is 6 + 4 = 1.5^o, or a one-and-a-half degree field of view. This method works with any type of telescope and any combination of eyepieces. If you write down the FOV for all your eyepiece combinations, you'll probably only need to make this calculation once. Moreover, some observers cut out a set of circles in pieces of clear plastic to use as ready-made FOV scales with their charts.