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What's my true field?

Try these three methods to determine an eyepiece’s true field of view.
ChapleGlenn

Last summer, I received an email from 13-year-old Adriana Baniecki in Chandler, Arizona. She wrote, “I have just started viewing the sky with my new telescope, which has an aperture of 114mm (4.5 inches) and a focal length of 910mm. I have 25mm, 12.5mm, and 4mm eyepieces, as well as a 3x Barlow. I was wondering, given my eyepieces, what magnification would be needed to view the Moon and planets.

“Also, in your January Astronomy column, ‘January’s top 10 targets,’ I noticed references to both the angular size of an object and the magnification needed to view it. [Author’s note: I had mentioned that the Pleiades star cluster (M45), which spans 2°, is best viewed with low power.] Given the angular size of any object, could you simply calculate the magnification needed to view it? If so, how?”

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