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# Magnitudes with Binoculars...

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Hi All,

At my location on nights when naked eye seeing is around mag 5, I can see stars of just about mag 8 as pinpoints of light with my 10x50s.  Is there a simple formula to estimate what magnitudes I would see under the same conditions with larger binoculars?

Thanks

Mark

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Posted (edited)

Very odd - based on a NELM of +5.0, I would have expected a M(lim) at 50mm of around +9.0 to +9.5, depending on your pupil diameter.

Theoretical formula is [M(lim) at aperture D] = [M(lim) at aperture d] - 5 log d + 5 log D

Based on your 50mm M(lim) being +8.0, M(lim) = 8.0 - 5 log 50 + 5 log D, where D is the binocular aperture

Based on your NELM being +5.0, M(lim) = 5.0 - 5 log d + 5 log D, where d is your pupil diameter and D is the binocular aperture

They will give you different results, but should give you a clue. No formula can do it for you precisely, though  (no such thing as a "normal" observer ).

HTH

Edited by BinocularSky
typos

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4 minutes ago, BinocularSky said:

Very odd - based on a NELM of +5.0, I would have expected a M(lim) at 50mm of around +9.0 to +9.5, depending on your pupil diameter.

Theoretical formula is [M(lim) at aperture D] = [M(lim) at aperture d] - 5 log d + 5 log D

Based on your 50mm M(lim) being +8.0, M(lim) = 8.0 - 5 log 50 + 5 log D, where D is the binocular aperture

Based on your NELM being +5.0, M(lim) = 5.0 - 5 log d + 5 log D, where d is your pupil diameter and D is the binocular aperture

They will give you different results, but should give you a clue. No formula can do it for you precisely, though  (no such thing as a "normal" observer ).

HTH

Thanks for the info Steve.. I'll have a go at getting my head round that formula (failed physics at school)

Yes, with the 50s I can see dimmer than mag 8 but they do begin to fall into a different bracket which eventually ends up being the 'small grey pixel' bracket..

Mark

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