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Hi guys,
For astro work, you ideally want a 7mm exit pupil. This is because at night, your pupil dilates to around 7mm. Anything less is just a waste. This is why the purpose-built astro binouclars come in sizes like 9X63 (63 divided by 9 gives you 7). The other good size is 7X50. As a general rule of thumb, the most magnification that someone can usually hold steady is 10X.
Clear skies,
Shane
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