Visual limiting magnitude

[Tiki]

The other night 24 Canes Venatici (visual mag 4.68) was not visible as a steady light but as somewhat intermittent light. What therefore was the VLM?

Thanks in advance.

[acey]

There is no universally agreed rule on how limiting magnitude is defined. It's up to the individual observer to decide whether it's the faintest star seen with direct or averted vision, seen steadily or intermittently, viewed with one or both eyes, seen at the zenith or corrected for atmospheric extinction, viewed with or without glasses....

And that's before we even consider star colour. Also, when checking the listed magnitude of a star, you need to know if it's V-magnitude or some other scale, and also how the figure has been obtained (some stars are variable, of course).

All of this makes "limiting magnitude" a very fuzzy concept. I would say that the main thing for any observer is to decide on a method of assessment and then use it consistently. Then it will at least be possible for the observer to compare different sites or nights. But when it comes to comparisons between different observers, we've all got different eyesight, so one person's 4.5 could be another person's 5.5.

[KevUU]

I saw the title and came to give you my thoughts, but after reading acey's post there's nothing else to say! What he said ^

[Tiki]

There is no universally agreed rule on how limiting magnitude is defined.......

.... But when it comes to comparisons between different observers, we've all got different eyesight, so one person's 4.5 could be another person's 5.5.

Thanks Acey. I have hitherto assumed that when i hear VLM figures quoted that there is some absolute scale to which people are referring.