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Stepping off with fields of view is another handy method almost everyone uses sooner or later. A question that needs answering before this can be used to best advantage is, what is the angular field of view for the telescope and eyepiece I am using? I first used the following method over 25 years ago. This method is also mentioned in the January issue of Sky and Telescope. It works great for me! A star on or near the celestial equator moves westward at the rate of 15 degrees every hour or 1 degree every 4 minutes. To find the field of view of your telescope and any given eyepiece or the field of view of your spotter scope, perform the following procedure. If you have an equatorial mount, set the scope up and polar align it as you would for an evenings observing. Turn the scope to point at a right angle to the polar axis. It will now be pointed 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 degrees of angular field of view! EXAMPLE: If it takes 6 minutes for a star to drift across the field, the angular field of view is 6/4 =1.5 degrees. If you have an altazimuth mount you will need to use a star chart to find a star along the celestial equator. This method works with any type of telescope and any combination of eyepieces etc. |
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