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Old 11-07-2019, 04:41 PM
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astro744 is offline
Join Date: Apr 2007
Posts: 805
At night time you pupil dilates more so the percent of central obstruction to pupil diameter is less. E.g. For a particular eyepiece perhaps 2mm of 6mm is obstructed (33% by diameter). At daytime the pupil is smaller say 3mm. At 3mm you have the same 2mm central obstruction for the same eyepiece, so (66% by diameter).

The effect is more noticeable at low power. I should really be talking about area not diameter but the ratio is the same and is easier to follow.
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