Click on image to Enlarge (Source: NASA - Chandra X-Ray Observatory )
The simplest method of measuring the field of view relies on the use of a star chart.
Knowing north and east in the sky, you can easily turn your star chart so that the image in the eyepiece corresponds to the chart. Look for two stars that just fit in your field of view, and locate these stars on the star chart. You can now measure this distance on the map and compare it with the scale on the margin of the map to convert your linear measurement to degrees or arc minutes.
Remember that 1 degree (°) = 60 arc minutes (60′) = 3600 arc seconds (3600″). Binoculars typically have fields larger than 4degrees , and telescopes normally give a view smaller than 2degrees.
It is essential to be able to judge angular distances in the sky. The following table lists some angular estimates:
Solar / lunar diameter: ½°
Width of index nail at arms length: 1°
Orion’s Belt: 3°
Short arm of Crux: 4½°
Long arm of Crux: 6°
Width of clenched fist at arm’s length: 10°
Long arm of Diamond Cross: 10°
Everyday objects can also serve as angular gauges. To determine the apparent angular size of anything in degrees, divide its linear width by its distance from your eye, then multiply by 57. For example, a 30cm ruler held one metre from your eye measures 30 ÷ 100 x 57 = 17°.
A more accurate method to determine the diameter of your field of view involves measuring the time it takes for a star to drift across your field along the east-west line.
This method is only useful for telescopes, since a star will take ages to cross the large field offered by binoculars. Choose any bright star, preferably far from the south pole – a star in Orion’s belt would be a good choice.
Centre the star in your field of view, turn off the drive, and place the star just outside the eastern edge of the field. As the star drifts into view, start your stop-watch. When the star dis appears at the western edge, stop the watch and note down the elapsed time. Repeat this measurement several times and take the average.
If this average time, T, is measured in minutes, then: field of view in arc minutes = 15 x T x cosine( D ), where D is the declination of the star (taken from a star catalogue, or estimated from a starmap).
For example, suppose you measure several transits of Canopus and calculate the average time to be 3.5 minutes. Canopus’ declination is roughly –52.7°. The field of view is then 15 x 3.5 x cos(–52.7) = 15 x 3.5 x 0.6 = 31.5 arc minutes. Thus the field of view is roughly half a degree across.
Make a note of the size of each eyepiece in your logbook, since a given eyepiece used on a specific telescope has a fixed field of view.
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Deepsky Observer’s Companion (P 13)
Director: Deepsky Observing Section,
Astronomical Society of Southern Africa