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These are the rough guidelines for selecting Messier like objects.
Nebulae are visible in a 6" telescope.
Open clusters are brighter than magnitude 8. (Poor OC are omitted)
Globular clusters are brighter than magnitude 9.
Galaxies are brighter than magnitude 10. (Gxy with low SB are omitted)
Planetary nebulae are brighter than magnitude 11, and larger than 12" across.
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