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How to determine limiting magnitude?

derherr65

By derherr65
in Observing - Discussion

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derherr65

derherr65

Posted
Posted

What is the easiest way to determine the limiting magnitude of your favorite viewing spot? Does averted vision count? Not knowing the stars especially well I'd need a computer or red light and map to look for specific star examples... and wouldn't that destroy my night vision and affect the test?

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Chris H

Chris H

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The easiest way is to use the constellation of Ursa Minor (the little dipper) because it's always visible from the UK. Go out get your eyes fully dark adapted, this should take atleast 45 mins to an hour, then see which of the stars you can see in the constellation.

See this link for magnitude of stars of Ursa Minor Limiting Magnitude Exercise

Using a red light to see the chart won't destroy your night vision, don't use a laptop though just print out a copy.

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Altair40

Altair40

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Thanks from me, too. I'm off to download a planetarium prog - one reccomended from this site...

Again, more very useful info.

Wouldn't have been able to see too much last night because of the seeing. Just looking through the newt at the moon I could see loads of shimmer like a heat haze.

Must have been the wind stirring up different air densities/temperatures. Non laminar flow!

Maybe on a still night it'll be better. I'm also not sure of the level of light pollution here so a least observable magnitude would help with guesstimating that.

Just pleased I got out at all! Had 5kg dive weight on my tripod tray - did help with the wind.

:)

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Chris H

Chris H

Posted
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Little dipper isn't a stringent enough test, I'm pretty sure I can see all of those, but I like the concept. I'll make it work and report back.

If you can see all those stars then you have a limiting mag of 6.5 which is about as good as it gets.

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  • 2 weeks later...
derherr65

derherr65

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With more than half a moon, walking the dog night vision, leaves off trees making neighbors lights more visible, and a high light haze which is unusual for here, I can see a 5.0. Waiting for a new moon to really do the test.

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acey

acey

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I use Ursa Minor for visual limit and a Sky Quality Meter to measure sky brightness. The meter reading can be converted to an equivalent visual magnitude limit using this Conversion Calculator - NELM (V) to MPSAS (:) systems

In practice I always get a higher estimate from the meter than I can actually see: typically 6.3 when I can only see down to about 5.9. Everybody's eyesight varies.

Strictly speaking you should be taking Zenith Limiting Magnitude (with the meter you just point it straight up and press the button). This can be tricky when doing things visually, unless you have access to software that will give you the magnitudes of whatever stars you can see. In practice Ursa Minor is the most useful because it's always at roughly the same elevation and is visible all year round. And unlike the meter it's free.

BTW the stars in Ursa Minor go down to 7,8, 23... any magnitude you want!

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pete_l

pete_l

Posted
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One thing that's important to recognise is that there's no such thing as an absoluite visual limiting magnitude. All you can do is make an estimate for your eyes, in a particular set of conditions.

Because of that, I favour an SQM (per acey's post). One thing I've noticed with mine is that even with the ability to take objective measurements, the darkness of the sky is not a constant. One (moonless) night I can take a measurement that is +/- a significant amount from what it was under apparently identical conditions the night before - of after. I get variations of at least 0.3 MPAS, which translate to nearly half a magnitude according to the NELM-MPAS conversion in the link above.

However, even making the numerical conversion isn't the end of the story. There are many different models for plotting the sensitivity of the human eye, depending on what assumptions and starting conditions you plumb for - or even whether you assume the observer has both eyes open, or one eye closed. For example, using the Knoll/Schaefer model, an MPAS reading of 19 corresponds to a NELM of 4.77. Whereas using the Blackwell/Clark method¹, the calculation tells you the NELM is 5.8 - nearly a whole magnitude darker! [ref: http://w1.411.telia.com/~u41105032/visual/Schaefer.htm ]

For all of the above reasons, I've become extremely skeptical about claims that people make about the NELM for their pet observing site. Even with instrumentation, there is no single correct answer to the question and darkness is only one attribute that goes towards making a good site for astronomy.

[1] much criticised, but if you *wanted* to promote a site as being "better", which scale would you choose?

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ozark1

ozark1

Posted
Posted

At this time of year, the Great Square of Pegasus is a favoured way to determine limiting magnitude

NELM for # of Visible Stars In

The Great Square of Pegasus

30

Mag 6.5

17

Mag 6.25

13

Mag 6

9

Mag 5.75

7

Mag 5.5

5

Mag 5.25

4

Mag 5

3

Mag 4.75

1

Mag 4.5

0

Mag 4 (or worse)

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RichardJBartlett

RichardJBartlett

Posted
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I'm with Kris and Ursa Minor, however, here at 38 deg North in sunny Kentucky, UMi can be too low... in which case, I'll do my homework beforehand, pick a familiar constellation with some dim stars and look to see which ones are in the mag 4-5 range. I'll go out, wait 30 mins, and look for the faintest star I can in that group.

As ozark has said, Pegasus is a good choice for this time of year. I've used Hercules and Ophiuchus in the Spring and Summer.

Regarding laptops - download DarkAdapted - http://www.aquiladigital.us/darkadapted/ - great little tool for making your laptop screen red and avoiding the loss of your night vision :)

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