How can i tell what magnitude my skies are?

[Ccolvin968]

I've seen some people say their skies were magnitude 16, 17, 18, etc. How do you determine this? Is there an app or website? Does this tell you the minimum magnitude that you can see from your location or something totally different?

[Bizibilder]

those numbers sound like the faintest objects that can be "seen" photographically by taking a long exposure (maybe hours).

Normally the figure quoted is the faintest star that can be seen with the naked eye on a clear night.  If your skies are really dark then mag 5-6 is possible.  From urban areas maybe mag 2 would be the limit.  The easiest way to tell is to use a star chart and slowly work your way around the sky trying to see progressively fainter stars.  If you are in a rural location mag 4 should be easy enough and you can work your way fainter from there.  don't forget that nights do vary due to high cloud and haze as well as light pollution.

You should be able to find a light pollution map for your area like here:  http://cleardarksky.com/csk/prov/Minnesota_map.html  or maybe easier to read here:    http://www.jshine.net/astronomy/dark_sky/

[JamesF]

There's a method for estimating naked eye limiting magnitude by checking which stars you can see in Ursa Minor.

One example is here: https://darkskydiary.wordpress.com/2012/02/25/naked-eye-limiting-magnitude-redux/

James

[Ccolvin968]

It probably is with a camera. Didn't think of that. 4.4 seems to be my limiting magnitude. Closer to 5 on a perfect night.

[YKSE]

The 16,17,18 etc you saw are probably the other way of measuring the sky darkness, meauring Magnitude-Per-Square-ArcSecond(MPSAS), instead of the commonly used Naked-Eye Limiting Magnitude(NELM). That 18 MPSAS equals 4.0 NELM, not so good sky for DSO hunting. You can easily do the conversion using this link

[Ccolvin968]

Thanks ykse!

[Stu]

That's right, those figures are normally measured with a Sky Quality Meter (SQM). I bought one recently and find it really interesting to see how it affects what you can see. Of course transparency also plays a big part.

Your skies sound similar to mine, around mag 4.5, down to mag 5 on a really clear night. From my measurements this is around 19 with the SQM

I find this link useful for calculating the visibility of extended objects, I believe it uses the same basic algorithms that Yong (YKSE) linked to.

http://www.bbastrodesigns.com/VisualDetectionCalculator.htm