Eyepiece and telescope pairing
Tele Vue 3.5mm Nagler Type 6 in the Orion SkyQuest XT4.5 Classic Dobsonian: magnification and field
What the Tele Vue 3.5mm Nagler Type 6 does in the Orion SkyQuest XT4.5 Classic Dobsonian, worked from the published focal lengths and apparent field.
Field of view
ring = 0.32 degrees true
dot = 0.4 mm exit pupil
- Magnification
- 257x900 mm / 3.5 mm
- True field
- 0.32 degrees82 deg / 257x
- Exit pupil
- 0.4 mm114 mm / 257x
- Max useful mag
- about 228x2x aperture
Computed optics
- Barrel match
- 1.25 in eyepiece in a 1.25 in focuser
Over-powered. At 257x this pair exceeds the highest useful magnification of about 228x for the Orion SkyQuest XT4.5 Classic Dobsonian, so the image dims and softens without showing more real detail. Use a longer eyepiece.
Magnification is the telescope focal length divided by the eyepiece focal length: 900 mm divided by 3.5 mm is 257x. The true field of view is the eyepiece's 82 degree apparent field divided by that magnification, 0.32 degrees.
The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 114 mm over 257x, which is 0.4 mm. That makes this a high-power lunar, planetary and double-star setting.
The eyepiece to buy
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