Eyepiece and telescope pairing
Celestron Omni 40mm Plossl in the Sky-Watcher Classic 250P (10-inch Dobsonian): magnification and field
What the Celestron Omni 40mm Plossl does in the Sky-Watcher Classic 250P (10-inch Dobsonian), worked from the published focal lengths and apparent field.
Field of view
ring = 1.33 degrees true
dot = 8.5 mm exit pupil
- Magnification
- 30x1200 mm / 40 mm
- True field
- 1.33 degrees40 deg / 30x
- Exit pupil
- 8.5 mm254 mm / 30x
- Max useful mag
- about 508x2x aperture
Computed optics
- Barrel match
- 1.25 in eyepiece in a 2 in focuser
Works, but the 8.5 mm exit pupil is wider than a dark-adapted pupil of about 7 mm, so the outer light is wasted and the sky background looks brighter. Fine for framing, but a slightly shorter eyepiece uses the aperture better.
Magnification is the telescope focal length divided by the eyepiece focal length: 1200 mm divided by 40 mm is 30x. The true field of view is the eyepiece's 40 degree apparent field divided by that magnification, 1.33 degrees.
The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 254 mm over 30x, which is 8.5 mm. That makes this a low-power, wide-field and deep-sky setting.
The eyepiece to buy
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