# Question on EP Field of View

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#### bbrock

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A couple of nights ago I got the chance to do something I've wanted to do for a long time. I measured my EP's Fields of View using my 10" reflector. <br /><br />I'll explaine how for anyone interested after I ask my question. <br /><br />Question. All of my EP's FOV are reduced by 41% to 45% using a 2x barlow. I would think this should be precisely 50%. <br /><br /> For example, the 10mm Plossl gives me a FOV of 0.4053 arcdegrees and using the same EP with a 2x barlow gives 0.18116 arcdegrees. Well, 0.18116/0.4053 = 0.44692, 44.7 percent, not 50 percent. Can anyone explain this?<br /><br />Measurement Method. If you locate a star on or close to the Celestial Equator ( Starry Night would help identify one. ) The rate of rotation is 15 arcseconds per second. Using a Hyparcus star about 1 degree above Mintaka ( Orion Constellation ) gives you a star extremely close to the equator. <br /><br />So if you time the star as it travels from one end of your eyepiece -- through the center and to the other side of your eyepiece. you know that each second represents 15 arcseconds. Using a stopwatch and the above mentioned star, I calculated the Fied of View of my EP's , including with a 2x barlow. <br /><br />Example: Orion Expanse 6mm<br /> Traverse Time for star on Celestial Equator = 87.195 Seconds (time).<br /><br />87.195 x 15 arcseconds/sec = 1307.925 arcseconds<br /><br />1307.925 / 60 = 21.789 arcminutes<br /><br />21.789 / 60 = 0.36331 arcdegrees FOV<br /><br />Bill

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#### heyscottie

##### Guest
Ah, yes. Barlow magnification is rarely exactly as marked, because the magnification depends on the eyepiece design somewhat. The distance of the lens in the Barlow to the focal plane of the eyepiece affects magnification. As you increase this distance, magnification will increase. In fact, you can really have a 'variable' Barlow by putting extension tubes between your Barlow and your eyepieces!<br /><br />In practice, of course, it's probably best to avoid this practice, and use the lenses as they come, but the option is there.<br /><br />Anyway, that's why your Barlow factor is not exactly 2X.<br /><br />Scott

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