Binocular Limiting Magnitude

[Cygnus_X1]

I wish to test my new Olympus 10 * 50 ( yes, I know they are cheap, but they seem pretty good so far ) by seeing just what lowest magnitude they will display. The question then is...what is the actual limiting magnitude for a pair of 10 * 50 ?

I've seen varying claims on the web that limiting magnitude is anywhere between 9 and 10.5. However, I've also had a 60mm refractor for years and have seen down to magnitude 11 with that ( under ideal conditions  ) despite its supposed limit being around 10.

Alas the skies have been too hazy in recent nights to perform any reliable test, and I've yet to see any star lower than magnitude 7.5. At least...I hope it IS the haze and moonlight and not the binoculars. But when a good clear night does finally emerge, what magnitude should I see down to ? Is there some official formula for it ?

[AlexB67]

The moon would very much have a bearing on it. Sounds about right that is how far you would get. I ground some numbers through the magnitude estimator I have, making a binocular correction to sky background brightness calculation, since you now have two objective lenses, there may be some other assumptions you need to make I've not considered, so lets say a little bit crudely I estimate at zenith you should get to about 9.5 - 10 in a pair of bins for NELM skies like 6 or so, depending how much light is lost inside some of the components and overall quality of the bins lenses . A city sky or of NELM 4 and around 45 degrees from zenith would put you in at about 8 without a moon at best when well dark adapted.  So, with a moon I'd say you are doing very well to see anything that low mag, I would not worry about the bins anyway. 

[michael.h.f.wilkinson]

Difficult to say, as many factors play a role. I think I could reach mag 10 with my 10x50 bins (rather more upmarket Bressers than the Lidl ones), but only under good conditions. Under suburban skies I tend to need my 15x70s.

[BinocularSky]

Limiting magnitude tables are universally misleading: too many variables:

• Observations and assumptions from which the table was constructed may be inapplicable to other conditions
• Atmospheric seeing can render small instruments able to see fainter objects than larger ones
• Most binoculars/telescopes and all observers are not "normal"
• Magnification will affect the outcome
• Direct or averted vision
• Bright field objects affect dark adaptation

Easiest way to reliably do it is to count the stars in a designated area of sky, e.g the region of the Pleiades surrounded by Alcyone, Maia, Electra and Merope. The number of stars (not including the four already mentioned!) relates to LM like this:

No    LM

6       +9.0

7       +9.5

9     +10.0

12  +10.5

Lots of other regions of sky can be used -- do a search.