Measuring your dark sites skies...

[Mike73]

Theres a few threads in our group about dark sites in the Devon and Cornwall area but you don't know how dark your site is until you measure it somehow.

If you click on the link below and go to my blog you'll be able to download and print a PDF which will give you NELM (Naked Eye Limited Magnitude) so if everyone did this at there dark site in Devon and Cornwall we'd have a pretty accurate 'map' of where to go rather than relying on simulators like Needless.org.

I tried adding the PDF to the forum but for some reason the forum wouldn't allow it.

http://darkskysketch...-magnitude.html

You can also register and add your findings to https://www.mydarksky.com/home.aspx which will help visiting astronomers.

[sologuitarist61]

Thanks Mike will have a look at that. Strange isn't it that you cannot seem to add a Word doc or PDF, tells me it is the wrong sort of file whenever I try.

[JamesF]

I wrote a couple of paragraphs on the same subject in mid-March: http://www.tanstaafl.co.uk/2012/03/measuring-naked-eye-limiting-magnitude/

I'd think it's a useful thing to do though obviously there are a few variables, the seeing on the night being one (I'm looking forward to repeating my test on a night when the seeing is spot-on) and the visual acuity of the observer. It can also be a useful thing to do every night you're out because it means you then have a reasonably good record of the seeing for comparative purposes.

James

[Mike73]

I wrote a couple of paragraphs on the same subject in mid-March: http://www.tanstaafl...ting-magnitude/

I'd think it's a useful thing to do though obviously there are a few variables, the seeing on the night being one (I'm looking forward to repeating my test on a night when the seeing is spot-on) and the visual acuity of the observer. It can also be a useful thing to do every night you're out because it means you then have a reasonably good record of the seeing for comparative purposes.

James

I recently bought a Sky Quality Meter (although not had much chance to use it!) to give me a more accurate reading. Should be interesting comparing SQM results alongside sketches in different seeing conditions over time.

[JamesF]

Ah, well, if I could afford an SQM I'd have spent it on something else already

James

[LukeSuppalot]

I thought I'd try the NELM test from my garden around Midnight. It seemed to be 5.7 looking directly at the stars ( about 6 using averted vision). Pretty good I thought. Need-less caculates about 5.5' so pretty close really. It stayed like that until about 1.30ish. Milky way was the best I've seen since Feb.

[Mike73]

Blimey!! NELM of 6 from your back garden int too shabby! Mines 5.3 from home but I'm probably about 4 miles from Plymouth city centre.

[sologuitarist61]

Can't even see Ursa Minor from my garden!!! You lucky guys. :smiley:

[LukeSuppalot]

Blimey!! NELM of 6 from your back garden int too shabby! Mines 5.3 from home but I'm probably about 4 miles from Plymouth city centre.

Trouble is I only have views from W to NE.

[webboid]

Very interesting information. As I am an urban astronomer I'll have to check this out

Could see quite a lot of stars last night mind you

[DrRobin]

Hi,

I used the method in the following link to measure my VLM at around 5.5. http://www.pbase.com/image/37608572

If I use the Bortle scale I would say I was a Bortle 6, which is VLM 5.1 to 5.5.

If I use need-less it comes out much lower, only around 4.7.

[acey]

Very good idea to collect limiting magnitude data, and SQM where available. The Sky Quality meter is not expensive compared to a decent eyepiece, and I use my meter a lot more than some of my eyepieces.

As has been said, there's a lot of variability in reported limiting magnitudes, for all the obvious reasons, so if you're gathering data it's worth specifying to contributors what exactly you're looking for. One definition is:

Faintest star at zenith seen steadily with any search method (i.e. with or without averted definition).

Another is:

Faintest star definitely glimpsed at zenith (need not be seen steadily) by any method.

And there are other permutations with or without averted vision. The first definition would, I think, be the most widely preferred. Also, many people don't appreciate the effect of atmospheric extinction: as well as good transparency (no haze), the star should be as near as possible to zenith, or else some other standard elevation should be used. Ursa Minor varies little in elevation and is the standard sequence many people use.

The Sky Quality Meter is obviously more accurate and reliable but it's useful to compare meter readings with limiting magnitude estimates as a test of theoretical models. If you're gathering data then it's also useful to ask contributors to state their age as this has an obvious effect on limiting magnitude.

The Need-Less site says it uses "a complex program" to estimate limiting magnitude at a given location: I'm guessing it uses Garstang's light-polliution model, and maybe Schaeffer's limiting magnitude formula. The latter is used to convert SQM readings to estimated limiting magnitude at the Unihedron site.

These models can often differ from what people actually see. For example, at a particular site there may be little sky-glow but a lot of ground-level glare from a relatively small number of bright lights. Getting data on what people actually see is therefore very useful for the purposes of selecting a good location.