Magnification = telescope focal length ÷ eyepiece focal length · true field = apparent field ÷ magnification · exit pupil = aperture ÷ magnification. Preset scope specs are typical values — adjust the boxes for your exact instrument. Target angular sizes are approximate.
The recommender ranks the eyepiece case for the chosen target: it rules out anything past 2× aperture or ~250× (Singapore seeing), or with an exit pupil over 7 mm, then scores what is left on framing, image scale and — for faint objects under Jurong's bright sky — exit pupil, since roughly a 3 mm exit pupil keeps the background darkest. Treat it as a starting point, not gospel: the sky, the target's altitude and your own eye all get a vote.
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