What would be the highest possible magnitude $m$ of some star that could be viewed with a naked-eye? I am acquainted with this question, but mine is about the ideal conditions.

In order to achieve maximum possible efficiency, I would suggest these points to be considered:

  1. Find (or better, construct) a monochromatic star with the peak wavelength at the maximum of the human perception ability (in order to avoid bolometric correction and ensure maximum efficiency).
  2. Find the most experienced astronomer possible with extraordinary viewing abilities.
  3. Place him out of the atmosphere, therefore in the middle of nowhere (just an astronomer and a star of magnitude $m$).
  4. Let him use the averted vision technique.

Do you suggest any other points that should be considered? What is then the maximum possible magnitude $m$?

  • $\begingroup$ Do very close supernovae count ? ;-) $\endgroup$ Commented Mar 5, 2022 at 0:17

2 Answers 2


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.


There has been a series of papers regarding this throughout the first half of the last century. According to an extensive review paper by H.F. Weaver the limiting magnitude under ideal conditions is normally about 6.5, but this can be increased to about 8.5 if one observes the star through a hole in a large blackened screen (thus eliminating the sky background), or if one just uses an artificial star in a blackened lab. The latter value should then also apply if you observe for instance in space, provided you avoid star regions affected by the zodiacal light.

And yes, averted vision should help here, because at low light levels you rely on the rod cells in the retina (which are much more sensitive to light), but there are not many in the central region of the retina (where the focus is).


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