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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?”

I responded by suggesting that she always start with the 25mm eyepiece, which yields 36x with a 910mm focal length scope, because its large field of view makes it easier to key in on sky objects. I also recommended this eyepiece for deep-sky objects wider than 0.5°, such as the Pleiades and the Andromeda Galaxy (M31), adding that the 12.5mm eyepiece (73x) and 25mm eyepiece with 3x Barlow (109x) would work fine on the Moon and planets.

What about the magnification needed for an object of given angular size? I pointed out that an eyepiece’s true field of view, not its magnification, is the ultimate determining factor. This caused me to do a little soul-searching. During my nearly five decades as an avid backyard astronomer, I’ve become familiar with the magnifying power all my eyepieces produce with my various telescopes, but I knew next to nothing about their true fields of view. With my 10-inch f/5 reflector, for example, I normally use three eyepieces: a 32mm (40x), a 16mm (79x), and a 6mm (212x). Spurred to action by Adriana’s email, I went outside with scope and eyepieces and went to work determining their true fields of view.

ChapleMoon
The Full Moon has an apparent diameter of about 0.5°, a convenient reference for calculating an eyepiece’s true field of view.
John Chumack
True to your field

There are three basic ways to figure out the true field of view an eyepiece provides. One is to use the Moon, which has an apparent diameter of 0.5°, as a measuring tool. So if an eyepiece captures a chunk of sky three Moon diameters across, it has a true field of view of 1.5°. With each eyepiece, I measured how many Moons, or what fraction of a Moon, I could fit in the field. Because this is the most subjective of the three methods and I didn’t want to bias my results, I used this method first.

My tool for the next method was our spinning planet. Because Earth rotates once every 24 hours, covering 360° of sky, a star near the celestial equator will drift 1° every 4 minutes. If you time how many minutes it takes the star to enter the field of view, cross the center, and exit on the opposite side, then divide that time by 4, you have the true field of view in degrees. If the transit time is 6 minutes, for example, the true field is 1.5°. My star of choice was 3rd-magnitude Sadalmelik (Alpha [α] Aquarii), located just 19' south of the celestial equator. I ran several trials for each eyepiece.

The final method can be done mathematically while indoors. An eyepiece’s true field of view equals its apparent field of view (AFOV) divided by the magnifying power it yields with a given scope. An eyepiece with a 60° AFOV that magnifies 40x has a true field of 1.5°. Calculator in hand, I worked out the true field for each eyepiece. The results for all three methods appear in the table below.

ScreenShot20171229at3.00.23PM
Some final thoughts: Because the Moon’s angular size varies with its distance from Earth (0.49° when farthest away, 0.56° when closest), it’s not an ideal measuring tool. Nevertheless, the ballpark figure you get is better than nothing, as my results show. Also, by the time this issue reaches the newsstands, Sadalmelik won’t be as easy to use for star-drift timings. Work instead with 2nd-magnitude Mintaka (Delta [δ] Orionis), the northernmost and westernmost of the three stars in Orion’s Belt and just 18' south of the celestial equator.

Finally, the AFOV of an eyepiece depends on its design. Here are approximate AFOVs for traditional types: Huygens or Ramsden (labeled “H” or “R” on the barrel), 30°; Kellner, achromatized Ramsden, or modified achromat (“K” or “Ke,” “AR,” or “MA”), 40°; orthoscopic (“Or” or “Ortho”), 45°; Plössl, 50°; and Erfle (ER) or König, 60°. For newer designs, especially super- and ultrawide types, refer to the manufacturer’s website for specifics.

Questions, comments, or suggestions? Email me at gchaple@hotmail.com. Next month: another 13-year-old and her eclipse adventure.

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