Society for Popular Astronomy

Limiting Magnitude
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Author:  chrispldexe [ Fri Jul 29, 2016 2:20 pm ]
Post subject:  Limiting Magnitude

I understand that the limiting magnitude from a clear dark sky site is 6.5.
I live in a city centre where the limiting magnitude is 3.5 if I, m lucky. I have read in astronomy guides that 10x50 Binoculars can reach down to magnitude ideal condtions.Similarly a 150 mm telescope can reach down to 13.6
Presumably frpm my light polluted site I can only get to magnitude 7 in binoculars or 10.6 in the 150 mm telescope.
Is this correct or am I completely wrong.


Author:  David Frydman [ Fri Jul 29, 2016 2:40 pm ]
Post subject:  Re: Limiting Magnitude

Chris, you are correct.
However, with mag 3.5 maybe 3.0 unaided eyes, there are so few stars, you may have difficulty locating what you want.
A planisphere will help. Perhaps larger size.

The mag 13.6 with 150mm is with high magnification, say 120x or 150x so you need a very steady mount. A cheap mount will not go so faint.
So I would expect mag 10 with a 150mm scope where you live.
But again high magnification might let you get to mag 11, maybe fainter as high magnification darkens the sky background.
With the Pleiades you can go faint as you have fixed companions. In an empty sky, more difficult.

Also glimpsing is one magnitude fainter than holding the star continually.
Experience is important and averted vision.


Author:  brian livesey [ Tue Aug 02, 2016 3:26 pm ]
Post subject:  Re: Limiting Magnitude

I'm on the edge of a small town Chris, but being on the edge offers no escape from light pollution; only occasionally is it possible to see stars below about mag 3 with the unaided eye. There are rare nights when the band of the Milky Way can be seen overhead, but not all the way down to the horizon.
These nights of improved clarity usually occur when water vapour and particulate pollution in the atmosphere are lower than usual. Most of the sky glare from light pollution is caused by light be reflected back to the ground and into our 'scopes from water droplets and sooty particles in the air. A night of good clarity indicates relatively clean atmospheric conditions.

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