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James K
James K
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This is probably not answerable in simple terms.

Generally, magnitude 6 is the limit of normal vision under ideal conditions, though some have reported seeing objects dimmer than this. Julius Schmidt reported that he counted 102 stars in the square of Pegasus meaning; he was seeing down to magnitude 7.4.

However the rod cells of the eye can respond to a single photon. A source that has single photons, with long periods between photons could have arbitary high magnitude. (eg a magnitude 20 source has about 1.0 photons per second per square-metre, per nm of wavelength, so allowing for a visual band of about 200nm and an iris area of 5e-5 m² results in about 1.05e-52000×5e-5×200 means about one photon every 100 seconds. - but if the eye can respond to single photons, it could be "seen", if there were no other sources of light. In practice it would not be possible to separate a magnitude 20 source from other objects in the sky and it would be impossible to be consciously aware of it)

But it is probably reasonable to estimate that a skilled and trained observer could see down to about magnitude 7.5.

This is probably not answerable in simple terms.

Generally, magnitude 6 is the limit of normal vision under ideal conditions, though some have reported seeing objects dimmer than this. Julius Schmidt reported that he counted 102 stars in the square of Pegasus meaning; he was seeing down to magnitude 7.4.

However the rod cells of the eye can respond to a single photon. A source that has single photons, with long periods between photons could have arbitary high magnitude. (eg a magnitude 20 source has about 1.0 photons per second per square-metre, per nm of wavelength, so allowing for a visual band of about 200nm and an iris area of 5e-5 m² results in about 1.05e-5200 means about one photon every 100 seconds. - but if the eye can respond to single photons, it could be "seen", if there were no other sources of light. In practice it would not be possible to separate a magnitude 20 source from other objects in the sky and it would be impossible to be consciously aware of it)

But it is probably reasonable to estimate that a skilled and trained observer could see down to about magnitude 7.5.

This is probably not answerable in simple terms.

Generally, magnitude 6 is the limit of normal vision under ideal conditions, though some have reported seeing objects dimmer than this. Julius Schmidt reported that he counted 102 stars in the square of Pegasus meaning; he was seeing down to magnitude 7.4.

However the rod cells of the eye can respond to a single photon. A source that has single photons, with long periods between photons could have arbitary high magnitude. (eg a magnitude 20 source has about 1.0 photons per second per square-metre, per nm of wavelength, so allowing for a visual band of about 200nm and an iris area of 5e-5 m² results in about 1.0×5e-5×200 means about one photon every 100 seconds. - but if the eye can respond to single photons, it could be "seen", if there were no other sources of light. In practice it would not be possible to separate a magnitude 20 source from other objects in the sky and it would be impossible to be consciously aware of it)

But it is probably reasonable to estimate that a skilled and trained observer could see down to about magnitude 7.5.

Source Link
James K
James K
  • 125.8k
  • 6
  • 314
  • 440

This is probably not answerable in simple terms.

Generally, magnitude 6 is the limit of normal vision under ideal conditions, though some have reported seeing objects dimmer than this. Julius Schmidt reported that he counted 102 stars in the square of Pegasus meaning; he was seeing down to magnitude 7.4.

However the rod cells of the eye can respond to a single photon. A source that has single photons, with long periods between photons could have arbitary high magnitude. (eg a magnitude 20 source has about 1.0 photons per second per square-metre, per nm of wavelength, so allowing for a visual band of about 200nm and an iris area of 5e-5 m² results in about 1.05e-5200 means about one photon every 100 seconds. - but if the eye can respond to single photons, it could be "seen", if there were no other sources of light. In practice it would not be possible to separate a magnitude 20 source from other objects in the sky and it would be impossible to be consciously aware of it)

But it is probably reasonable to estimate that a skilled and trained observer could see down to about magnitude 7.5.