Visual Limiting Magnitude

[iceman (Mike)]

How do you determine the limiting magnitude of the sky at your house or your observing location?

I've never bothered to really actually consider it at my house and to work out a true and tested value.. I know that after 10 minutes of dark adaptation I can see the clouds of the milky way, but I also know that at my dark sky sight (25 minutes drive out west) I can see TONNES more stars and milky way clouds..

So, what do you use as your tried and tested method to determine the limiting magnitude?

[iceman (Mike)]

Anyone?

It'll help the newbies, and me!

[rumples riot]

Most people can see 6th magnitude stars on clear nights, now if you look at the coal sack there is a 6th magnitude star in it. When you can see this, you can say you have a limiting magnitude of 6. Now if you cannot see it, then you have above five, so then you have to look for a 5 magnitude star and the same applies. Keep going down until you find the one you can see. So depending on your site, you can determine what the visual limiting magnitude of your site is.

Paul

[iceman (Mike)]

I'm more interested in what stars/constellations <b>you</b> use to determine it, rather than the theory

I've always known the theory, but never bothered to actually find a constellation and map the magnitudes of the stars, and find which ones I can see.

It will be useful as a statistics and log type of thing, to determine good nights from bad, and whether there's increasing light pollution around you etc.

and also to compare the VLM's when you go from your home to a dark sky site.

[seeker372011 (Narayan)]

In the November issue of Sky and Telescope-page 130-it suggests that if you can see two stars inside the great square of Pegasus you are reaching visual magnitude of 4.6.

Guess what, I can't usually see any stars within the Great Square at all from my home so I suppose my suburb is pretty badly light polluted at least to the North

Earlier this year my son did a senior geography (Year 11) project for school where he counted the number of stars visible in the Southern Cross from various locations across Sydney and used that as a measure of the light pollution in various suburbs.

Anyway from our backyard he could see a 5.31 magnitude star (HR4830) which I suppose must be considered the limiting magnitude.

On the other hand all I could see -on the same night-was Epsilon Crux -which is the 3.59 magnitude star that Aragorn also refers to-even with my glasses on.

So age and visual acuity have quite a bit to do with limiting magnitude. I suppose that CCD measurement of stellar magnitudes may be more repeatable and reliable.

[ving (David)]

I cant actually pick a number for magnitude but for some indication of visibility I use the LMC and SMC. On f ull moon i cant see them at all and on a moonless clear night they are quite visible.