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TRUE FIELD OF VIEW IN A TELESCOPE

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TRUE FIELD OF VIEW IN A TELESCOPE

Eyepieces have a characteristic called Apparent Field¡ªthis is essentially the width of the field of view as seen by the observer. When you look in an eyepiece, there is an edge to what can be seen and it is somewhere off to the side. Narrowfield eyepieces have apparent fields from 30 to 52 degrees. Widefield eyepieces have apparent fields from 60 to 75 degrees. And Ultrawidefield eyepieces have 78 to90 degree fields of view. This last field of view is so large, it is often necessary to rock the head from side to side to see the edge of the field when looking through the eyepiece¡ªjust as you would have to do when looking out a ¡°porthole into space¡±.

The actual field of view seen will depend on this apparent field AND the magnification of the eyepiece. Low power eyepieces typically have larger true fields (the real size of the field in the sky) of view. There is a simple relationship, and it is expressed by the following formula:
				AF (Apparent Field Width)
	TF (True Field Width) = --------------------------------
				M (Magnification)

If you know two of those numbers, you can easily derive the third.

Or, you can use our Calculator.

How important is it to know the True Field of a given eyepiece in your scope? Objects in the sky come in many different sizes, and you want to be sure the true field can accommodate a view of the entire object.
Every book or observing guide will list a size for each object that can be seen. This size will determine how high a magnification you can use and still see the entire object, preferably with some surrounding dark sky in the image to ¡°frame¡± the object viewed.

If you don¡¯t know the Apparent Field of your eyepieces, there is another way to measure it. You can determine the actual apparent field using the formula by entering a measured True Field. Take a star on the Celestial Equator (look at a planisphere or Atlas to find a relatively bright star on the equator) and, with drive motors in your telescope turned off, time how long it takes for a star to go from one edge in the eyepiece to the other edge of the field:

The True Field, in degrees, is the number of minutes timed (in tenths) divided by 4 (i.e.4 minutes timed = one degree), or the number of seconds timed divided by 240 (240 seconds timed = one degree of arc).

The True Field, in minutes, is the number of seconds timed divided by 4 (60 seconds timed = 15' of arc), or the number of minutes timed times 15 (1 minute timed = 15' of arc)

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TELESCOPE EYEPIECES

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Explore Scientific 25mm 100 Degree field waterproof 2 inch eyepiece
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Explore Scientific 25mm 100 Degree field waterproof 2" eyepiece
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Manuf. No. Explore Scientific EP10025-01

Explore Scientific EP10025-01 25mm 100 Degree field waterproof 2" eyepiece
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Meade Series 5000 Ultra Wide Angle 30mm 2 inch Eyepiece, 82 Degree Field of View
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Manuf. No. Meade 07744

Meade UWA Eyepiece 07744, Series 5000 Ultra Wide Angle 30mm 2" Eyepiece, 82 Degree Field of View
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Meade Series 5000 Ultra Wide Angle 20mm 2 inch Eyepiece, 82 Degree Field of View
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Meade UWA Eyepiece 07743, Series 5000 Ultra Wide Angle 20mm 2 inch Eyepiece, 82 Degree Field of View
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Meade 14mm Series 5000 1.25 inch Ultra Wide Angle UWA waterproof
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Manuf. No. Meade 07742

Meade UWA Eyepiece 07742, 14mm Series 5000 1.25" Ultra Wide Angle waterproof
MSRP: $162.00
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Meade 8.8mm Series 5000 1.25 inch Ultra Wide Angle waterproof
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Manuf. No. Meade 07741

Meade UWA Eyepiece 07741, 8.8mm Series 5000 1.25" Ultra Wide Angle waterproof
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Meade Series 5000 Ultra Wide Angle 5.5mm 1.25 inch Eyepiece, 82 Degree Field of View
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Meade Series 5000 Ultra Wide Angle 5.5mm 1.25 inch Eyepiece, 82 Degree Field of View
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Manuf. No. Meade 07740

Meade 07740 Eyepiece, Series 5000 Ultra Wide Angle 5.5mm 1.25" Eyepiece, 82 Degree Field of View
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Meade Eyepiecs Kit Series 5000 HD-60, 07736 w/ aluminum carrying case
Meade Eyepiecs Kit Series 5000 HD-60, 07736 w/ aluminum carrying case
Item No. 712-18020
Manuf. No. Meade 07736

Meade 07736 Eyepiecs, Series 5000 HD-60, six of the HD-60 eyepieces in an aluminum carrying case
MSRP: $598.00
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Televue Ethos SX Eyepiece 4.7mm 1.25 inch 110 Degree
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Televue Ethos SX Eyepiece 4.7mm 1.25" 110 Degree
Item No. 704-11008
Manuf. No. Televue ETH-04.7

Televue ETH-04.7 Eyepiece, Ethos SX, 4.7mm 1.25" 110 Degree
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Takahashi UW Flat Field Series Eyepiece - 10.0mm
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Takahashi UW Flat Field Series Eyepiece - 10.0mm
Item No. 711-10610
Manuf. No. Takahashi TAK-10UW (TEA0100)

Takahashi TAK-10UW TEA0100 Eyepiece UW Flat Field Series 10.0mm
 
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Takahashi UW Flat Field Series Eyepiece - 7.0mm
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Takahashi UW Flat Field Series Eyepiece - 7.0mm
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Manuf. No. Takahashi TAK-7UW (TEA0007)

Takahashi TAK-7UW TEA0007 Eyepiece UW Flat Field Series 7.0mm
 
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Takahashi UW Flat Field Series Eyepiece - 5.7mm
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Takahashi UW Flat Field Series Eyepiece - 5.7mm
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Manuf. No. Takahashi TAK-5.7UW (TEA0057)

Takahashi TAK-5.7UW TEA0057 Eyepiece UW Flat Field Series 5.7mm
 
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Takahashi UW Flat Field Series Eyepiece - 3.3mm
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Takahashi UW Flat Field Series Eyepiece - 3.3mm
Item No. 711-10603
Manuf. No. Takahashi TAK-3.3UW (TEA0033)

Takahashi TAK-3.3UW TEA0033 Eyepiece UW Flat Field Series 3.3mm
 
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Explore Scientific 40mm 68 degree field waterproof 2 inch eyepiece
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Explore Scientific 40mm 68 degree field waterproof 2" eyepiece
Item No. 737-00940
Manuf. No. Explore Scientific EPWP6840-00

Explore Scientific EPWP6840-00 eyepiece 40mm 68 degree field waterproof 2"
MSRP: $450.00
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Explore Scientific 34mm 68 degree field waterproof 2 inch eyepiece
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Explore Scientific 34mm 68 degree field waterproof 2" eyepiece
Item No. 737-00934
Manuf. No. Explore Scientific EPWP6834-00

Explore Scientific EPWP6834-00 eyepiece 34mm 68 degree field waterproof 2"
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Explore Scientific 28mm 68 degree field waterproof 2 inch eyepiece
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Explore Scientific 28mm 68 degree field waterproof 2" eyepiece
Item No. 737-00928
Manuf. No. Explore Scientific EPWP6828-00

Explore Scientific EPWP6828-00 eyepiece, 28mm 68 degree field waterproof 2"
MSRP: $300.00
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