Visual limiting magnitude

[Tiki]

From my back garden on a good night, i can just about make out The Milky Way with direct vision providing it's at the zenith. What is the approximate VLM of my zenith skies under these conditions?

Thanks in advance.

[umadog]

You might want to take a look at the Bortle scale: http://en.wikipedia.org/wiki/Bortle_scale It places more emphasis on the overall sky quality. This is to get around some problems with visual limiting magnitude: namely that different observers can come up with quite different VLM for the same site on the same night. It sounds like you're probably Bortle 6, which means a VLM of about 5.0 to 5.5.

[Tiki]

Thanks for the link umadog, many questions answered.

[acey]

Best way to check your naked eye limiting magnitude is to identify faint stars near the zenith then check their V-magnitude (e.g. with Stellarium). Or use the stars of Ursa Minor as a rough guide. I would be interested in knowing what your NELM is at times when you are just able to see the Milky Way.

Another way is to use a Sky Quality Meter. I personally find that a reading of about 20.3 corresponds to the Milky Way being faintly visible. According to the conversion calculator at the Unihedron website this corresponds to a limiting magnitude 5.7, though I've found the estimates from that calculator to be rather high compared to my own observations. Everyone's eyesight is different, the Milky Way is not uniformly bright, my own estimate was not made with the Milky Way at zenith, and my eyes are getting old. But as a rough guide I'd say that faint Milky Way visibility should correspond very approximately to 5.5 (to the nearest half magnitude). Certainly once you get to 6 the Milky Way is brightly and easily visible right across the sky, and at 5 I reckon you'd struggle to see the Milky Way.

[moffer]

This is to get around some problems with visual limiting magnitude: namely that different observers can come up with quite different VLM for the same site on the same night.

That was certainly the case when my son and I decided to check out the NELM for the first time at our local dark site. Transparency was excellent on the night and we used Ursa Minor as our guide. I went first and my ageing eyes could only manage with certainty 3 of the 4 stars that make up the rectangle - I struggled with the 4th star at mag 5.0... pathetic or what? I knew my son would do better but was more than a little surprised when he managed to see a star of mag 6.75 situated within the rectangle. Fortunately for me we did the NELM just before we packed up for the night, so I only had to put up with the proverbial take from him until he dropped me off at my home just a short drive away.